Evening! I'm in the middle of building my first engine and have a question. To make a long story short I ripped the engine out of my 1964 Grand Prix back in December for a full rebuild and just received everything back from my machinist two days ago. I'm waiting on a couple things to come in before full assembly, so in the meantime I decided to check the gap on my rings and prep my pistons, though I ran into an issue. For context, my block had to be bored to about .060" over, as it had previous been bored .040" out and needed a bit of help. My machinist supplied me with a set of Hastings 2M639060 rings that, to my knowledge, are a non-file to fit set of rings for my specific application. Not sure exactly what pistons my machinist supplied me with, though they are original-style flattop pistons that I think may be hypereutectic aluminum? Though when I double checked the gap on my compression rings, I'm pondering over whether my gaps are a hair too wide or not. Doing the math using a .004" multiplier with a bore of 4.122", I came out to a minimum gap of .017" on the top compression ring, and .021" on the bottom compression ring. Though when I checked my rings in the bore, the top compression ring measured out to .024-.025" max and the bottom ring about .027" max. After bombing around the internet a bit, I came across a database (Ron's Pontiac Page: Piston Tolerances) that says on a stock 389, a good top compression ring gap is (minimum) .021" +- 0.05". Considering my overbore, I cross-referenced the listed gap for a 400, which came out to (minimum) .019" +- .010", -.005". So theoretically they're within spec? I know, at least from what I've heard, having a slightly larger gap is good insurance when the rings heat up and expand in a street application, though I don't want to deal with blow-by issues. Maybe I'm just paranoid considering my lack of experience, though I definitely wanna do this right. Was gonna call my machinst, though shop's closed 'til Monday... just need some peace of mind, haha. Any help is greatly appreciated. Thank you!
Keith Black pistons typically specify a slightly wider gap on the top ring. SpeedPro and others don't...usually. KB puts their top ring land closer to the top of the piston and it sees more heat.....needing more gap. Follow the piston manufactures recommendation. Find out from your machinist what slugs he sold you and/ or look them up based on any markings you find on them. If all else fails use the calculate gap method and maybe shoot for the higher side of the spec. ....I'd rather have a little extra gap then butted rings anyday. Sent from my SM-G950U using The H.A.M.B. mobile app
If you want to get everything right, please double check the tolerance you quoted here.... The gaps are a little wider than they possibly could be, but still within spec. I'd use the rings as they are.
http://015ef8d.netsolhost.com/TechPage.aspx#trGaplessPistRings The last few Pontiac 455's I built all have file fit rings set to .018" - .019" top ring gap, second rings are .018"-.020". One of the second rings had a ring gap of .024" due to my mistake, Total Seal said to run it, as it wouldn't hurt a thing being a little wider. Don't worry about the engine brand, rings don't care about that, look at ring gaps verse bore size, see chart above from Total Seal. I would install what you have as is and never look back.
If your worried you could buy a second groove Total Seal, not the standard style and go for near zero leak down but on a street engine a lot of extra $$$.. I would fun what you have and make sure you run the engine at 180 + degrees which we all need to do anyway.
Whoooops, haha that's what I get for making a post at 3 AM. But thank you, and everyone else of course, for the advice.
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