1928 Mod A/B restoration help needed

[Roger Gwynn]

Standard clearance would be .0055, taking it out to the top clearance of .0065 should do it and don't worry if it seizes as it is usually minor and the high spots can be removed with a fine file. Just whip the clutch in, wait 5 minutes for it to cool down and once the engine is free ride off. My VA used to seize when ridden hard, after the third time I honed the cylinder out half a thou more and it has been as good as gold since. Modern pistons expand at the same rate as the 'good' old pistons, many years ago I found the expansion rate for Heplex pistons and compared them to the modern material to discover that there was negligible difference.

[RayBlack]

Thanks for the advice Roger and Nev. I will inform the machinist to go with 0.065 clearance and keep my fingers well and truly crossed it all works out in the end (which may well be a long way off )

[nevhunter]

The correct place to measure the Piston size is at the Bottom of the skirt at right angles to the gudgeon Pin. Piston types are solid skirt, T slot and W slot and "invar" strutted (some car pistons). I prefer full skirted and solid. Highly relieved at the sides( Called Slipper pistons) need more oil cooling that the ARIEL SINGLE oil system provides.. The slotted Pistons crown (and RINGS) runs hotter than solid skirt, due to less area of cross section for the High temp to flow through. They are also more inclined to crack and are More suited to Medium output family cars. (water cooled). Nev

[RayBlack]

All good points Nev, but the fact remains the only available pistons for a '28 SV are the repros from JP, so that is what I am working with.

[nevhunter]

JP will make whatever you want. Nev

[tony.barnett]

I'm using a JP in my 1930 Model B after multiple ring breakages with the original piston (the ring grooves had worn allowing the rings to chatter a bit). I had it bored to +30 and fitted the JP. It has been fine and I've probably got 2000 miles on it now. But the bike vibrates a bit more than it did with the original piston. Not really a problem, but not as smooth as it was (and it was wonderfully smooth). I wonder whether there is scope in trimming a little excess material off the JP to make it more like the weight of the original...but then I'd have to take apart a perfectly good bike and I'm not too keen on that.

[Roger Gwynn]

In my experience you can spend hours with a rotary rasp removing metal from inside the the piston with little difference. Most of the extra weight comes from the gudgeon pin, so use and old or rather lighter one if you have one in good enough condition. Some modern pins have a constant wall thickness rather than being tapered towards the ends. It may be possible to machine this taper into the ends.

[RayBlack]

Thanks Tony. It is reassuring to know that the piston will at least be fuctional and reliable.

Roger: I do not have a used gudgeon pin as the engine came with what looks like a car piston fitted and a pin that is smaller than 1 inch diameter. I think I will go with what JP have provided for now and see how it runs rather than trying to figure out how much extra weight to remove (and possibly making it worse).

The barrel went into the machine shop a couple of days ago to get the new sleeve fitted and bored to suit the new std JP piston. I am presently dealing with lots of crossed fingers hoping that the process is successful