Commando 850 Piston Clearance

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VR880

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Dec 13, 2005
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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?
 
I have always had my 850's fit to 4 thousands when fitting new pistons.
 
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
 
Jason,

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

Debby
 
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
 
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
 
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