I seem to remember seeing somewhere that you could up the compression by building up an area of the head by welding .
Just a wondering if this was worthwhile on an M21 + any pointers on where and how to do it , its an alloy head ? .

Maybe i should just wait to see how i like riding the old girl as she is .

Cheers
Chilly

['Maybe I should just wait to see how I like riding the old girl as she is..']

That would probably be the best plan initially...

Although the M21 falls into the 'slow ride' category they are quite relaxing to ride and have a certain character in that format which definitely grows on you I have found...

Certainly for running about locally you might find it quite sufficient...

If not, it's quite interesting and challenging to try and coax a few more brake horse power from what is essentially a stationary engine that has had wheels fitted...

Really though, those sort of things are usually done once someone has already decided they like sidevalves, even with all their limitations...

Fitting a standard B33 engine, or even a mildly tweaked B31 engine would give you more power than you are ever likely to extract from an M21, with better acceleration and top speed and vastly improved fuel consumption... ...Ian

email (option): ian@wright52.plus.com

I have a B33 piston but my barrel has a liner , would this piston fit ok ? , not that i'm going to do it at the moment , i've just had a nice NOS + 40 piston fitted

Not recommemded unless you remove the original liner and rebore the barrel to suit a larger diameter liner that will accept the B33 piston...

If you chose that course of action the liner wall thicknes should not be more than 2.5mm...

Essentially BSA either used a low quality barrel casting with a quality liner fitted or, later on, a better quality barrel casting that didn't need a liner.

So if you use an early cylinder but without a liner fitted it's a definite gamble...

Regarding compression ratios if you alter that don't go beyond 7-1 or you will very likely have pre ignition problems...Ian

email (option): ian@wright52.plus.com

Thanks Ian , all food for thought

Domed piston in the 16H Ian but not sure how much more effective this makes it to the M20............I know you've built a few 16H engines in your time, but ownership and running are the true test..........

The 16H engine is a far older design than the M20.........the latter is "finer" engineering............but a well set-up 16H is still pretty damn good.............

Always worth seeing if you can coax a few more BHP out of your sidevalve......regardless of the make........but my own 16H will still hit a genuine (timed) 70mph on standard cams and (uprated) gearing.................bit wobbly at this speed tho, steering damper needs to be screwed down and a tight grip on the 'bars !

I like Norton's, but equally have done many, many miles on M20's too and no preference between the two........... However, I like the simplicity of the 16H - v - the slightly increased complexity of the M20..........

I've concluded the M20 and the 16H represent two slightly different ways of achieving a similar result and each design has its good and bad points...

I have a preference myself for engines with valve timing marks...Something I have yet to find in a Norton engine...or at least in the Big 4's I've had apart...

There's no doubt though, both bikes gave good service.....Ian

email (option): ian@wright52.plus.com

The 16H engine bereft (usually) of timing marks can be a pain......even if marks are present, there's no guarantee of accurate and proper timing..........it has to be set up either "by eye/feel" or with a degree disc..........

I noticed that the 16H additionally does not like changes in the carb' needle position.......I altered mine once on a NOS carburetter and it just would not "rev" nor perform........when I put the needle back to the original position (just one notch) it transformed it.........

A well set-up 16H can be rather quick and I always found it far easier to start than the M20............but in balance, the last M20 that I used on a regular basis was also very quick on acceleration and far more mechanically quiet.............even though my 16H is properly set-up and in good mechanical condition on tick-over it still reminds me of someone "chiseling stone".........

Ian, Commandos have timing marks (but I suppose they're really AMC 'Hybrids' and not proper Nortons)...the trouble is the factory manual refers to 'ten rollers apart' on the timing chain and the illustration shows ten pins with the marks aligning with the roller pins... but in real life, the marks are on the 'peaks' so there are ten rollers in between but it looks nothing like the drawing....and once the oil pump worm is on the crank pinnion, it's almost impossible to visually check as the mark on the pinnion is hidden...apart from that, it's a perfect system and much less rattly than a Triumph twin with all those backlashing gears !

Whereas with the BSA a 'dot' lines up with a 'dot' and a 'dash' with a 'dash'..simple...

However, this is just one aspect which is plainly a better design on the M20 than the equivalent arrangement on the Norton...

The Norton also has some features which are more soundly designed than the BSA...

I like the rebound (damping) springs on the Norton forks for example which are far more effective than the virtually non functioning damper on the M20....

