I have an old OS 61 four stroke that seems to be a little low on compression. I was wondering if a Magnum/APS 61 four stroke ring will fit in this engine.

I the bore is the same and the ring thickness is the same, I see no reason why it wouldn't. If the bore is larger (longer ring) in the magnum by a small amount, you can file the end of the ring to make it fit the smaller bore. You would need to test fit the new ring in the bore (not on the piston) and ensure it doesn't bind anywhere in the liner anyway.

U, is this after a recent run or after sitting a while? Thanks

It was after a recent run, after I tached it at 9,300 RPM on a MA K series 12x8 on 10% nitro.

Wow, unless I'm screwed up in my thinkin that would good numbers for a .91.

I get 10,000 with a 14x6 on a Magnum .91

We're talking 12x8 here I thought.

Those numbers do sound good for a 61 on 12x8. The ASP/Magnum 52 I have barely can turn a 12x6. I had to switch to a 12x4 for reasonable performance, on 20% nitro. The OS 70 can turn a 13x6 with no trouble. Also, I would make sure that the valve gaps are within specs, just in case.

Those engines often feel "soft" on compression but run well, don't worry about it.

True, and the two Magnums I have owned always sounded loose and clunky too.

That is good news. Thank you all.