I am doing a rebuild of a 1930 BSA SV Sloper. It was originally built up from a basket case and is fitted with M20 girder forks, M20 rear wheel, M20 flywheels, crankpin and conrod. Although orinally 87mm bore, I sleeved it 82mm and fitted an M20 piston. As it had previously siezed and to avoid future problems, I was advised to do two mods:
The first was to file off metal either side of the piston below the oil ring, paricularly in the gudgeon pin area.
The second was to machine a 0.5mm deep x 4mm wide groove directly below the oil ring and drill additional holes in this groove.
What are the forum opinions as to the need and/or desirability of such surgery? Also what is the recommend piston/sleeve clearance. I have 0.008"

email (option): pvlietstra@gmail.com

Was it a genuine BSA piston, or Hepolite or was it a pattern one? The 0.008" clearance seems a little bit excessive I'd say 0.0035" to.005" would be better, but if it is a spurious pattern one I'd stick nearer the 0.005" measurement. The machining/filing would really be only applicable to the pattern piston as they do tend to nip up just under the rings indicating that they have a higher expansion rate or that there is not enough taper on the skirt.

email (option): davmax@ntlworld.com

I agree with Daves suggested clearance figures and his comments about the origins of the piston...It shouldn't be necessary to do any modifications, standard pistons are both tapered along their length and oval, with the smallest dimensions being on the axis of the gudgeon and above the rings. This is done to restrict friction to the thrust faces primarily. No further oil holes are needed. Gap the rings to about .008" before refitting......Ian

The piston was a GPM made in Italy.

email (option): pvlietstra@gmail.com

Then I'd definately stick to the higher clearance of 0.005". I really find it hard that in this day and age that they cannot produce a piston to the same quality as the 1940's ones!

email (option): davmax@ntlworld.com

dave its because they are not interested how good it is only how much profit they can make

email (option): roger.beck@node6.com

Dave, its called a dumbing down of society. I can do trig, but my kids can't. My kids can do freehand writing but my grandkids can only type on the computer.

email (option): pvlietstra@gmail.com