As a matter of interest is there an accurate figure for the number of Nortons produced during WW2?...It seems to vary anywhere between 70,000 and 100,000 depending what you are reading...

I doubt the latter figure myself..so what do the records show?..Ian

email (option): ian@wright52.plus.com

Ian, there are some things that I'm not prepared to discuss on open forum....we have to keep these myths alive, you know.

Any mention of Norton WD production inevitably includes pre-war sales...Do the BSA figures include all KM & WM20s ?

Some of you Norton boys are very sensitive...

I'm not looking to draw any comparison or any conclusions from the relative production numbers....I'd just like to know the figure for Norton wartime production out of interest...A variation of around 35% in the quoted figures is a lot after all...

Since you ask though ...the BSA production figure often quoted of 125,000 or so is given as the wartime production figure...So that wouldn't include K M20s if strictly accurate...

I wouldn't be at all surprised if it included all of the 1939 production year as well though...(August 38-August 39)...Ian

email (option): ian@wright52.plus.com

It's an interesting point Ian/Rik..........did these figures include those early war machines from 39-40.....?

I must hold my hands up and say that I've never tried to add up the total numbers of BSA, Norton and AMC production.........I'd assume that the published figures also include other models such as the BSA B30's, C10's, C12's, Norton Big 4's, Matchless G3's and G3L's, etc.............? The total wartime production from all manufacturers was published as over 425,000...........nearly half a million........

The calculation is frought with errors though, and the true figures are unlikely to ever be established............for example, Triumph were effectively out of the game between the end of 1940 and April 1942 after their factory was flattened in the Coventry blitz.........trickle production of the 3SW, 5SW and 3H continued from temporary premises until the new factory at Meriden opened in 1942 and then for 3HW production only..........

I'm fairly confident however, that BSA had the majority, followed by Norton and then very closely by AMC..............Ariel, Triumph and Royal Enfield were next, followed by Velocette, James, Excelsior, etc.........

The Norton fork wears well.......but the downside is that they are not bushed ! When worn, oversize spindles are required and those side check-springs, which do work well, are bar-stewards to fit !!! Worse than the Ariel W/NG, and don't get me started on Enfield's with their peculiar left and right-hand threaded fork spindles....

Steve Madden

and don't get me started on Enfield's with their peculiar left and right-hand threaded fork spindles....

email (option): ronpier@talk21.com

I did an article a couple of years ago on tuning my M20, one of my first jobs was gas flowing and welding up some of the combustion space and it does work though I had to retard ign a bit and fit a solid copper head gasket (this was a cast iron head)I also did this on my mates M21 but just welded up head, he says it romps up hills now. .

Thanks Baz , would that be Shauns bike with the ally head ? seem to remember reading somewhere about your endevours with the flat heads , be interested to know what you did to gas flow it as well.

Cheers
Chilly

And what the compression ratio is after altering the two heads you have done...Ian

email (option): ian@wright52.plus.com

Nearly 6-1 on M20. 6.2-1 on 21 P.S Chilly lad, it was Shauns bike. however my gas flowing is top secret, but just for you...leave exaust alone, but try to get as much of the 90o bend out of inlet tract, shape or cut off valve guide, radius valve and seat, radius edge of bore,A10 double valve springs, 389 amal plus long inlet manifold, then if your mad like me fit some modified goldie cams....if your bottom end is up to it!

Interesting..Did you try to go beyond these figures?...and if so what were your results?

The standard volume of an M20 head is approx. 127cc which gives a nominal 4.9-1 compression ratio on a standard bore...

The 712cc engine I used in the bike I carried out compression ratio tests on, without any head alteration, resulted in a 6.6-1 compression ratio due to the increased bore size..

This allows for the addition of roughly 8cc of material to stay within a 7-1 limit if no other material is removed during the reshaping process...

With a standard M20 this would rise to 44cc for 7-1...

I did a week of alterations and road tests to prove (or not) the theory that the practical limit for a sidevalve is 7-1 before you hit pre ignition problems...

I started off at 8-1 and then took the bike to a local 'test hill'...It 'pinked' badly so I removed the (alloy) head, ground out some material, recalculated the compression ratio and headed out to the hill again...

More pinking lead to multiple removals of the head and incremental reductions of the C.R...until I reached a stage where the engine would only pink under a heavyish load and a drop to a lower gear and more revs, or a little retard on the lever would stop it....

After all this the final compression ratio was indeed 7-1 as received wisdom indicated...so a point proven...

