Piston Ring Size

[sickswan]

I'm rebuilding a bsa c15 at the min the bore is 67.13 which I believe to be +5 tho however u cannot get that size rings closest is standard or plus 20tho.. what should I buy? Cheers luke

[hencam]

Hi Luke,

this is my suggestion based on opinion and limited experience of the unit BSA single. personally i would measure the bore to piston clearance by sliding the biggest feeler you can get between the piston and bore in the area the piston travels. measuring using a digital caliper, vernier caliper or bore micrometer is ok, however the feeler gauge method is more reliable and in most cases is what an engine reconditioner would work from. similarly to when having your barrel bored if you are prepared to run it in you can ask for it to be bored tighter clearance, perhaps 0.002" or 0.003" thou as opposed to 5 thou if you need to get on and race the bike. a tight bore will give better performance and overall will last longer as the piston cannot float about or slap.

so back to the original question. my suggestion would be measure piston to bore clearance, check what the service limit is, i can't remember for C15 about 0.009" i think if it is within this then a light hone/glaze bust will remove the polished surface and help new rings bed in. This will also tell you whether you need a re-bore or not, if its close to the limit save the grief and hassle and have it bored.

if it is well with the piston clearance limit, in my opinion the best way to check which rings you need is to purchase both and try them in the bore, most supplier will accept the ones not needed back if you are prepared to pay the postage. Again there is a piston ring end gap service limit, so with your new rings either size, for this explanation start with the standard rings, place one in the barrel half way down the bore, take the piston, place it skirt down on the ring and turn it 180° back and forth to level the ring in the bore. then look down the bore and measure the ring end gap using feelers, again if it is close to the service limit, personally, i would forget it and try the +0.020" rings. do exactly the same with the piston, it is most likely these rings will either touch on the ends or have slight over lap. if this is the case you need to file the ring end back equally on each one until the ring end gap is the what it should be with a new piston rings and bore. use a fine file and have patience, it is worth it. keep trying until you have the desired ring end gap. if you only want to buy one size of rings I would go for the +0.020" in my experience of the unit BSA you will be lucky to get away with std. you may find some NOS 0.010" rings as these were once available in the good old days!! only one other thing, speaking from experience the ring thicknesses on modern pattern pistons, sometimes differ from the good old Hepolite type. so if you have an original piston try and find some NOS rings. if it is a modern type GPM etc then buying the rings should be easy.

Hope this helps

cheers H-Cam

[b40rt]

Have you checked Rupert Ratio ?

[naichuff]

Check out Draganfly motorcycles They do parts for C15s I use them for my B31 parts and they have a good service

Edited January 13, 2015 by naichuff

[sickswan]

cheers for replys