I am currently rebuilding my CB500K0 engine and had the barrels rebored ages ago to +0.25mm by a very reputabe outfit via Trigger.
I tried the piston rings (genuine Honda -323- parts) in the bores prior to refitting the top end and every ring, except one gives a perfect ring gap in the bores (around 0.009"). The one ring that doesn't fit (second compression ring) overlaps in the bore by over 0.5mm, which would suggest it is actually a +0.5mm ring wrongly packaged by Honda (ring set was sealed when I bought it NOS).
I then tried some --383-- rings from a CB125 single (the CB125S shares piston style with CB500 except the valve cutouts in the crown are slightly different but the bores and ring grooves are identical). I have six sets of NOS -383- +0.25 rings but every one gave the overlap of greater than 0.5mm in the 500 bores....so it would indicate they are actually +0.5mm oversize. I can't believe that six sets of Genuine Honda rings, could be wrong ..... so a bit of a puzzler there.
A good friend and very occasional poster on here, has donated me another set of genuine NOS +0.25mm ---323-- rings, so hopefuly that will sort out my single problem ring.
Question is though ...how can six sets of rings all be wrong in genuine Honda sets? Here is the relevant data from the CB500 & CB125S Shop Manuals (BTW just putting the CB125S/ CB100 Shop Manual into my Dropbox links)
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