I ran the bike like that for 38,000 miles before selling it...

Very high compression ratios aren't really desirable anyway with a sidevalve as ultimately compression ratio is gained at the cost of gas flow...A trade off that doesn't make for the best power...Ian

email (option): ian@wright52.plus.com

Thanks to you both for the info , but i'm now confused ,Baz says gas flo and up compression , Ian says upping compression interferes with gas flo , but you both have upped the compression and rode the bikes with what seems like good results with the mileages you have since accomplished

I have had a rebore + i have a liner so can't up the size at the moment , but if i can improve the engine to above its original standard for good road riding and maybe lugging my camping kit around sometimes , i reckon thats got to be a good move , if it reduces fuel ecnomy or makes it to fragile then at this stage of my rebuild then maybe i'll leave alone , but i just love improving over the standard set up on most of my bikes .

Hi Chilly..To summarise..

Compression ratio increases are OK up to a point...Beyond that gas flow is reduced, which is detrimental to power production.. (There's no point having a high compression ratio if there's nothing to compress due to gas flow restrictions).

I'm interested in what Baz has done as he seems to run less compression than I do...though still more than standard...

Also, beyond 7-1, in my experience so far, pre ignition/pinking is likely...

You can do what you like in terms of power production within the limits of a basically standard layout...You will never get enough of the stuff to overload the bottom end...

Welding up the head will raise the compression ratio AND improve the gas flow with some additional shaping work....and you need to get the inlet port to pass as much gas as possible, so some work is needed there...A bigger inlet valve can be beneficial as well..

A final compression ratio of approx. 6-5 to 7-1 is OK...Maybe the lower figure for the iron head....

My own enlarged M20 is actually better on fuel despite being 720cc now, as it is more efficient at the engine speeds employed....Ian

email (option): ian@wright52.plus.com

Thanks Ian , that sounds a positive move for me , i know that the bike Baz worked on made good power after his work , just need some more detail from him before i bugger something up

Carb size to match flowed inlet port or is it just same carb with better flow in the inlet track + how much to relieve the bore , wanna know a lot don't i .

Is this type of tuning similar to the old ford car flat heads ?

The first job is to do some work on the inlet port..It is too small, a poor shape and too short...and the inlet guide protrudes into it a long way which also interrupts gas flow....A carb to match the reworked port can then be fitted...

The basic principles of tuning sidevalves apply across most SV engines...

However, structural details, layout, cam and crank specs., valve sizes, port angles and other factors that vary between different designs mean you can't necessarily just 'lift' a feature from one engine and apply it successfully to another...

If you can get any information on tuning sidevalve Harleys that can be instructive..and there were some good articles written by an English chap whose name escapes me at the moment but I'll look it up.....Car stuff can be helpful as well to a degree...

Tuning sidevalves is now a bit of a 'black art'..They are more difficult to tune than OHV engines and after the arrival of the OHV engine no one really bothered other than Harley Davidson.....

Most people who wanted to improve the power output (of car engines) simply bolted on a supercharger.(with the Austin for example)..That can easily double power output pretty rapidly...

There is relatively little literature about unfortunately..I have read everything I could get my hands on for over 10 years, have trawled various patents and looked at any new developments like the Gas Gas trial bike...along with gradual experimentation and alteration to my own engines...

That process, along with some lateral thinking, allowed me to work out the spec. for my sprinter engine...

Your road bike will be a lot easier thankfully...Ian

email (option): ian@wright52.plus.com

My bike's performance seems to improve a few months after Christmas without any engine modifications.

My leathers fit better too.

email (option): sacombsashtrees@hotmail.com

Ian I havn,t gone higher than 6-1 yet as it goes so well, with 22 engine sprocket it cruises at 65-70 on motorway but loses out on hills so I stick with the std 18 or 19, top speed with 19 was about 85 using a bicycle speedo set to accurate. To tune side valves I read Phil Irvins book "Tuning for speed" plus "Speed and how to obtain it" and used some info on tuning Harley side valves, if I lost 2 stone in weight it would be quicker! I would like to bore out bigger without going M21 crank etc, but yet to find suitable piston to suit, also cams are goldie clubman altered for more overlap and clearances set to 16thou both.

Great stuff the both on you , thanks for leading me astray , i now have old car books out of the back of my sheds cupboard + Tuning for Speed and speed and how to obtain it , but to be honest from what i've read so far , i reckon you two have just about covered all they describe .

