piston working clearance

Usually the information comes with the piston.

Who made the piston, the clearance is determined by method of production ie forged or cast and the type of alloy. Look inside the piston for a makers mark, AE is Hepolite, JCC is Taiwan and you can use the Hepolite clearances. 

In the Dominator Service Notes, the recommended clearance on a 650 is 0.0035" to 0.0040". With 0.0045" advised if you intend to go racing a lot. A 750 engine  is probably going to need at least least 0.0040" .  As John says the amount of advised clearance will depend very much on who made the pistons and how good the rebore is. Plus if any honing is added to the equation. I had my 650 barrels honed which enabled me to begin long distance riding, at a good cruising speed, after just a couple of hundred miles of running-in. The trade off for this being the rings did not bed properly for thousands of miles and the motor drank lots of oil before they did.

The old 'rule' for aircooled engines of 1 thou per 1 inch of bore is as good a starting point as any, especially as you are in the dark as to the pistons origins.  May be a little tight though looking at the above for a nearly 3" bore of a 750.

However it is a lot easier to go bigger than smaller!

Don't forget to check the ring gaps too- 2 thou per bore inch is a sensible starting point for them.

On the general point about clearance, I have just had my 850 Mk3 rebored. The modern GMP pistons come with no clearance information whatsoever. They expect you to finish the bore to EXACTLY the nominal, metric bore dimensions. A standard 850 bore is 77.00mm and a plus 0.020 is actually 77.50mm (rather than the imperial addition of 0.508mm).

Modern pistons are not symmetrical cylindrical shapes. Measured in line with the gudgeon pin my GMP pistons were from top to bottom 76.98, 77.10, 77.20mm, and at 90 degrees to the gudgeon pin 76.96, 77.34, 77.43mm. Measurement precision around 0.02mm. Thus they are smaller at the top, and smaller at the gudgeon pin and below it. These are the places with most metal and when you warm them up they change shape. So, measuring cold clearances is difficult and you need to know that the bore is 77.50mm. Actually, those metric dimensions translate to cold clearances (all around) of about 0.005" at the piston top and 0.003" below the gudgeon pin, and 0.0006" for the skirt at 90 degrees to the pin.

Sorry this is not information for a 750, but I explain it to help you judge those 750 pistons. I don't know if NOS 750 pistons might be more exactly spherical. I agree with Philip's comments from the 650 documentation for 0.0035" - 0.0040" at the piston top, and his suggestion of 0.0040" - 0.0045" for a 750, and note that for my '828' it is 0.0050".

If anyone is having the barrels rebored make sure that the workshop mounts the barrels on the crankcase flange joint and not on the bottom of the flange, which might not be parallel to the joint. I found my original Mk3 bore was 0.003" out of square fore and aft (which does not matter), and 0.003" out in line with the crankshaft (which put a strain on everything but the bores survived nearly 50,000 miles). I was tipped off to look for this by Ashley and it is apparently common on Mk3s.

... but I always err on the large size when it comes to rebores. An extra thou or so will make no difference to performance but will mean you don't spend the rest of your life running the damned thing in.....

I restored a 1931 Subbeama couple of years ago and it had been rebored by a previous owner. It ran fine till I gave it any work to do when the piston tightened in the bore - it was OK after letting it cool off for half an hour or so. I had a further .0015" honed out and it ran just as well, didn't burn oil, but more important I could give it some stick without worrying. It's worth remembering that engines like my ES2 were only thought (by Edgar Franks) to need a rebore when the clearance was .007" over the standard .005".......

Norman's posting reminded me of many years ago while attending a Norton celebration where I was able to talk with some of the original foundry workers who actually cast the Norton and other vehicle barrels. Apparently the force of the molten iron  filling the mould would push the bottom of a cylinder slightly to one side thus changing the allignment from parallel and perpendicular which is possibly what happened with the set on his 850 engine. You have to ask why nobody at the Factory checked the machined and bored product for accuracy???  Another cost-cutting???

HEPOLITE Service manual 1961 may be of assistance . Takes some interpreting and is only a guide.

GRAHAM

My thoughts:

Anything that would run would be seen as ok.

Scrap=money lost and when you have no money you will do anything to get some.

For most, the warranty would have expired before the issue came to light so it was 'ok'.

I always reckon on .005 thou at the bottom of the skirt at right angles to the gudgeon pin, plus 2 thou for high spead. Commando pistons are VW Beetle, lots of sizes in the catalogue. Japanese engines came in with half a thou clearances, and British clapped out machines could not do that. We used to do them on a very expensive Swiss jig-borer when the boss was not present.

George is right as to attitude. 850 Commando barrels split at the skirts, as well as offset sleeves, I have two of them, and not much meat to salvage them.