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post #1 of (permalink) Old 09-21-2003, 09:57 AM Thread Starter
 
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Oversize piston ring question.

I need to pick up some oversize rings for my YJ that is slightly out of spec on the bore size.
What I'm confused about is the actual measurement...

Let's say that I need to get .010 oversize rings. Will the rings be .010" wider than a "standard" piston ring or will it have a .010" bigger diameter?

For example - a standard ring has a theoretical ID of 3.275" and is .455" wide.
Will the oversize ring have the same ID of 3.275" and be .465" wide, or will it have an ID of 3.285" and be .455" wide.
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post #2 of (permalink) Old 09-21-2003, 10:28 AM
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Re: Oversize piston ring question.

I can help, but I'm not sure what you are trying to achieve.

+010" oversize rings are for +.010" oversize pistons and will have the same thickness, but be +.010" on the ID and the OD

Do you have a bore that is .010" over standard?
If so, you can't just put larger rings in with standard pistons. The piston will rattle itself to death in that bore. You need "ten over" pistons and they should come with rings. Don't use standard pistons in a +.010" bore!

Are you trying to use standard size pistons in a ~slightly~ over standard or out of square bore?
If this is the case, most quality ring sets (speed pro, Ross, etc. are oversize a bit and need the gap filed to fit your particular bore. Make sure the rings fit with the correct gap through the entire range of the stroke by pushing the ring down the bore and inch at a time (with a piston to keep it square) and measuring the gap at each interval. File the ring gap to be minimum at the smallest point in the bore.

I hope this helps, if it's not what you were looking for, give us a little more info and we will be happy to help!

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post #3 of (permalink) Old 09-21-2003, 10:46 AM Thread Starter
 
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Re: Oversize piston ring question.

The reason I'm asking all this is because I re-ringed my 258 about 3 weeks ago when I was trying to eliminate the horrible case of blow-by that I have. At that time my daily driver needed a fuel pump so I had to take my YJ everywhere.
If I got over 50MPH for any longer than about 5 minutes continuously, then I would literally start shooting oil out of the breather filter and around the PCV hose. If I went the 35 miles or so to my girlfriends house on the freeway, I would end up losing roughly a quart of oil during the round trip..most of that oil would be covering my entire engine.

Anyway, I'm trying to use my stock pistons in my 258 which has 170k miles on it. Just through normal wear, the bore has obviously been increased.
In this case, using a bore gauge, it's about .007" - .009" out which is pretty darn close to .010".

When I went down to Napa to ask them about ordering some piston rings, they asked if I needed standard or 10, 20, or 30 over.
I was thinking that the .010 rings would be good since my bore is almost that far out of spec. But that's when I started thinking about the actual physical size difference of the rings.

As for measuring the ring gap, what tool(s) do I need to measure that while the piston is in the bore?
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post #4 of (permalink) Old 09-21-2003, 10:58 AM
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Re: Oversize piston ring question.

You cant measure ring gap with the piston in the bore. You have to pull the piston, remove the rings then put the new ring into the bore. Push it down the bore with a bare piston to keep it square, then pull the piston out. Without moving the ring (in the bore by itself) check the gap with a feeler gauge. If you even think you moved the ring, stuff the piston back into the bore to square the ring back up. You MUST do this all the way down the stroke of the piston so you don't set ring gap at a 'big' spot and then have it bind at a 'tight' spot.
If you get +.010 rings, you can theoreticly file the gap to fit your bore, but ring tension against the walls of the bore will be to great and the distance from the ID of the ring to the groove in the piston will be too great.

If you are .007 to .009 over standard on the bore, I don't think new rings alone are going to help. The out of square bore (taper and out of round) isn't condusive to good sealing. You are looking at boring the block +.020 (to make sure it all cleans up) and new pistons/rings. Sorry.
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post #5 of (permalink) Old 09-21-2003, 01:41 PM
 
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Re: Oversize piston ring question.

Rings need to be matched to piston size, e.g. .010 rings go with 0.10 pistons. .020 rings go with .020 pistons. The rings need to seal not only between the piston and cyl. wall but also between the rings and piston. Even if you put .010 rings on stock pistons to tighten up to the cly. walls they wouldn't seal around the piston , or ride so far out on the ring landings the pistons will self destruct in short time.

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