Thanks for your time and input , now wheres me welder and dremel

you could get the block altered like this for more power!

Baz you can bore out bigger using a BSA W6 piston I am using one at the moment on my 41 WM20 .The piston is STD @ 85MM so I am at 533cc , but will be replaced by a long stroke B33 piston once I have built up my spare M21 bottom end.. Dave

Baz...yes,a 'long rod' B33 piston will fit into the M21 OK to give you 635cc...

It would also make it 6-1 with a standard M20/21 head...and a bit more with your head which I work out to have a volume of 113cc...(Based on 6.2-1 when fitted to an M21)

If that is correct it would come out at 6.6-1 more or less on a 635cc cylinder.(the calculations don't factor in the head gasket thickness which would lower the figure a little, dependant on the thickness used).. Ian

email (option): ian@wright52.plus.com

I did fit a liner barrel with an 85mm B33 piston. Before running the engine I sold the project, a plunger M21, to a chap in Tennessee. I understand from his cousin that it ran well.

Much depends on the concentricity of the bore.

Jeff

email (option): jjbandoo@aol.com

Ian, I measured the combustion space using a burette after welding and it came out at 101cc, piston is 15thou down from blockface and a .6mm solid copper gasket fitted. p.s Shauns head was 110cc after welding and that was also an iron one.

Baz,

My just-completed 720 got the full treatment. I did not flow test the intake and combustion chamber mods, just copied the H-D WR and KR development work. With Ian's guidance I fitted bespoke 2 inch intake valves with from Manley Valve here in the US. I modified Ian's 90mm piston with a .216" pop-up to improve the internal choke point where the mixture flows from the intake valve into the cylinder. Also relieved the barrel deck to just above the compression ring and radiused the edge.

The intake port is out to 32mm, with the valve guide ground flush with the floor of the port. The original guides are very long compared to OHV applications, and since there are no rockers to induce side loads, shortening the intake guide by .30" should not pose a reliability penalty. I had a 10TT9 carb that had been over-bored to 32mm and matched the port to that diameter.

Much detail work went into the contour of the intake valve seat, using a NeWay hand cutter and hand-finishing with a Dremel.

I had 2 alloy heads welded up by a professional and set the piston-to head clearance to a very tight .028" initially, later .033. The intake valve area of the head required some relief to fit the 2" valve. It seems the Harley tuners assumed negligible flow from the shrouded side of the valve and also used a tight fit there. Compression ratio measured at 7.1:1

Farther down, I lightened the steel M21 flywheels as Ian suggested, but probably not enough. The rod is NOS, polished. Cams are 65-2436 and 2438, Goldie "touring cams." I'll try other cams after the engine is run in. So far it has run 16 easy miles, after 2 piston seizures which I blame on the ceramic insulation applied to the combustion chamber causing excessive heat transfer to the piston (presumably.) As I've posted recently, I do not advise ceramic coating the entire chamber. Only the valves, ports, and piston crown. While grinding off the ceramic coating I removed enough metal to bring the CR to just below 7:1.

With a balky 2 to 1 gearshift and New England winter it will be a while until I can comment on the performance, other than to say it pulls strongly on part throttle.

I have attached a link to some photos of the engine work, posted in no particular order.

email (option): jjbandoo@aol.com

Very nice Jeff, I,ll stick with what I have at the moment, since doing the work on mine have completed 3000mls, 4 speed hillclimbs on tarmac, and many long freeway runs at 65plus with no hint of seizures, though I still lap in the exhaust valve every 2000mls.

Interesting stuff folks ,its good to see , don't think i'll be going too far at the moment with the tuning , but it looks to me as if you can easily be sucked in with it , luvly jubbly

Got to admit i didn't realise there was so much going on engine wise with these ole flat head motorcycles , blimey ! 55 years on bikes and i'm just getting into Flatheads

Thanks for the encouragement

Great stuff Jeff...That's the most detailed account of your work I've heard to date (well, all in one place anyway )....How much did you machine off the flywheels...?
I was fortunate to pick up a pair of those flywheels recently so will probably 'rejig' my engine at some point...

Baz, Using your measured cylinder head capacities produces the following result...

Your M20 (101cc) has a compression ratio of 5.9-1..

Shauns M21 (110cc) is 6.3-1..but with your M20 head on it that would be 6.8-1

With the capacity increased to 635cc (110cc) it would be 6.7-1 and with your M20 head fitted (101cc) it would be 7.3-1...Ian

email (option): ian@wright52.plus.com

Does anyone have knowledge of, or ever done, re-locating the position of the spark plug on one of these heads? If yes, did it improve 'performance'?

