I'm getting my '58 M21 barrel professionally assessed to see it it needs new valves, guides and piston.

Looking back at some of the posts here it seems I'm better off trying to find NOS pistons (mainly because most modern ones are overweight) but conversely valve and guide technology has moved on so much in recent decades that I'm better off with with something newer.

My barrel is standard (82mm) and looks in good order but I'll be surprised if I get away with a hone and a standard piston so I'm probably looking for a rebore and +20 piston. I'm guessing that these will be hard to find - any recommendations for a source?

Valve (and valve guide wise) I've been looking at GS valves but see that Alpha Bearings sell these too, does anyone have any preference between these two suppliers, Draganfly also have them but I've no idea on the quality supplied. Unless I should stick with NOS?

Any advice and guidance gratefully received.

Gary.

email (option): gj.owen@hotmail.co.uk

Recently somebody wrote that he was going to uw nos guides to replace a worn guide.
But i wonder if guides of 70 years old are still good....?
New guides should be made off better, modern steel, dont you think ?

Furthermore, someone told me that new valves are made off steel ment for water cooled engines mostly. Not for bikes like m20's.
Who knows....

G&S valves are very good quality and are my choice...No problems running them in the original, or pattern, cast iron guides...
From other sources you are likely to get Indian ones which work but aren't as good...

The G&S valves are very reasonably priced as well...(They don't do guides by the way)

Alpha guides are good...I use those in the B Series engines and also for M Series engines if I can't get NOS...I don't know if Russell Motors still have original guides as I am still using ones I bought a while back...

Regarding any advances in guide technology that revolves largely around material specification..
High silicone, low expansion bronzes such as 'Colisbro' are pretty much in favour these days
Unless you are planning to experiment with closing up the stem to guide tolerances and are interested in producing 'specials' I wouldn't bother though..
There are none commercially available that I know of and cast iron guides will do the job perfectly well in a standard sidevalve...

You won't go far wrong with original pistons..Try Russell Motors, I think they still have them. Alternatively Bantam John...He has the largest stock of original pistons in the UK...

I might have a spare NOS original but would have to check what sizes I have...Best to get your barrel checked first to determine your requirements...

Take care with the guide/stem clearances and piston/bore clearance as well...Many engine shops dealing with newer engines have a tendency to tighten up the tolerances as they are used to applying less clearance...

Take my word for it...don't do it. Stick to BSAs recommended figures...

Personally I would recommend fitting a hardened valve seat on the exhaust side as well..It means longer seat life and less maintenance..and fit an annealed solid copper head gasket to improve reliability there as well...Ian

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

Sadly although modern stuff may to made to better tolerances (?) they are seldom made of 'better' material - they usually use standard off the shelf stock rather than some of the specialist CI's produced in the past.

My own opinions -

DONT USE STAINLESS STEEL valves, find som high quality 'non' stainless ones - GS valves seem good, i've just used some SiC plated SS ones from them in the B25 (from an A65) - buggers to machine down to size!!!!
Alpha valves are generally non-stainless.
Modern valves tend to be composite construction with the head friction welded onto the stem, to allow materials for the different mechanical/thermal requirements.
That being said BMW flat twins had these and yes the heads did drop off ...... expensively. When this first happened to me in 81'ish, it was cheaper to buy KAUSER heads than genuine BMW!!!!

Valve-guides - find some Cast-Iron ones, poor quality CI is better than the 'brass' generally supplied as 'Phos-Bronze' !!!!!
Don't run unplated SS valves in CI guides .... the guides will 'eat' them.
CI guides are best where there is minimal lubrication - hence its use on pre-war bikes

Colisbro - (should b spelled Colsibro for Silicon Bronze?) is supposed to be the best, in reality few guides are are actually made of this stuff - usually its a Phos. Bronze, as its a similar colour.
You know its Colisbro when you try and machine it - it needs a VERY sharp tool.

Piston - NOS is good, but watch the clearances - too many machine shops think old brit bikes run JAP clearances !
No idea about reliability of modern pistons in M20/21's - some are total crap (JP) in Triumph T100's and BSA 25/44/50's - i know as i've been busy replacing/repairing a three engines which tried to run them.

['Modern valves tend to be composite construction with the head friction welded onto the stem, to allow materials for the different mechanical/thermal requirements.
That being said BMW flat twins had these and yes the heads did drop off ...... expensively. When this first happened to me in 81'ish, it was cheaper to buy KAUSER heads than genuine BMW!!!!']

