48 coupe- what pistons to use?

Bolts

Bolts

Well-known member
I have this 59AB block stripped down. It was stuck.

I don't know if the deck was machined but the pistons are .060 oversize.

I'm not planning on machining the deck but I have to buy new pistons because I ruined the ones that were in the engine when I "unstuck" it.

I have measured the deck to the crank center and calculated the distance from the wrist pin to the top of the deck.

After all this I think the top of an 8BA piston (with a compression height of 1.561) will sit about .054 below the deck at TDC.

Does this sound like it's in the ballpark of where it should be?

Thanks
 
Bolts said:
I have this 59AB block stripped down. It was stuck.

I don't know if the deck was machined but the pistons are .060 oversize.

I'm not planning on machining the deck but I have to buy new pistons because I ruined the ones that were in the engine when I "unstuck" it.

I have measured the deck to the crank center and calculated the distance from the wrist pin to the top of the deck.

After all this I think the top of an 8BA piston (with a compression height of 1.561) will sit about .054 below the deck at TDC.

Does this sound like it's in the ballpark of where it should be?

Thanks
Click to expand...

I unstuck a block marked 59 on the bell last year. Also .060 over, but it's a 221 cu in replacement block, so be careful there. You need to measure the bore. I bought 2 nos ford manufactured pistons from Bob Selzam in NJ. 732 928 9421. They came wrapped in cosmoline.
 
It seems like the different manufacturers of pistons have differences in their products. You will have to measure your cylinders to see where you need to go. If it is a 41A block (221 replacement block with 59 on the rear bell) then it should have some of the characteristics of the old 221 type blocks. A 41A will still be less in bore diameter than a standard 3.1875" 239 bore size at .060" oversize so watch out for that. Sonic testing will tell you how much cylinder wall you have to work with but a 239 can be bored out quite a ways. This test would also tell you how far you can go if it is a 41A 221 cid block. A .060" oversize 239 should be at 3.248" or better.

If you have a lot of wear on and old flathead Ford block from where it was last bored, I would recommend getting it bored to the next common oversize minimum. The old flatheads like to wear on cylinder walls with a 4-ring piston so they need some good meat to start out with at overhaul.
 
The bore measures right at 3.250 as best as I can measure using calipers. The pistons were marked .060. They were the three ring type which I haven't found yet in my searching. The crank is 3 3/4 stroke.

My question is about the compression height of the pistons.

If the piston top is below the deck about .050 would that be considered an acceptable fit?

Thanks
 
I have a 59 block in my shop I'm tearing down, the pistons measure about .079" down from the deck on the outside edges. Measured with a digital caliper, at top center by eye. I'm not saying that is correct, just what I have.
 
I don't consider myself an engine building guru, so maybe some of our members who are more knowledgeable will step in and answer your specific question on compression. However, I have built several flathead engines and usually shoot for a final assembled top of piston to cylinder head clearance of 0.040" - 0.050". So your measurement of 0.050" from top of piston, @ tdc, to top of deck surface does not sound correct to me, assuming we are talking dome top pistons. But then I also wonder how you would know you have 0.050 between top of piston and deck surface if you don't have your new pistons to make that measurement?
 
Three ring pistons are not common as replacements for the four ring types but they are out there. The forged pistons have three rings but they need more clearance and I don't know what the dome to deck dimensions would be for them. Speedway had some three ring hypereutectic types but they may not have been any good or at least they quit selling them. Ol' Ron has mentioned some pistons that he uses but I'm not sure which they are.

Mercury pistons are set up for 4-inch stroke and they would set down into the cylinder too much if accidentally used with a Ford crank.
 
Last edited:
Bolts said:
The bore measures right at 3.250 as best as I can measure using calipers. The pistons were marked .060. They were the three ring type which I haven't found yet in my searching. The crank is 3 3/4 stroke.

My question is about the compression height of the pistons.

If the piston top is below the deck about .050 would that be considered an acceptable fit?

Thanks
Click to expand...

Are you saying the piston dome is .050 low? If so, this would not be acceptable. Ideally the edge of the piston should be at or very near the block deck.
 
I would use an Egge 3 5/16 3 ring pistons, very light and you not only have 258 Ci you also have another point in CR. This is my favorite engine. With all the stock 48 components bolted to it. It should run better than a blown Hemi?
 
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