From what I've read in various articles... the best position for the spark plug is considered to be on the centreline between the valves but approx. in the centre of the combustion chamber...

In addition some sources recommend a slight bias from this position towards the exhaust valve...

That's where I'm putting it when I make the head for my sprinter...(Next job after the inlet port mods are finished and then the engine will be completed..)

I haven't moved it in a more 'standard' engine but I'd like to try it when I have time..and would also like to hear from someone who has...Ian

email (option): ian@wright52.plus.com

Here are the photos I was unable to upload last night:


https://www.icloud.com/photostream/#A4GWZuqDGnHbzi

email (option): jjbandoo@aol.com

I have an alloy head I am tempted to modify, having been reading this thread. While it is being welded to change the combustion chamber it would seem a logical step to get a 'plug' reinforced position fitted at the same time. Decisions, decisions! :-)

Do it and don't worry...It's the only way to really learn anything.. ..

Great pictures Jeff...I always like the extra care taken over the preparation of the parts...

I'm afraid I've got to the point that I do that even on standard engines....Ian

email (option): ian@wright52.plus.com

Jeff

Do I read correctly that your 2 inch Manley intake valve was a 'special', or is it a stock item from another application with a standard Manley catalogue number? Thanks.

The Manley valves were a special order. They require bronze inserts to fit the BSA guides since the stems are narrower. I had a heck of a time getting keepers, collets and suitable springs, but the puzzle is solved and the installation is slick. Also, I recommend the use of a Manley exhaust, same size as stock or perhaps a bit OS to clean up burned seats. 60 years of metallurgy for better reliability.
If there is any demand I will order some sets to sell.

Jeff

email (option): jjbandoo@aol.com

Hi Jeff...Put me down for another 2 pairs if it comes to pass...Ian

email (option): ian@wright52.plus.com

cheers guys, this is fascinating stuff. i doubt i will mess with any m20 that i buy, but im defenately planning to have a fettle with my 53 m21.

email (option): johnrregan@yahoo.co.uk

I carried out the tests on the WDM20 (720cc) that I use quite a bit for travelling...

Consequently I restricted fuel to the 97 octane 'lead free' available from all garage forecourts...

A fuel with better 'anti knock' qualities would have allowed a slightly higher compression ratio but other factors must also be considered...

The problem with the side valves is that reductions in head volume to increase compression ratios can lead to reductions in gas flow as well...

Harley Davidson found this to be the case on the KR racers which suffered a drop in power output as the compression ratio was increased...

Harley expended much effort on refining the combustion chamber shapes and developing 'pop up' pistons to maximise compression ratios without adversely affecting gas flow...Thus the complicated shapes applied to the head and the top of the cylinder on the KRs...

However, this still resulted in relatively moderate compression ratios being used...H.D. found that a bias towards gas flow rather than compression was the best way to elicit more power...There's point in having a high compression ratio if there isn't much to compress!...

So, the gas flow limitations also limit compression ratio and probably more so than 'pinking' caused by poor combustion chamber shape...

I would think it most likely that H.D. were running 100 octane fuel at that time....Ian

email (option): ian@wright52.plus.com

Have read all this tuning info with great interest , i'm now at the stage of building the top end of my M21 , had a re-bore and built up the bottom end, now want to make it reasonably efficient without doing radical stuff at the moment , any ideas on what size carb to use , Concentric or Mono is not an issue ,i'm at the stage where i can open up the inlet port to accept a larger carb , i can reduce the inlet guide length , just wondered if there was anything else to consider such as inlet manifold length etc ?.
Would these small alterations be worth doing or should i just run it as is ?

Cheers
Chill

Always better to have a 'base line' so run it as is, then start fiddling - unless you have tuned that particular engine before.

You do need to get a few miles on her to loosen things up, jet her rich, whatever the carb and make sure the ignition timing is something like.

Then when the snow comes (?) start fiddling .............
Better still have a spare head and/or cylinder and work on them

Thanks and wise words indeed, but i just wanted to do at this stage some simple tuning that the factory didn't have time to do hoping this would help make the bike run more efficiently , maybe these changes i'm thinking of doing are not worth it at this stage tho ? .

Thanks
Chill

What little i know about SV's tends to be secondhand, however a theme seems to be that you have to get very radical to get any significant improvements - other than perhaps Comp-Ratio.

