Piston ring gap
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Cargo Master
Joined: Jul 2015
Posts: 2,063
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From: tucson
Piston ring gap
I'm getting ready to assembly my 300. It's a '93 that is going into my '66. Rv cam, hypereutectic pistons and Hastings rings. Do you use the ring gap specs in the '66 shop manual, or diff specs. The machine shop said that the top ring should be gapped wider or at the high end of the spec. What does everyone else suggest?
Motor will be around 9.1:1 compression.
Thanks.John
Motor will be around 9.1:1 compression.
Thanks.John
Fleet Mechanic
Joined: Nov 2002
Posts: 1,643
Likes: 69
I have always used the ring gap manufacturer's recommendations on ring installation. I have never had a problem in engines as high as 13.4:1 CR.
I once did a dyno study where we built an engine and dyno tested by placing it in a cell at -20F and starting it up and as soon as good oil pressure was established went to WOT and held it there, trying to get the rings to butt.
Then disassembled the engine, installed another set of rings at .002" tighter and repeat.
We continued this until the engine seized or temps stabilized.
I got down to single digit clearances and nothing happened.
After that experiment I always went to the tight side of the mfgr.'s recommended specs.
Note that this experiment was done on an NA engine. With boost all bets are off.
I once did a dyno study where we built an engine and dyno tested by placing it in a cell at -20F and starting it up and as soon as good oil pressure was established went to WOT and held it there, trying to get the rings to butt.
Then disassembled the engine, installed another set of rings at .002" tighter and repeat.
We continued this until the engine seized or temps stabilized.
I got down to single digit clearances and nothing happened.
After that experiment I always went to the tight side of the mfgr.'s recommended specs.
Note that this experiment was done on an NA engine. With boost all bets are off.
Thread Starter
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Cargo Master
Joined: Jul 2015
Posts: 2,063
Likes: 137
From: tucson
After doing some checking, KB pistons have a chart that gives you a ring gap factor that you multiply by your bore diameter. For a normal build
it's 0.0065. For me, 0.0065 x 4.030 = 0.026 for the top ring only.
it's 0.0065. For me, 0.0065 x 4.030 = 0.026 for the top ring only.
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