A guy has asked me to fit new piston rings to a set of used Pistons and install them along with conrods back into the used barrels. So after a clean up and a glaze busting job on the bores there is a tiny discernable wear step on one side of the bores, half a fag paper which I assume has been on the thrust face of the bore. Seems a good idea to assemble the Pistons in order that the thrust is now on the opposite side of the bore to balance out further wear. This will be determined by which con rod goes in which barrel. I have studied the diagram in the manual showing piston offset, direction of crank rotation and camshaft position and can't decide which is the bore thrust face.

The thrust face is the one towards which the conrod small end is pointing on the power stroke, if that's the question.

You can't change which side of the bore is the thrust face, that is determined by the rotation of the crankshaft. Unless the barrel can be fitted the other way around?

Rings can wear so that they are shallower than new, and new rings will then hit the wear ridge, snapping them. A good manual might advise on maximum wear, as a rebore might be necessary.

If the con rods have to be fitted a particular way around, don't change that. Some are shaped to counter thrust forces, some have offset gudgeon pins, some have diagonal split main bearings. Whatever you have, they should go back in the way they came out.

If the gudgeon pins are retained with circlips, ensure the circlip gap is at the top or bottom. Not unknown for gaps at the side to allow inertia forces to close them down, allowing them to fall out.

portland_bill said:

You can't change which side of the bore is the thrust face, that is determined by the rotation of the crankshaft. Unless the barrel can be fitted the other way around?

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The only thing I can think of is to swap the pistons to the opposite con-rod and to also flip the positions of their barrels so that the pistons are the right way round in the correct barrel but the barrels will have been rotated through 180 degrees.....but I really wouldn't recommend it.

Thanks for the input guys and good point there about the circlips. I should explain more though. The Pistons, rings, barrels & conrods all came to me as separate items along with a cylinder head to fix. The guy who wants this done is a mate and decent bloke but has the mechanical ability of a butterfly. So I am trying to do the best I can for him as well as make things as foolproof as possible. Idea being to have the parts ready for a top end overhaul when needed. So I have conrods 1 & 2 which only go in 1 position and one way round with the stamped numbers on the big end facing the camshaft. The Pistons have been fitted with with new rings and only fit the conrods one way round because of the gudgeon pin offset. But as Peter points out the barrels can turn through 180 degrees and fit in either position. So the very slight wear inside the bores can either face towards the camshaft or away from it. There is nowhere near enough wear to threaten any ring damage. I know it is splitting hairs but should I assemble the bits with the wear facing to or away from the camshaft side or which side of the barrel takes the most thrust?

The thrust side in this case is opposite the camshaft side.
It's easy to picture, as the big end begins its descent to one side on the power stroke, the conrod angle pushes the piston against the opposite (thrust) side of the barrel.

What ring gap do you have ?

"Do the right thing", get a new set of barrels & pistons/rings. Just saying : )

You could fit a thicker gasket or spacer under the cylinder barrels to raise them slightly so that the new rings cannot make contact with the tiny wear lip.

But this of course, will lower the compression ratio which afaik is strictly VERBOTEN to do on the classic 500

AL.

I reckon that if your finger nail doesn't catch on the wear lip, it should be fine and not cause damage to the top rings.

If there is a tiny lip, you could always smooth it out using a Bearing Scraper or Dremel Tool with a suitable bit, etc.

Don't worry, it'll be fine.

And, - Good on you for helping out a mate!

AL.

giardini said:

The thrust side in this case is opposite the camshaft side.
It's easy to picture, as the big end begins its descent to one side on the power stroke, the conrod angle pushes the piston against the opposite (thrust) side of the barrel.

What ring gap do you have ?

"Do the right thing", get a new set of barrels & pistons/rings. Just saying : )

Click to expand...

That is what I wanted to hear
Like I said the wear is negligible but just visible and the ring gap was within the tolerances when I checked it before fitting the rings.
As for new stuff well I am having to work with what I have been supplied with and trying to do the best job possible. I am confident that all will be well. Thank you again for all the input

I'm with AL,
if its a case of make do and mend then I don't think it will make much difference to which way you fit the barrels.
However I would try and remove the lip. just use a smaller sander in a drill. you know the tube type that fits over a mandrel. You could remove enough of the high spot then give it another hone.
The rings will find their own position which ever way you put the barrels.
The only trouble will be a bit of oil getting by the rings if the barrels aren't perfect