Commando 850 Piston Clearance

VR880 · Dec 13, 2005

I have rebuilt many Triumphs but am doing my first ground up restoration on a 74 Norton 850 Commando. My question, is .004 piston clearance enough for an 850?

pommie john · Dec 14, 2005

I use 3.5 thou on my 750 racer. I'd say you were in the ball park with 4 thou.

Ron L · Dec 14, 2005

I have always had my 850's fit to 4 thousands when fitting new pistons.

Jason Curtiss · Dec 14, 2005

Wow – 0.004” clearance between the piston and valve sure sounds scary!

I didn’t realize you could run these old Norton engines that close to the “edge”; I’ve always used the thickness of a piece of corrugated cardboard as acceptable clearance. Well, it sounds like I can use thin shirt-cardboard instead and still have plenty of clearance.

Jason

debby · Dec 14, 2005

Jason,

He didn't really specify, but I'm guessing he meant piston to cylinder clearance.

Debby

Jason Curtiss · Dec 14, 2005

Deb,

You must be right. But 0.004" sounds almost sloppy for piston-to-cylinder clearance?

I would expect a good forged piston to have a low expansion rate and therefore require a tighter fit. The clearance typically varies from one piston manufacturer to another and is usually specified in the instructions with the new piston.

At any rate, I’m back to using my corrugated cardboard for piston-to-valve clearance!

Jason

Ron Hulton · Dec 14, 2005

Using the factory min. / max . specs you can be anywhere from . 0035" to .0054" .

Cylinder Grade A 3.0315 / 3.0320

Grade B 3.0320 / 3.0325

Piston Skirt Bottom 3.0280 / 3.0271

Commando 850 Piston Clearance

VR880

pommie john

Ron L

Jason Curtiss

debby

Jason Curtiss

Ron Hulton

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