Bore size-- quick question- need quick answer

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Jerry Doe

May 21, 2003
The same reputable company who screwed up my valve guides also rebored my barrels to +40. I am just inspecting things and about to re-assemble. When I remove rings from pistons and put pistons in bores to measure gap it is 8 thou on both, between piston and bore. Is this OK? seems a bit excessive to me (only 2000 miles since they rebored it)

So you have .008 on one side when you push the piston to the other side? If so, that gives you about .004 all around? A buddy and I who are rebuilding a Matchless G9 and a Dominator 88 just went through this piston clearance issue a few months back. The consensus number we came up with from the Matchless and Norton experts was 0.004 piston clearance. However, this number seems to depend greatly on the type of piston fitted. We both fitted new oversize Italian GPM pistons to our bikes and these pistons have a reputation for seizing (apparently the problem is solved now) but may require more clearance than other pistons. All depends on what the pistons are made of and how much they expand. What kind of pistons have you got? The piston manufacturer should be able to give you a clearance to shoot for but it sounds to me like you are just about right. My GPM pistons listed a bore size (including clearance) on the box so the machinist who bored my barrels used that figure. I am sure there are other people on the forum with much more experience with Commando pistons who can chime in.
Sorry-- false alarm-- cancel thread.

I was not measuring it right. It is 3 thou so is perfect--Cheers
if you can fit an 8thou feeler guage down the side of the piston,at the bottom then it has 8 thou clearance not 4,you dont 1/2 you measurement.
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