Forum

[Les H]

Hi Dennis the only trouble is that from what I can make out from Andy's details is that the top of the bore is perfectly central but the bottom of it has wandered over to one side. So you can imagine the forces against the sides of the piston as it flies up and down, constantly being pushed sideways in one direction then the other at an average of about 50 times a sec and as much as 100! I agree if the offset was linear and only amounted to just 3mm, then things would probably work out Ok, but the impression I get backed up by Andy saying an engineers square is not in contact all the way along, is that the bore is sloping to one side which seems to be potentially quite a problem.

[Dennis C]

Hi Les, I fully understand your concern, but on these old bikes the engineering is quite basic and a bore that is slightly off true is an all too common find,(even when they where new).----- As you know the greatest force is applied to the rear of the cylinder wall and should be the part that wears the most, in reality when stripping an old BSA engine most of the wear is normally found on one side of the bore (usually the left side), this is partly caused by the crankshaft end float which always runs to one end and exerts sideways pressure on the piston, and partly caused by the spark plug being at one side of the head giving a burn which starts at the left of the piston as the piston is forced down the pressure increases towards the bottom of the bore causing this slight slope, unless you can find an "old school" engineer to do the rebore then this slope will be followed by the boring machine.---- Our hosts often have for sale Enfield cylinder blocks at discount prices because they do not pass quality control due to the bore not being true, it would be interesting to know how far out they are before they reject them. ------- Just as a matter of interest this falls within the makers original spec for the bike.---- I restored my Triumph T90 22 years ago now and the correct pistons for these are a very rare find but I found a pair of new Hepolite plus 10 at an autojumble, to accommodate these due to the bores being worn at one side, I had to rebore it slightly to the left I still have this bike and have done around 25000 miles on it without any problem. Kind regards Dennis.

[Les H]

Hi Dennis. I agree about the low tolerances these engines inherently have but 0.3mm is 12 thou. The bore service limit on most old classic bike engines is around 5-6 thou. With this amount of wear the cylinder is deemed to be worn out and requires a rebore. The point I'm making is that 12 thou misalignment or out of tolerance even in an old engine is quite substantial. You say that a gasket could give a 12 thou tilt, but I have to say I have never found any gasket to differ as much as even a quarter of a thou let alone 12 thou or say 9 thou if used on a long stroke barrel to tilt it 12 thou. I am not saying the engine would blow up immediately, but I can't believe it would run for too long before he experienced piston problems. If it was so obviously out that it caught Andy's eye then I personally would take it to a reputable machinist and get them to run their borer down the barrel with it switched off to get a real clue how much the tilt is and get their opinion. Andy may well decide to use it as it is and that is that. Can't say no more!

[Alan R]

Hi guys----------Yes, DENNIS C has hit the main problem on the head (not Cyl. Head, I hope ?)---with regards to any re-bore ie the Boring machine used in the trade for re-boring a supplied block or barrel will follow the existing alignment. I suppose I can count myself as one of your "Old School" engineers( I mentioned my time with the SVR recently) as I would put this job on a Keates Angle Block mounted on a faceplate in a centre lathe, suitably bolted for rigidity. Given the overall size of the cyl. I would expect a faceplate in the order of about 2ft and a lathe of matching proportions----The use of the biggest possible boring-bar can now be employed with an indexible carbide tip for good cutting. Using this set-up you can now manually run the tool as close to the bore as possible and see any run-out etc. You might also want to re-bore to + 0.005" and hone to finish. ANDY W, do you know anybody on either of the 2 steam railways in Devon ?? Maybe their workshops can help ??

[Alan R]

--------------- ooops !! TYPO again------that should be " - 0.005"

[Dennis C]

Hi Alan, as you say "the Boring machine used in the trade for re-boring a supplied block or barrel will follow the existing alignment." this is absolutely the case and one reason why so many rebored blocks have this slight lean. ----- To throw another thought into the equation, if the bore has worn like this then it must follow the natural path of the piston?, ---- is it better to rebore to this path or force the piston off its natural path?. ---- Unfortunately we do not live in a world where everything is clear cut and perfect. ---- I have personally rebuilt engines with more lean on the bore than Andy thinks he has, these engines have covered many thousands of miles without fuss or trouble.

[andy w]

Thanks again for the advice and information.I will take my barrel to the local auto machine shop and see what they say.The comments about a rebore following the existing bore line confirms my suspicion and would also apply if the same borer was used to fit a liner.I will see what I can find out locally.
Thanks all,Andy.

[Alan R]

Hi ANDY---- I've just spoken to my mate with the ex-WD B40 re}-a spare barrel. He says that there are two types ie an earlier 4-stud and the later 6-stud. Can you tell me what type you have ? We may have a spare in case things go " Breasts Heavenwards".

[Les H]

Dennis...the barrel is from a different engine! Alan, are you sure a modern rebore machine will not bore perfectly vertical, but merely follows the original bore?

[Dennis C]

Hi Les, Yes I know the barrel in this case is from a different engine that's why I didn't throw this in earlier, just general chat following Alan's post, Interesting thread though, any machine work is only as good as the machinist and in my experience most do tend to run a clock down the bore (sorry used the term clock in a previous post as well this may just be a local term it means dial gauge) and set it up to this, which means any rebore will follow the existing bore. A T B Dennis.