Nipping up

[A10Boy]

Bill
I have quickly read thru the thread and didn't see any mention of "gapping" the rings. Remind your friend that he should have about 10 thou end gaps. The old rule of thumb was 3 thou per inch across the piston.

Also, I'm not in favour of high compression pistons in iron engines, it can cause a hot engine.

Also, the engine should be "run in" on mineral oil not synthetic as they don't bed in properly.

[bsa-bill]

Thanks all - spoke to Bob this afternoon (pointed him in our forum direction so maybe he has already read this)
The bore should be ok with a hone but he will need a piston.
He was also told by the shop the rings should not need gapped, I know I have been led to believe this too but always check and found while sometimes some of them are correct others need a rub with a file, may well be true for modern stuff with tighter tolerances.
   

[Brian]

When you say the shop told him the end gap didnt need to be checked do you mean the shop where he bought the pistons or the machine shop that did the rebore. Either way I wouldnt use that business again.

[A10Boy]

Bill

If the bores are at the right tollerance, then the advice that the rings dont need gapping has almost certainly caused this siezure.

Your friend should go back to whoever gave him that duff advice and tell them that they are responsible for the siezure and if it was the piston supplier demand a replacement free of charge. How can a piston supplier know what the bores are, and how can a cylinder borer know what the ring gap will be unless hes checked them after boring?

EVERY component used on an engine build should be checked for tollerance otherwise this kind of thing is bound to happen.

[bsa-bill]

Good point Brian and A10 boy I'm not sure who it was told him, he was looking for the receipt for the pistons but has lost it so not sure where he got them but I have had two rebores done by the local shop without problem and they never mentioned rings so I just assumed it wise to check and gap as necessary, but I do know tractor pistons our workshop have installed came ready to drop in ( having said that we did have a combine engine pick up after a rebuild but that was due to wrong assemble, offset on rods IIRC)
The shop we use is the only one for some distance ( yep distance here in UK is not the same a distance US or OZ I know).
To be fair they are generally pretty good and mostly do good work and will put right anything they get wrong, thing is lots of the engineers that know about our bikes and old engines will be retiring, no disrespect to present day engineers but mostly they are fitters, plug in their laptop and replace the part it tells them to, just like many industries these days - reduce the skill level reduce the labour costs.

rant over (good to have one a day I find)

[A10Boy]

I suppose the lesson here is not to believe what these people tell you. As I said, EVERY component used on an engine build should be checked for tollerance otherwise this kind of thing is bound to happen. I cant imagine putting pistons in any engine without checking the ring gaps, it just dont seem right.

PS a few years ago, I was part of a team restoring a Merchant Navy steam loco. When we fitted the pistons which were 22 inches across [from memory], I did the 3 thou/inch calc and gapped the rings accordingly - the biggest pistons I'm ever likely to fit.