I had my barrel re-bored to take a +40 thou piston and rings (NOS), now got to checking the ring gaps before build and they are around 12 thou.
I see the advice in the tech section is 8-10 thou, so the question is can I get away with this or do I go back to the machine shop and have a "conversation"? (And if so recommended option to fix please)
Saged advice most welcomed....
Scott
email (option): larkesrATGEEmail.com
Scott the bore should be rebored according to the piston size. That is, it should have a piston skirt clearance to allow for heat expansion of the piston. BSA recommend 3.5 to 5 thou which is measured at the widest part of the bottom of the piston. I tend to ask my engineer to use the upper end 4.5 to 5 thou.
So before you worry about the rings, I would take it to a proficient engineer to measure the piston and then the bore size. I'm not saying that yours did? But no engineer should rebore a barrel without the piston in his possession for measuring. Ron
email (option): ronpier@talk21.com
Agreed....Additionally, it should be noted that there are some original, aftermarket 'split skirt' pistons about and these run a much tighter clearance than the solid skirt type as the 'split' limits the overall expansion of the piston when it warms up...
I have also found the ring gap to be oversize after a rebore on occasion.. However, in this case .002" shouldn't cause you any real problems even though it isn't the ideal....
If the cause is an incorrect bore size there is no solution short of fitting a new liner and carrying out the rebore again. If the bore size is OK then trying another ring set is the only option. The results of that could not be predicted...
Checking the skirt clearance (bore size) as it stands now would at least determine whether it's bore size or a ring problem that's causing the trouble...
Unlike Ron I tend to go for the minimum tolerance followed by careful running in...Which method is the best (if either) is open to debate!...:laughing: Ian
email (option): ian@wright52.plus.com
Thank-you gents very much appreciate the responses to my predicament I shall break out the vernier tonight, wish me luck!
I will refer back with the details.....
Scott
email (option): larkesrATGEEmail.com
Not wishing to piss on your firework Scott. But in my opinion you will never measure them accurately with a vernier! You will need internal and external micrometers and someone who knows how to read them correctly....... Especially the bore! Ron
email (option): ronpier@talk21.com
Again I have to agree with Ron...A vernier will not achieve a sufficient degree of accuracy either for the piston or the bore....
For the piston an appropriate outside micrometer with .0001" increments and for the bore either a professional set of bore gauges which once set can then be measured using a micrometer or, more ideally, internal micrometers made for such a job...
Here's an illustration (for reference) to show where the bore should be checked along the various axis before a rebore...This will pick up uneven wear and ovality etc....Obviously after a rebore the bore should be parallel and one size all the way through!!...Ian
email (option): ian@wright52.plus.com
Ok, so I do have a bore micrometer and know how to use it (built gas turbines for 20 years). Don’t possess a micrometer big enough to mic the piston but my vernier is digital so repeating a dimension is easy to find/confirm.
Cylinder is 3.2705 down the line so at least it is straight!
Widest point I could find on the bottom of the piston was 3.2630, I make that 0.0075 thou. A bit on the high side, what do we think gents?
email (option): larkesrATgmail.com
From memory the standards book gives .007" as acceptable at overhaul and condemned at .010" so I think .0075" is beyond acceptable. Ron
email (option): ronpier@talk21.com
A few years ago I ordered a set of +60 rings from one of the well known BSA suppliers only to find the gap on the compression rings to be some 65 thou. Replacements were not a lot better so I gave up on them.
The answer I got when I queried this compared to the BSA quoted end gap of 8-10 thou was “The quoted end gap is only a minimum to prevent the ring gap from closing up when hot. A larger gap is not a problem”. :confused:
There was also a calculation of the percentage increase in ring gap area as a percentage of the total bore area…… obviously I am too thick to understand the significance of that!
email (option): petercomley@web.de
Whilst its nice to have a fully equipped workshop,all the expensive measuring gear etc - In this case all that is needed is a set of feeler gauges and a bit of patience.Ring end gaps can be checked,bore to piston clearance and also bore for parallel. If the bore was done without the piston being present I would also be concerned about the surface finish of the bore - Is it honed correctly ? I have seen some very poor examples of "rebored" barrels,just recently my brothers G3l came apart for excess blue smoke,it had clearly only just been built before he bought it. When we took it apart the bore had an almost mirror finish ! No way the rings were ever going to bed in and seal. I also recently measured a pair of Triumph barrels that had just been bored,one side was 2 thou larger on dia than the other and they were both tapered.
Finding a machine shop that will do work properly I think can only be found by reputation,not by advertising.The price is irrelevant too,compared to the hassle of a poor result.
Just to clarify for all...
They had the piston and the rings, bore is honed not mirror finish and also parallel as mentioned above.
From the wisdom of Ron and Ian it seems to be that issue is they rebored to +40 thou and not the widest point of the bottom of the piston plus 5 thou (please correct me if I am wrong here Ron/Ian).
So as I understand the advice, it’s OK, but could have been tighter?
