Clearances: Rings & Valves

[Ron/IA]

My block required the crankshaft mains be replaced, so while I have the engine tore down; I am redoing other parts also. I.E. - new stainless steel valves with one piece guides, and new piston rings.

My question is: What does the Fordbarn population prefer to use for clearances on the ring gaps and between the valves and lifters? The measurements in the "Service Bulletins" are slightly tighter than the measurements in "Les Andrew's Vol. 1".

Valves: Service Bulletin - .010"-.013" (both exhaust & intake)
Vol. 1: Exhaust - .015", Intake - .013"
Rings: Service Bulletin - top ring, .011"-.013"
middle ring, .009"-.011"
oil ring, .007"-.009"
Vol 1: - top ring, .012"-.015"
middle ring, .010"-.012"
oil ring, .008"-.010"

Are the larger clearances in Vol. 1 stated because of the materials used today verus during the Model A years? Does it make any difference? Is tighter quieter than larger? Is performance better with one verus the other? When engine rebuilders rebuild an A engine; do they tend to use the tighter end of the stated measurements, or the larger?

Thanks, Ron/IA

[Russ/40]

A general rule of thumb that provides good service and longevity is .003" per inch of bore. I would use that rule for every ring. It's a Model A, not an Offenhauser.

[Ron/IA]

Russ/40 - To make sure I understand your reply; all three rings (top, middle, & oil) should have a gap of .011"-.012" (3-7/8" x .003" = .011625")

Can you tell me where this general rule of thumb may have come from? Why wouldn't there be less ring gap clearance from top to bottom? Most printed ring information I have seen states this. (Just wondering.)

Thanks, Ron/IA

[van Dyck]

A lot depends on how hot the combustion temperatures get. If you have an inefficient cooling system (anywhere in the block), any given cylinder or cylinders can run too hot causing the minimum installed ring end gap too butt together, causing cylinder wall scuffing or worse, broken rings. I prefer to go "high side" on specs when setting end gap, especially on the top ring which experiences combustion heat first hand.
Break-in should be a consideration as well. As the rings "nest" into the cylinder wall crosshatch, the end gap on the rings increases - give this at least a couple hundred miles of light duty driving for this to take place, carefully avoiding overheating. Drain the oil to remove the microscopic wear particles created by friction from the rings and cross hatch. Replenish with a good quality zinc rich motor oil.

[Ron/IA]

van Dyke - Thank you for the in site, it makes sense.

Ron/IA

[Kurt in NJ]

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I got taught a lesson many years ago---loose is safe, it was a Mercedes diesel, made everything to the min clearances, --going to last forever, the day after it was delivered the owner tried to go as fast as it would go --all day, scored up the pistons, cyl walls, the replacement engine I made good and loose---much looser than max clearance, that engine in his use outlasted the car, went into another car and outlasted that car

Piston clearance varies with design of the pistons ---use mfg specs
with ring gaps too small is bad, causes big problems, a little too much gap is not usually a problem
valve clearance is determined by camshaft design and valve materials--stainless expands more than steel, but the camshaft lobe design means the most---use camshaft grinders specs

There was a period in my younger years I made everything to the tight end of specs, now I try to make clearances in the range of a broken in car---most of our restored cars don't get enough miles to fully break them in---I now try to make things fit like a 15,000 mile car

[Russ/40]

Originally Posted by Ron/IA

Russ/40 - To make sure I understand your reply; all three rings (top, middle, & oil) should have a gap of .011"-.012" (3-7/8" x .003" = .011625")

Can you tell me where this general rule of thumb may have come from? Why wouldn't there be less ring gap clearance from top to bottom? Most printed ring information I have seen states this. (Just wondering.)

Thanks, Ron/IA

You will note! All the responses after mine basically state "loose is better". I don't really know where the general rule of thumb came from, but I can explain it. My experience, and the experience of the majority of builders has shown that as you go to the larger gaps, compression and oil consumption rates do not appreciably change, if they even change at all. You will hear stories of so called by the book builds that ended up with scored cylinders. There is nothing to gain by going to the tighter specs, and potentially much to lose. As to the .003" rule, it basically comes from backwards calculation from bore size to the loose end of the specs. It is true that top rings run hotter and theoretically should be looser to accommodate thermal expansion, but my point is the difference is not worth the effort.
Think of the millions of rebuilds on these flatheads, where they didn't even bore, but put the same pistons in with bigger rings, and they did not burn oil. They are so forgiving, and we tend to over think everything we do with them. Make 'em looser, and you won't have to worry.

[Ron/IA]

Russ/40 - Thank you so much for this reply, and to those above that feel the same as you do. My goal with these questions was to find out what the majority thought and why. I gained that information. One of many reasons I participate on Fordbarn.

Thanks, Ron/IA

[Jim Brierley]

Piston clearance is perhaps the most important thing to worry about. Ford said .002" max, this will not work with modern pistons and they will seize in the bore. I won't set them closer than .0035" I set valves at .012" intake and .014" exhaust, on a stock cam.