Jason Curtiss wrote:Thrasher - I think you mean 0.0015-inch per inch of bore, don't you?

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Yes thanks. I'm always doing that!

Just resurecting this thread as I was speaking to Norman White today about this subject, and he said that 0.004' to 0.0045' was the optimum gap to have. I queried him with regards to the details in the factory manual (as I understood that the mathematics equate to 0.002' to 0.0029' piston to barrel clearance) and he seemed to think that they were erroneous!!!

Somebody said that's .002 for each side, or .004 total...

(I don't knowe about that, I'm just digging up a foggy memory)

Anyway, I use .0045 total on most aftermarket pistons.

I've always used the rule of thumb of .0005 per inch of dia. Forged pistons of course...

I've used .0040/.0045" for cast pistons and .005" for forged pistons in the 750 race bikes for years, and it seems to work well. When I started building 920 engines back in the '70s, I tried forged pistons from Forgedtrue, Arias, and Venolia. All recommended really tight clearances, around .0025" to .003" IIRC. They seized every time with those clearances. I worked my way out in increments to .005", and that's where they stopped seizing. I use .005/.0055" for all the big bore JE forged pistons I use now. That's in iron cylinders and alloy cylinders with iron liners. If you had alloy cylinders with Nicasil bores, you might be able to run less clearance, but I've never tried it. Air cooled Kawasakis in the same bore size range do indeed work at tighter clearances, around .002", I think, but that will not work in a Norton. I've had JE making Norton pistons for years now, and I've talked to their engineers in detail about clearances. They said they normally would recommend .002" for the 73 mm bore, but had no experience with old British Iron, and were very clear that their numbers were only a suggestion, and that I should use whatever worked for my application. The pistons I get from JE are ground for .002" clearance for the 750 and .003" clearance for the 850 and larger sizes, based on whatever bore size I give them. That's their normal practice for this type of piston. I could have them made for .005" clearance by giving them a different bore size spec, but I like them the way they are, because it allows honing worn stock cylinders out another .003". Frequently that will clean up the bore enough that the next oversize isn't required. I only sell the pistons as race parts, and the engine builders normally expect to have to hone the cylinders to fit the pistons, not to a standard bore size.

Other folks might have other experiences. These are mine, for whatever they are worth.

Ken

Ken sold me a set of aluminum jugs for my 750 Norton some 10 years ago, after sending them back he nicely traded out for a spare set I got this all straight. With a .004 total clearance the piston slap was heard by gent who knew his stuff BRIAN HOZIGAL. That was at the 1999 rally in Virgina. I had to make up two inch torque plates for the top and bottom of the jugs and fully spec tighten the bolts Brian than fit up the pistons with .0025 total clearance and they have run that way many miles I rev the motor to 7000 all the time. So yes if you use torque plates you can fit Alum. jugs much tighter on cast pistons. That said with stock jugs and cast pistons .0035-.004 is right in there. Having them coated on the skirts seems to help them tolerate the break in better. Buy good rings for your cheap pistons and all will be good.

This is an interesting discussion
I have a couple of SR500 Yamaha's. Forged J&E's in them. I run them at .0025 to .003
I have a 470 bored and stroked DRZ motard with a forged CP piston. It's a nikasil cyl. It runs .0022 clearance. 94mm bore.
All of the BSA singles I have around here have cast pistons in them. I think they are about .0045. My 850 Norton is .0045. I don't remember what my 750 is but I'd guess it's close to that.
I also have to agree that torque plates make a difference.

Back in the '70s when we were still making 920s by sleeving the stock iron cylinders, I had to make up torque plates and bore and hone the cylinders with them, or the rings wouldn't seal well, no mater what the clearance. The problem was distortion at the top of the bore where the counterbores are for the four long through bolts that hold the cyinder to the cases. The rings seated OK, but the leakdown measurements were poor. They still ran OK on the street, but used a bit of oil, and didn't make the horsepower on the race track. Using the torque plates solved all that, but I still had to run .005" clearance to keep them from seizing. Again, this is on race bikes that were seriously abused on a regular basis. I never made any for the 750 bolt pattern, but the 750s didn't seem to need them. I doubt if I would even have noticed a little piston noise on the race bikes. I ran a 920 on the street for a while, put together using the torque plates, and it ran fine, and didn't smoke or use oil. It may have also had some piston slap when cold, although I don't remember any, but I think I just expected Nortons to be noisy, and wouldn't have noticed it unless it was pretty bad.

Ken

When I bored my '74 850 the first time about 30 years ago the Norton dealer told me to have the machinist shoot for .003 clearance. .003-.004 is OK, .005 is loose.