Next question, given the above, what about the 12 thou ring gap, what do we think (8-10 thou being the guidance on the tech page)?
email (option): larkesrATgmail.com
Personally I wouldn't worry now about the extra 2 thou gap on the rings, just treat the whole thing as a part worn engine, but if you want to tighten them up, you can use +60 rings and gap them accordingly. Or go the whole hog and get a +60 piston and start again:disappointed: Ron
email (option): ronpier@talk21.com
[So as I understand the advice, it’s OK, but could have been tighter?
Next question, given the above, what about the 12 thou ring gap, what do we think (8-10 thou being the guidance on the tech page)?...]
Firstly, the engine will run OK if the dimensions you quote are correct, both bore size and ring gap...
However, assuming you gave the engine shop the correct BSA skirt clearances they have obviously gone approx. a couple of thou oversize on the upper tolerance limit...I would deem that to be careless and unacceptable as they have effectively shortened the working life of the cylinder. I'd therefore tend to consider someone else for any further work...There is little point applying a tolerance if a machinist can't work to it!
The engine shop I use bores the cylinder and then hones to the finished size and you get the size you asked for down to a tolerance of + or - .0005".
Quote...[Whilst its nice to have a fully equipped workshop,all the expensive measuring gear etc - In this case all that is needed is a set of feeler gauges and a bit of patience. Ring end gaps can be checked, bore to piston clearance and also bore for parallel...]
Regarding the type of equipment used for the job the highest accuracy will be achieved using the equipment described in my previous post and particularly so when taking multiple bore measurements...If a comparable level of accuracy could be achieved with feeler gauges etc. then no one would bother to buy micrometers...It's really a case of going for 'best practice' first if at all possible...
Accepted, not everyone has all the equipment needed, or perhaps the skills to carry out this work but most engine shops do, so it is possible generally to have the most accurate measurements done...As we are discussing discrepancies here of only .001" to .002" there is little margin for error....Ian
email (option): ian@wright52.plus.com
Absolutely Ian, I was badly making the point that with care in a garden shed it is possible to build an engine to a good standard.I'm lucky in that I have all the right kit to do it properly - As you say so should any decent machine shop.
All that said the M20 is a slow running,low performance side valve.They were pretty much thrown together during the war years,as long as they ran it was good enough sort of thing.The last M20 engine I rebuilt had 46 thou of side clearance on the con rod. Alpha bearings taper ground the crank pin,(For free,inc postage ! ) to get it back to 10 thou. The chap I spoke to there said it had probably been wrong since it was built as there were no real signs of wear on the flywheel faces and the rod eye was the correct size.
Going back to the original question,the extra end gap on the rings is no big problem,a (Very) slight loss of compression and a bit more oil consumption perhaps is all that is going to happen.Afterall few M20's are going to do 12,000m a year or be revved to the moon through the gears.
email (option): joncvsv8@yahoo.co.uk
I'm sure it will run just fine as is,see previous post plus on the credit side you will have reduced the risk of it "nipping up" during running in if the conditions get really hot.
I'm sure it will run fine! But I'm with Ian on this. What is the point of buying a brand new piston and then paying an engineer to present you with a semi worn out barrel.....You might just as well have left it as it was.:angry: Ron
email (option): ronpier@talk21.com
That hadn’t escaped me Ron, won’t be going back.
Subsequently found someone nearer and better but hey ho I have learnt a lot in the last 36 hours from you and Ian mostly.
Thankyou for sharing your knowledge, the build continues...
Scott
email (option): larkesrATgmail.com
The way I understand it is that when you have a 82 +1 mm oversize piston the cylinder has to be bored to 83 mm, the closer the better.
In this way it irrelevant if the piston is a plain piston, a split-skirt, low expansion etc. The clearance is determined in the size of the piston by the ones who know best, the makers of the piston.
How can an engineer who gets a piston in hand determine the clearance without knowing what he really has in hands?, the maker knows..
So in this case with your/Scott's cylinder of 3.2705 inch = 83.07 mm, I would say it is 0.07 to wide, or is this too obvious??
BR Michiel
email (option): m.wijbenga@hotmail.com
That's not really how it works...As an example a BSA B33 with a solid skirt piston being bored to +.040" (85mm standard bore)would have a clearance between the piston skirt and the cylinder of 0045"-.0065"....The same cylinder being rebored to +.040" but with a split skirt piston would be bored to a skirt clearance of .0006"-.00275"....A much smaller clearance to take account of the different piston design...
Additionally, the piston itself will have been manufactured initially with a production tolerance applied so there can be small but significant differences between piston diameters even when they are of the same type...That is why each piston must be individually measured and the correct tolerance then applied to produce the final bore size...
Also, as piston diameter increases so does the skirt clearance as the bigger size pistons expand proportionally more....Thus an M20 piston of 82mm has a .004"-.006" clearance but an 85mm B33 has a clearance of .0045"-.0065"...An increase to say, 90mm with a piston of the same material would require a bigger clearance again...So, it can be seen that boring as close to the piston size as possible is not the way to go....
