Hi there everyone. I have a BB34(not GS) with a standard piston BSA part number 65-1265. To try to cure a smoky engine I was going to have the barrel rebored but was advised that it only needed honing. I had this done and the bore does now look good. I don't want to put the old piston back in the nice smooth bore but a replacement appears to be unobtainable. Chris at Autocycle Engineering has done a search for me but drawn a blank. Any suggestions or help will be most welcome. Regards. Dave.
Were the barrel and piston measured for clearance? Were the ring lands on the piston checked against new rings for clearance? If you are within tolerances it would probably be fine to use the old piston with new rings.
The coating is slippery and anti-seizure. HPC promote it for other reasons the required here (increase the size of the piston) and they also do a ceramic top coating (also able to be used on combustion chambers and valve heads) for minimising heat transfer). We've used it many times (usually just the side coating) on racing 2 strokes where controlling bore clearance to very tight tolerance is essential. The ceramic top's benefits are arguable both ways in a two stroke - I have used it but no longer bother.
On a 64mm 2 stroke piston we get somewhere between 1 and 2 thou increase in size. As the above website suggests, there can be up to 0.003" increase in diameter (or reduction in bore clearance).
It's cheap (by comparison with new pistons) and is available (unlike some pistons). We have used it on used pistons - the ring lands are masked off and the skirt blasted (not sure what with) to clean it and then the coating sprayed on and baked. The coating has a very fine 'stopped' effect that slowly flattens off a little (more visual and dimensional).
No generalisation is wholly true, not even this one. Oliver Wendell Holmes
Thanks for your replies. The "old" piston has a few scratches and in the past has appeared to have "caught" on the barrel. the barrel was also marked but the honing has resolved this so the damage was slight. I have some new original Hepolite rings for the piston but don't feel comfortable putting a lightly scratched piston in a nice smooth bore. Regards. Dave.
don't feel comfortable putting a lightly scratched piston in a nice smooth bore.
The iron/steel of the cylinder liner is many times harder than the cast Al of the piston so there's no chance the piston will deface the bore. Also, scratches typically are into a material leaving little or no raised area at the edges. Even if the piston were made of a much harder material than Al it only would be the areas raised above the surface of the piston, not the "grooves" of the scratches, that would be of concern.
Originally Posted By Boomer
Were the barrel and piston measured for clearance?
This is the essential point. A new, perfectly smooth piston that has the wrong clearance would be very much worse than an old scratched piston that was the correct size for your current cylinder. It is because of small variations in size that machine shops bore and hone cylinders to the diameter of pistons supplied to them with the cylinders. They don't bore to whatever nominal size the pistons are listed as having.
Of greater importance on the piston is the ring side clearance. That is the clearance between the ring and the land face. To much and the rings can "flutter", which can cause ring breakage, which then scores the bore and destroys the piston at the least, and maybe gets pieces down into the internal workings, which can cause worse damage. The BMS book cites 1 to 1.5 thousandths max clearance as acceptable, any more requires a new piston. A feeler gauge should go all the way in between the ring and land, with no appreciable widening of the gap out at the outer edge of the land. Taper in the groove is as bad as to much clearance. (worse, in my experience).