Well it was good while it lasted

We know that a standard Commando will seize at two and a half thou when lightly worked. At one and a half thou pretty much any old air cooled engine will lock up.

Glen

LD, you sound like maybe you've been working with modern water-cooled, aluminum block, small bore engines. Some of them run quite well with .0015" cold clearance, but not our Nortons, particulary the ones that are run hard, like Jim's (comnoz). .004" for 750s and .005" for 920s are pretty much the minimum you can get away with for cast pistons in iron Norton cylinders on a race bike. Add another thou or so for forged pistons. Take away a little for alloy cylinders, but not much. Depending on bore size and piston design, you might run a little more, or a little less, but we're talking differences like half a thou here. If you're really easy on the bike, and do a serious break-in, you might get by with a little tighter clearance on the street, but only because you're giving the piston skirts time to callapse down to a clearance that works. With a stock 750 with original cast pistons, you can get by fine with .0035" for a street bike ridden normally. On a race bike, you'd want at least .004" with the same pistons. I was never able to keep a 920 with cast pistons and iron cylinders from seizing at anything under .005", and at that clearance they would end up squeezing the skirts down after a bit of use, until the clearance was more like .0055" - .006".

Ken

I performed the quick and easy leak down again this morning with the total seal rings. Standing on the kickstart with a cold motor gives me about 30 seconds before the pistons lose compression and the kickstart falls through. With the motor warmed up on the stand and the oil circulated for a few minutes it takes twice as long to leak down - about a minute. Thats a big diff between a cold and warm motor and I don't know if its the circulated oil or the warmed up tighter piston making the diff.

[video]http://youtu.be/rzp1nEGWLBk[/video]


I think you have a hell of a lot of ring wear there and that gap was too wide. If you had .0007" thou wear on the cylinders in 10,000 miles then you would have over a thou of wear in 20,000 miles and thats getting up there when you don't want any cylinder wear at all.

Before I put in the lightweight pistons I ran Hepolites for about 10 years. They finally got sloppy and noisy. When I measured the cylinders there was only about .001" wear near the top but the skirts were worn, the ring lands were loose and it was starting to smoke.

Its not a perfect world. You have to balance piston weight against crankshaft breakage. The cost of carbide cylinder impregnation against having to re-bore the cylinders. Someday there'll be a glass piston that weighs nothing and lasts forever in a ceramic bore. Until then we do the best that we can.

Thank you for that info, it was clearly written.
I have experience with many engine designs, from air cooled Japanese road to land speed efforts to dirt bikes, and everything in between, but my Norton experience was long ago (when I was racing one in California club races).
600cc honda singles (air cooled) run .0015 clearance, and that's an old design now. Our land speed engine (Aprilia RSV) needs .0012" with forged Pistons in steel liners, this in an engine making 380 "dynojet" hp at the wheel (Google Team Punisher/AF1to see details), and those a large clearances by modern standards.
F1 engines musty be heated before starting as they are press fits.
It's been so long since I built a Norton road race engine that I have forgotten every thing I knew about them.