The Ford Barn

goshencars · 10-20-2025, 04:57 PM

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

41Fordor · 10-20-2025, 05:26 PM

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.

rotorwrench · 10-20-2025, 05:27 PM

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.

Brian · 10-20-2025, 06:36 PM

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.

34fordy · 10-20-2025, 06:44 PM

Quote:

Originally Posted by Brian

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.

Would this be a safe bet for any 37-38 21 stud blocks?

Kube · 10-20-2025, 06:48 PM

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Quote:

Originally Posted by Brian

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.

Brain, I too have done this with great results. However, I would consider myself lucky.

Ideucev8 · 10-20-2025, 06:49 PM

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.

UpNorthGreg · 10-20-2025, 09:45 PM

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".

Bored&Stroked · 10-21-2025, 07:28 AM

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.

goshencars · 10-21-2025, 06:37 PM

Are the wrist pins the same diameter? What is that diameter? Thanks to all

Ken/Alabama · 10-21-2025, 08:31 PM

I’ve done it many times .

deuce_roadster · 10-21-2025, 08:47 PM

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.

37 truck · 10-22-2025, 09:31 AM

Did a 1940 221 that I bored to use 20 over 239 standard pistons from Egge. Worked out great.

Bored&Stroked · 10-22-2025, 10:17 AM

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).

10-20-2025, 04:57 PM	#1
goshencars Member Join Date: Jun 2010 Location: Ohio Posts: 95	239 pistons in a 221 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

10-20-2025, 05:26 PM	#2
41Fordor Senior Member Join Date: May 2010 Location: Cheyenne WY Posts: 201	Re: 239 pistons in a 221 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.

10-20-2025, 05:27 PM	#3
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 18,006	Re: 239 pistons in a 221 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.

10-20-2025, 06:36 PM	#4
Brian Senior Member Join Date: May 2010 Location: Masterton, New Zealand Posts: 4,096	Re: 239 pistons in a 221 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. __________________ Unfortunately, two half wits don't make a whole wit!

10-20-2025, 06:49 PM	#7
Ideucev8 Member Join Date: Jan 2018 Location: Kaikoura New Zealand Posts: 68	Re: 239 pistons in a 221 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.

10-20-2025, 09:45 PM	#8
UpNorthGreg Junior Member Join Date: Sep 2025 Location: Northern Minnesota Posts: 13	Re: 239 pistons in a 221 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".

10-21-2025, 07:28 AM	#9
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,906	Re: 239 pistons in a 221 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.

10-21-2025, 06:37 PM	#10
goshencars Member Join Date: Jun 2010 Location: Ohio Posts: 95	Re: 239 pistons in a 221 Are the wrist pins the same diameter? What is that diameter? Thanks to all Last edited by goshencars; 10-21-2025 at 06:55 PM.

10-21-2025, 08:31 PM	#11
Ken/Alabama Senior Member Join Date: May 2010 Location: Alabama Posts: 3,455	Re: 239 pistons in a 221 I’ve done it many times .

10-21-2025, 08:47 PM	#12
deuce_roadster Senior Member Join Date: Oct 2011 Location: Shelton, WA Posts: 4,043	Re: 239 pistons in a 221 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.

10-22-2025, 09:31 AM	#13
37 truck Senior Member Join Date: Jul 2011 Location: Omak, Washington Posts: 277	Re: 239 pistons in a 221 Did a 1940 221 that I bored to use 20 over 239 standard pistons from Egge. Worked out great.

10-22-2025, 10:17 AM	#14
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,906	Re: 239 pistons in a 221 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).