Oversized pistons and ring end gap

[Blood_of_Tyrants]

I am in the process of tearing down a F series engine and have a few questions.

First according to the specs, the bore is 3.125" and when I measure the bore it is indeed 3.125" and there is practically no ridge. Should come out with a little honing. But something doesn't quite pan out. The pistons all have 0.020 stamped into them. I assume that 0.020 means that the the bore and pistons are 0.020" oversized. Shouldn't the bore and pistons measure close to 3.145"?

Second the end gap on the old compression rings is HUGE. Like 0.030" or bigger huge. And, yes, I used the piston to push it down a little to make sure it was square. Isn't that way too much? The manual says the end gap should be 0,008". What size piston rings should I get? I guess I could try the 0.020 oversized and hope the end gap can be ground to fit with a Dremel.

Third, what is a good source for rings? I found them at JC Whitney. And the gasket set is available from OReillys.

 

[jnissen]

How did you measure the bore? A dedicated bore gauge or snap gauges you transfer to micrometers? The second method takes patience to get right and you need to measure side to side and not front to back. The piston will rock in the bore and will wear slightly oblong as a result.

Are you sure those numbers were not crank bearing values? The rebuild tag on ours had .040 for the cylinders and .020 on the crank and rods.

 

[Blood_of_Tyrants]

It has an arrow pointing to the front and 0.020 on the top of each piston. I measured the pistons and the bore across several diameters came out 3.125. I used a dial caliper. I know that isn't the best way to measure, but I'm pretty sure it's not off by 0.020.

 

[Blood_of_Tyrants]

The connecting rods have STD stamped on the back.

 

[Blood_of_Tyrants]

PS, The rings that came out had an end gap of 0.11" and the oil rings would fall out of the cylinder. It all adds up to zero or very little compression. And if it started, it would smoke like a freight train.

 

[GopherHill]

I suggest a set of 0.020 rings. You can try them and file the end gaps to suit. If they are way too large, sell them to someone. Not a large investment.

 

[Blood_of_Tyrants]

The did the calculations and 0.030 should give me a difference of 0.094" . That should take up most of the end gap.

 

[Blood_of_Tyrants]

I think someone put standard rings on an oversize piston.

 

[frank8003]

page 88 in here

Engine Professional magazine

look in here

Tips And Tricks For Filing Piston Rings With Real Street Performance

Filing piston rings can be a deceptively simple task. There are a lot of details which have to be addressed in order to get the most from your rings.

www.enginelabs.com

and here

A Basic Primer on Fitting Piston Rings

We give you a basic primer on fitting piston rings, helping you custom size the end gaps to minimize pressure while enhancing your power.

www.hotrod.com

here too

Engine Tech: Understanding Piston Ring Tension - OneDirt - The Dirt Track Magazine

Selecting piston rings based on tension properties can get a little tricky. Low tension improves horsepower but higher tensions keep the engine healthier. Experts at the School for Automotive Machinists (SAM) and Total Seal offers tips and suggestions for selecting the right ring package.

www.onedirt.com

here

Tech Tip: Piston Ring Installation Tips

File fit ring sets require filing of the top and 2nd rings to achieve the correct end gap. To properly measure the ring gap, the ring should be square in the bore one inch down from the deck.

www.import-car.com

Manual Piston Ring Filer | PRF-250

End gap rings squarely with this manual filer. Can also be used to butt grind valve stems. 120 Grit Silicon-Carbide abrasive included.

goodson.com

 

[cucvmule]

Rings wear and the gaps get larger. They may have been in spec and maybe someone fitted different parts to make it work. The pistons would not be the same measurement as the bore.

Find the proper engine manual for all the engine specifications. That will give you the correct ring gaps for all rings. 1st ring 2nd ring and oil control rings. Too tight of gap is bad as well as to wide.

Most ring packs unless file to fit only requires checking for verification. I would also replace rings while you are there.

Hastings makes good quality cast iron rings.

 

[cucvmule]

Chevrolet engines in the 60's and 70's sometimes had between 1 and 7 different pistons in different sizes. This made assembly in the engine shop easier as the final bore and honing tools ware.

Then when machine shops would get the engine they would "square up" on rebuilds as they are working on one engine and doing a "better" fit of machined tolerances, Blueprinting.

 

[jnissen]

I’m still struggling with your bore measurements. If pistons are 20 thou over your measurements must be off. I’d start there before moving on.

 

[Blood_of_Tyrants]

I ordered 0.030 oversize and the end gap was an interference fit. I filed the end gap to 0.014" as the manufacturer's instructions was to have an end gap og 0.004" per inch of diameter.

 

[GopherHill]

Sounds like you did well. At least that is how I'd handle it.