The original CR would more or less run on paraffin (i actually coaxed my lawnmower to run on 780% diesel 20%petrol - dont ask ...) and there is evidence that pre-war SV's run better with a bit of paraffin mixed in.

HOWEVER - on our modern petrols, i would have thought than an increase in CR should make them go better, or at least feel as if they accelerate/pickup better.
As Ian said 6:1 seems about the limit because as he says increases in CR usually screw up airflow into the engine - the result would be something that 'pulls' but don,t rev.

I can't say I agree with that totally...There are plenty of standard M20s around if you decide you have to experience the 'real thing'...

Personally I've done that for tens of thousands of miles so I understand exactly what they can do...

I 'tuned' my own M20 as I ride it on some reasonably long journeys and an increase in average speed over a distance can make a lot of difference to travelling times...

Perfectly logical and not based at all on the idea that I can make the M20 a fast bike...I have faster bikes if I want to use them, but they aren't suitable for WW2 events...

At the end of the day what's wrong with seeking better efficiency or better travelling times?...Many owners (thank god) are interested in exploring the possibilities inherent in most designs...

The M20 doesn't really fall into the 'sacred cow' category where they must be preserved at all costs...There are thousands of them out there...Ian

email (option): ian@wright52.plus.com

Whenever i think about SV's i keep remembering mate Marc's 16H - happy at 50-60, all day long.
Give it a long hill (climb out of Pickering}, usually with a +18 stone passenger after said passengers bike has broke and it starts to nip up.

It was re-bored, it dos'nt get much use (perhaps 200miles a year) so never has had time to bed in.
It also spends its time keeping up with a bunch of pre-war Ariels and Norton (OHV).

Its a lovely bike, in my opinion ruined by it being re-bored for no particular reason - so it smoked - a bit, but 'rebored' it was.
I've had it many times, playing with the ignition/carb trying to stop it nipping up - only problem is i cannot replicate the long slog up a high with a load, condition.

It really makes me (who will have a go at modifying ANYTHING with an engine) think - leave things alone ............


I'll now get back to my soon to be oversize/re-cam'ed B25 ..............

I can't understand why you have an issue with reboring engines...

I've done scores of British engines of all types over the last 45 years without any subsequent problems...

Thousands have it done every year and historically there have been millions...

Are you really suggesting old engines shouldn't be rebored...Ian

email (option): ian@wright52.plus.com

Problem is Ian that many don't warrant a re-bore and many are consuming oil through other reasons, such as valve guides.

If the engine is 'breathing heavily' due to blow-bye, then yes, you get some oil consumption and oil leaks.
Re-ringing (often using the next ring o/s up to get the correct gap) after glaze busting the bore usually is enough, particularly with the low mileage our old bikes get - not everyone does the mileage you do Ian.

Add that to many 'engine shops' seem to think they are dealing with high silicon Japanese pistons when they bore your cylinder and you end up with my mate Marcs problems.
Solved either by a lot more running or a light hone.

With SV's if you measure or even watch the shape of the bore as various re-boring cuts are made, you see that the bore is anything but round - not the usual wear towards the top of the bore, but an area on one side, coinciding with the valve chest area - this is the bit which nips the piston.
This is all well documented and known.

All engines i build for competition are re-bored, usually with a head-plate to allow for distortion, always with more piston/cylinder clearance than a road engine and always meticulously honed/finished.
There's nothing finer assembling a new piston into a new bore (sad aren't i!) ........ however that's no reason to do this with every engine - just my opinion.

I agree there is no point reboring prematurely generally, although this operation is frequently done on a full engine rebuild as part of the process of 'resetting the clock to zero'...

The fact is most customers expect/demand it....

There is also no dispute over the fact side valves tend to wear the cylinder unevenly...Particularly those that have a tappet chest cast onto the side of the cylinder with no air gap behind it..(Early M20, WD Triumph side valves for example).

Over a period of use the piston and rings will wear along with the cylinder and fitting new components to a barrel worn this way can cause trouble if not carefully run in...

Reboring the cylinder to restore it (at least temporarily) to a correct true shape shouldn't (and doesn't) cause any problems at all if the new cylinder bore, piston and rings are carefully run in on a correctly set up engine...

Problems at this stage more likely indicate a fault in clearances or settings, an underlying problem lying elsewhere or careless running in....Ian

email (option): ian@wright52.plus.com