Don't be put off by this...I have been running modified 'two part' IVECO turbo diesel exhaust valves in my M20s for years. Modern valves of this type don't fail frequently enough for it to be an issue...

However, I don't think it pays to get into this sort of thing unless you feel you need to..G&S valves for the M20/21 are 'off the shelf' items that don't require further work before fitting...

If you do elect to try a non standard valve at any time in an iron guide ensure it has a hard chrome plated stem...

Changes from standard should be considered carefully..

First you have to be confident you are improving things...Sometimes with the best of intentions it doesn't work out that way...
Second, the engine will then have parts that cannot easily be replaced without a lot more work..
Lastly, do you want to get involved in the work, time and expense of 'experimenting'...?

Personally, I like to try things to determine what works and what doesn't.
On that basis I have tried multiple specification and tolerance changes over the years on BSA heavyweight singles...but it's worth remembering, they work pretty well the way BSA made them...Ian

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

Ian's right about the two part valves - i should have not given the impression they where no good, but the BMW (and VW as well) head drop off is well documented, so not only i've experianced it.

If you go for one piece SS valves, as well as making sure they are plated, also (chromes OK, but as i say i have some Silicon Nitride plated G-S valves) make sure the tappet end is stellite (or other hard-faced) - though on the SV's wear should'nt be a problem, they will mushroom making them hard to remove.

Many commerial diesel valves are very high quality and nicely long (easy to cut to length and machine to fit) - definately something to look for.
I have used them myself many times and have several in stock for projects.

There are rally only 2 ways to make a piston,
Forged or cast.
I have seen pistons turned up from solid billet but these are a waste of electricity as they will fail if actually used due to the physical structure of the billet.

The 12.5 % Silicon controlled expansion aluminium alloy basically has not changed from the time it was developed because nothing works much better.
Like everything else , advances in analysing has allowed for tighter control of impurities and some increases in longevity due to the mireo- alloying effect of what was previously considered impurities.
Thermonics has allowed better gain structures to be obtained.
Casting is still and always will be the cheapest way to form metal so old pistons will be almost as good as a modern equivalnet.

Forging makes a substantially stronger piston allowing engines to rev to 20,000 rpm without the crown becoming a flying saucer or deisels to run massive turbo boost. Neither of which will of significant advantage to M20 owners.

The most important change is advanced machining technology has allowed much more consistancy so any two new pistons off the shelf will be exactly identical.

Now to guides.
Cheapest guide material is cast iron and cast iron is sufficient for the original intended use.
Now days with very infrequent use CI is not the very best material but even back in the 40's it was not the best, it was the cheapest that would do the job and the fact that most of us are still running the original CI guides 60+ years latter is all the proof you need.

Si Al bronzes have been around for a long time.
BSA could have fitted them except for the cost and the difficulty of machining them to the accuracy required with the production tooling available at the time.

Because nothing as cost efficient works better, and the current materials have not been found wanting in modern engines there has not been much developement in valve guide materials in the past 20 years.

Valves on the other hand are quite different.
Valves are forged and forging is the most expensive method of forming metal.
Thus a lot of work has been done & is continuing to be done to drive the cost of valves down.
Even worse every bit of a valve requires physically different machanical properties and maximizing for one, detracts from another thus the composite valve. The materials used are quite expensive and conditions they work under are very difficult.
Remember the exhaust valve will get up to forging temperatures in modern lean burn engines but has to resist being forged out of shape by the "hammer blow" of being slammed into the seat. The cold seat needs to handle a red-yellow hot ( welding temperature ) valve face being hammered into it without welding together and without heat checking.
The valve proper has to resist the transfer of heat from the yellow-red head end down the tiny stem to the tappet tip while transferring as much heat as possible to the guides.

Thus we have a lot of valve developement with little for guides & pistons.

email (option): bsansw1@tpg.com.au

I've just bought some M20 inlet valves from G&S by the way...The current price is £22.50 each (including the dreaded VAT.) plus shipping...Ian

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

Just contacted G&S for a set of valves, the inlet ones are in stock but they've no exhaust ones (a couple of months away apparently). Before I try tracking a set down elsewhere, has anyone got a set they can sell me please? I'm happy to pay a modest premium to save me the hassle.

Gary.

email (option): gj.owen@hotmail.co.uk