Engine Tolerance and Tightness

[RonLey]

After getting the engine together it is really tight. Without the pistons I can put my torque wrench on it and turns at less than 30 ft lb. But put the pistons in and it is tight.

The cylinders are bored out .060, the pistons are older with the side split but have never been used and have a have a scraper and expander on the middle ring.

The pistons rings are consistent with the top ring gap is .012, the middle ring gas is .010 and lower .080. So all are at the tighter end of the tolerance. There is not any shiny spots on the skirts, the top slides easily on the wrist pin and is not binding against piston. Measured the width of the connecting at the big end to make sure it wasn't hanging up. The piston clearance is a little tight at .002 (skirt not ring end) but can still pull out the gauge.

I thought I might have a bent connecting rod but all check out. Got a cylinder gauge and checked the bore diameters and checked fine. Loosened the caps just a little to make sure that they weren't too tight and binding. Measured ID of connecting big end to make sure no high spots in babbit.

But still when get the 4th piston in it is almost impossible to turn the crank. I have tried different orders of putting the pistons in. I am out of ideas. I am almost at the point of ordering new pistons and rings but I can't definitely say that it would solve the problem.

What am I missing.

[Tom Wesenberg]

Newly rebuilt engines are especially tight, and that's why I like to do several heat cool cycles before taking the car on the road. The front rope seal also adds friction until it's been run several hours.

[tbirdtbird]

let us know how you make out

[RonLey]

I took the rods over to a local model a engine builder that check them on his connecting rod jig that checks alignment.

[George Miller]

With out seeing what you have it is hard to say. But your ring gap is to tight for a Model A. Also in my book you need at least .0035 clearnce on your piston. Again .001 per inch of bore. You Will find that a long stroke engine with a big bore is hard to turn when rebuilt. There is a lot of ring drag and with a long stroke the piston speed is much faster than a short stroke. That makes it hard to turn.

[mike657894]

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measure the pistons and the bores. and if you dont have that equipment take it to who did your boring. because it sounds like either your bores or your pistons are not what you think they are.

[Chuck Sea/Tac]

I think .002 is trouble!!

[Fullraceflathead]

If it was mine I'd hone the cylinders until I had .003 to .0035 clearance.

[jfreddie]

Connecting rod straightness can definitely hang things up.

[James Rogers]

Better listen to George. This is another motor that will lock up when it gets to running temp.