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Message started by Red_Wine on 03/23/08 at 17:31:31

Title: Piston rings tolerances
Post by Red_Wine on 03/23/08 at 17:31:31

Hello all,

I have the following issue when reassembling my engine:

I have an old, good condition piston and new piston rings to match. Old piston pin looks fine, so I am going to keep it. The cylinder has been machined and looks mirror-like inside.

When I want to install the top piston ring, it goes in fine, but it has some play in the piston groove... with a feeler guage, a .06 gauge goes in freely, but a .08 does not. The .06 gauge drags in some points (over the piston pin holes) but still goes in fine.

Does any of you know if there is any tolerances allowed there?

Should the piston ring fit tightly or snug in place?

Or is some "movement" allowed in the piston grooves? If so, how much?

Any input or comments is appreciated.

BTW, mine is a 400 cc, chain drive... so maybe tolerances might be different... or not?

Thanks,

RW

Title: Re: Piston rings tolerances
Post by KwakNut on 03/24/08 at 05:32:25

Service limit for the 650 is 7 thou ring/groove clearance for the top ring, 6 thou for the 2nd.

The 400 won't be that much different - no rings on any motor are a tight fit, so I suspect yours are fine.

Title: Re: Piston rings tolerances
Post by T Mack 1 on 03/25/08 at 11:29:32

Piston rings fit tiny bit loose in the groove.

The measurement you need to check is the gap. Put ring in cylinder (no piston) and measure the little gap between the ends. Too much gap and you will get low compression and oil "blow-by". Too tight and there is no room for thermal growth.

Also, cylinder wall should have a X pattern honed in and not be mirror smooth. The X pattern helps to hold oil on the cylinder walls.

Title: Re: Piston rings tolerances
Post by Red_Wine on 03/25/08 at 20:40:44

Thanks T Mak 1,

I will check that gap and see what I get.

The issue here is that being a 400cc engine from the late 1980s, it is almost impossible to get any OEM parts for engine/transmission/electrical. Everything is either too expensive, or it takes 6 months to get here. :o

I am wondering if Wiseco or any other manufacturer would give it a try for a set of, say, 50 pistons for the 400cc engine, which is quite popular in Chile. Many 400 Savages are riding around... except mine :'(

Thanks again.

Best,

RW