239 pistons in a 221

G

goshencars

Member
I have a 1940 221 block bored .080 over. New pistons would cost way more than 239 standards. can I just bore the block to 3-3/16? will wrist pins fit the 29 A rod. Thanks
 
That's what my machinist said to do. The tin sleeves were already removed from mine and he said they usually just bored them to 239 after that.
 
Going out that far on a 221 doesn't leave enough cylinder wall. Folks have gone out to late Mercury 8CM specs with the 4-inch crank but they bored the block for sleeves and modified the crank for clearance. It can be done but it's expensive to do all that. They bored through the cylinder walls in places to get the sleeves in then used epoxy to seal it up and stabilize the decks. I always wondered how close the head studs were to the edge of the cylinder bores.

This was why they had to redesign the blocks for the 1939 99A engines. It was to get better head sealing and allow for much larger bores.
 
Possibly....it would pay to sonic test the cylinder wall thicknesses and do the maths. i have seen 81A blocks successfully run with 3 3/16 bore, I am currently running a 37-38 21 stud block with 3 3/16 bore.
 
Brian said:
Possibly....it would pay to sonic test the cylinder wall thicknesses and do the maths. i have seen 81A blocks successfully run with 3 3/16 bore, I am currently running a 37-38 21 stud block with 3 3/16 bore.
Click to expand...

Would this be a safe bet for any 37-38 21 stud blocks?
 
Brian said:
Possibly....it would pay to sonic test the cylinder wall thicknesses and do the maths. i have seen 81A blocks successfully run with 3 3/16 bore, I am currently running a 37-38 21 stud block with 3 3/16 bore.
Click to expand...

Brain, I too have done this with great results. However, I would consider myself lucky.
 
As Brian say's sonic test the cylinder wall thicknesses. I've been running a 81A bored 3 3/16" +.020 for over 20 years
Phil.
 
I'm in the same boat. Have a '41 221 that has had the sleeves removed and came with new .0825" over pistons and rings. Unfortunately, the block sat for 40 years and now a few of the cylinders have rust that will require boring. Would be nice to take it out to 3-3/16".
 
Have the block cleaned and sonic tested, then you'll now. Heck, we bore out 239 blocks .125 all the time . . . but I also sonic test them first.
 
As others here I have done it too. My late machinist (also rebuilt certified aircraft engines, was an A&P) would only bore to leaving a MINIMUM .100 wall thickness. You need to measure for core shift and cylinder taper to know if you will have enough to do that. I highly recommend a pressure test because there can be cracks where you cannot see them.
 
I'd like to see more than .100 wall thickness . . . I try to achieve at least .120.

Also, if the block is heavily rusted in the water jackets, then you may have a rust "pit" that eats into the cylinder walls in places that you have not checked with a sonic tester.

I measure each bore in 12 places - four at 3, 6, 9 and 12 o-clock at three different bore depths. 1" down, 3" down and 5" or so down the bores.

I try to stay away from heavily rusted blocks - if I want to bore them much.

Lastly, it is not a bad idea to pressure test the block both BEFORE and AFTER the initial bore work. Might save you thousands of $$$ if you end up finding issues before you continue to invest in it. This is especially true if you're going through the time and expense associated with heavy porting/relief and other "go fast" performance work! (and all the parts you may be purchasing - like forged pistons).
 
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