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#511711 - 05/22/08 10:38 PM
looking for piston ring gap specs
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New Member
Registered: 05/22/08
Posts: 1
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Hey folks,
first-time poster here, my father is working on replacing a piston in his 'dooodlebug' which was a Model A. He asked if I could try to find out what the gap on the rings should be.
He's just using a standard ring, didn't have any other specs to give me to relay here. If there's any info that can be provided, it will be greatly appreciated.
thanks in advance.
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#511728 - 05/23/08 12:08 AM
Re: looking for piston ring gap specs
[Re: B.R.]
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Member
Registered: 11/06/07
Posts: 406
Loc: Detroit (Royal Oak), Michigan
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Ring gaps are often a confusing and misunderstood part of a re-ring job. There are MINIMUM and MAXIMUM ring gap specifications which must be observed for the best performance of a new ring set.
Minimum gap tolerances must be observed in order to prevent the ring ends from butting together as the ring expands when the engine approaches operating temperature.
A MINIMUM of .0035 gap per inch of cylinder diameter is the formula. Example: 4" (bore) x .0035 = .014" minimum gap. There are variables to this formula:
Hyper-eutectic alloy pistons transfer heat to the cylinder walls. Those rings must have a minimum gap of .020" (for a 4" bore, using cast iron comparession rings). My 'worn' 5.0 Mustang measured .030" end gap, and still didn't use any oil. Since I had it apart anyway, I replaced rings with Total Seal.
Maximum ring gap is important because too much gap results in lost compression, power loss and ultimately poor oil control.
Any increase in the bore size, results in an increase of approximately .003" in ring gap for each .001" increase in cylinder diameter.
Rings should be placed at the lowest possible part of the cylinder (below ring travel area) as this is the portion of the cylinder which is unworn and which the ring is sized for. Checking ring gap in the worn portion of the cylinder will show a gap increase in direct relationship to the amount of cylinder wear.
Other measurements are important too. The space BEHIND the ring, between the ring and the piston should be within tolerance, and the 'top of the ring' to groove measurement is important too.
Piston rings travel around in circles as they wear. The ring needs to rotate freely in the piston groove. You can space the grooves upon initial setup, but later, they will travel, and that's ok.
Also make sure you have good (~45-degree) cross-hatch in the bore. A smooth bore, with little or no crosshatch, will cause ring-hydroplaneing (the ring lifts off the surface of the bore) and that cylinder will burn oil.
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