Further, if a bike was to be run in with a sidecar fitted for example it would be better to give the bore a little more clearance initially to avoid the potential for it to 'nip up' in the early stages (caused by the engine being more heavily loaded during initial running in due to operating with the extra weight of the sidecar)...
Taking this to a more extreme case where there is no time to run the engine in at all, when I used to build Triumph twin engines for pre 65 scrambling they were driven flat out straight off the bench...In that case used pistons were employed (with new rings) as they already had the high spots worn off and the bore was then honed to the point where it just smoked lightly when run up after assembly...In other words the clearance was such that the bore was nearly worn out but not enough to compromise performance!!....This illustrates how application can also affect the clearance selected in certain cases..Ian
email (option): ian@wright52.plus.com
Hi Ian,
From your first and third paragraph your pointing out that different pistons and use need different clearances, I fully understand that.
What I mean is that the pistons are always a little smaller that the size on the box, so what I think is that the maker has the intention the engineer bores the cylinder to the size which is on the box cq. on top of the piston and they have taken care of the correct clearance by making the piston a fraction smaller.
for example the diameter of a STD BSA 82mm M20 piston is 81.88 which suggest a 0.12 mm clearance
my STD 85 mm split-skirt B33 has a diameter of 84.93 which suggests a 0.07 mm clearance.
These clearances would off course be for normal use,
But your point about production tolerances in your second paragraph has partly broken my point above as i just measured a few similar pistons, most were exactly the same but one was 0.02 smaller.
Could the above be the theory but in practice it turned out that the tolerances for it could not be met, so boring clearances had to be included in the box. ???
Cheers, Michiel
email (option): m.wijbenga@hotmail.com
After reading all this, I don't dare to have a rebore anymore....
I think Ron is right in saying that a bit wider gap in the rings is OK and doesn't cause any trouble.
The story goes that if tou have piston trouble, it's mostly the rings (like a gap to small)
[Could the above be the theory but in practice it turned out that the tolerances for it could not be met, so boring clearances had to be included in the box. ??? ....]
I agree this may have been the original intention...However, apart from the practicalities of piston manufacture the tolerance of the piston is not the only variable...The rebore itself will also have a working tolerance...Virtually all engineering dimensions have a tolerance as manufacturing repetitively to a dimensionally exact size is, in practice, impossible at a reasonable cost...So, if a machinist was to bore a cylinder to 82mm for a pre sized piston he would not be able to guarantee an exact size and without a stated tolerance what size would be correct?...
Thus, even if the clearance was originally 'factored in' on the manufactured size of the piston then the manufacturer would STILL have to state a tolerance for use by the machinist doing the rebore. If the clearance was pre applied to the piston it could be assumed that this rebore tolerance would be the same for every piston size, type and material...However, a glance at a list of BSA quoted piston clearances shows this is not the case.
In conclusion I think you may well be right about the original intent but in practice, variable clearances calculated by measurement of an individual pistons actual dimensions, combined with an applied upper and lower tolerance have become the intended and correct method to use.....Ian
email (option): ian@wright52.plus.com
If one were lucky enough to find an NOS cylinder in perfect condition, and likewise a standard piston...would it be safe to use them ? In a wartime context, surely cylinders were bored to size and stored loose? It's difficult to imagine them being matched to pistons and then packaged together...I've seen no evidence of it anyway.
It must have been the intention that pistons, perhaps from several manufacturers including Wellworthy and Hepolite could be fitted and used in standard or oversized cylinders ?
A problem might arise with more recent pattern replacements as they may have unilaterally decided to tighten up on the clearances in line with modern practice and perhaps not made allowance for how air-cooled side-valves distort.
I bought a NOS barrel for my Triumph 5SW when I was restoring it and it came slightly undersize and required finishing to match the piston....Ian
email (option): ian@wright52.plus.com
It must have been the intention that pistons, perhaps from several manufacturers including Wellworthy and Hepolite could be fitted and used in standard or oversized cylinders ?
A problem might arise with more recent pattern replacements as they may have unilaterally decided to tighten up on the clearances in line with modern practice and perhaps not made allowance for how air-cooled side-valves distort.
Even now , I send a mower barrel in for a rebore & get it back with a new piston.
When you think about it, back in the day if the workshop needed a new barrel, then it would be a brand new std bore barrel from stores
They would then send the old barrel out for boring and the machine shop would supply the piston.
email (option): bsansw1@tpg.com.au
Thanks everyone for the contributions, looks like it fired the senses! :)
I start the bottom end build this weekend (while the missus is at work:wink:) and have decided to go for 60 thou rings to minimise the impact.
Wish me luck, built hundreds of gas turbines but this is my first M20 donk, will comment on the progress if people are interested?? :relaxed:
email (option): larkesrATGEEmail.com
Yes we are, Scott.