Piston Ring Gap Question

Modelacoupe3

Modelacoupe3

Senior Member
Hello Ford Barn members ,

I am in the process of assembling an original Model A engine .
The engine has been bored .030 over standard .
My question is related to the compression ring gaps.
The current gap is a loose .014 & a tight .015 out of the box .
Is this an acceptable gap ? If possible, I would like the opinion of those who rebuild A engines professionally .

Thank you !
 
Modelacoupe3 said:
Hello Ford Barn members ,

I am in the process of assembling an original Model A engine .
The engine has been bored .030 over standard .
My question is related to the compression ring gaps.
The current gap is a loose .014 & a tight .015 out of the box .
Is this an acceptable gap ? If possible, I would like the opinion of those who rebuild A engines professionally .

Thank you !
Click to expand...
Not a professional, but on my current worn 0.040" overbore engine the gaps on the new rings were 0.015"+. I don't recall if I got it in the instructions that Hastings included or somewhere else, but there was a Hastings spec of 0.012" - 0.018" for all three rings, different from what is published in the Red Book.

In the past I have gone with one size over on the rings and gapped those to spec. So for your 0.030" overbore you could use 0.040" rings and gap them where you want. In my case I was already 0.040" over and there isn't a 0.050". I felt that 0.060" would be a stretch, so I let it be.
 
Modelacoupe3 said:
Hello Ford Barn members ,

I am in the process of assembling an original Model A engine .
The engine has been bored .030 over standard .
My question is related to the compression ring gaps.
The current gap is a loose .014 & a tight .015 out of the box .
Is this an acceptable gap ?
Click to expand...

Yes.
 
Going to the next oversize ring set to make a "File fit" set, makes me nervous and to my knowledge is ill advised.
 
If the ring gap is a little bit (a few thou) oversize it's not a problem. You definitely don't want an undersize gap, that will lead to big problems.
 
I think .003 per inch of diameter is pretty clear. These specs are always for the minimum clearance. If fitting/xhecking rings in a new cylinder then an inch down from the top is enough for checking clearance. If fitting rings to a worn bore you need to get down below the ring wear which maybe half way down the bore. Buying oversize rings and filing the ends off for a model A makes no sense. You cannot fit the rings in the worn portion of the cylinder as the gap will close up and bind as the piston goes down in the bore.
 
You said "out of the box" and I just wanted to clarity how they should be measured. Plese be sure you're measuring the gap(s) when the ring(s) are installed into the bore. This will be the in-situ gap for the running engine. That will significantly close the gap so that you can take the correct measurement. Apologies if this was already obvious to everyone else, but i only learned it recently.
 
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