39 Merc flsthead

F

flathead48

Well-known member
I am stripping a 39 Mercury flathead ( at least I was told that, TT heads ) my question , are these blocks as good as the 59 series & worth the work to strip & rebuilt if not cracked ?

Thanks, Rick
 
Both of them have the same displacement and good blocks. The 39 Merc may be factory relieved which reduces compression by 0.39. The 59A valve angle was also changed for post-war blocks, with the valves moved approximately 0.090” further from the bore to allow more cooling water around the valve seats.

Further details are shown on pages 307 and 318 within the Techno document linked below.
 
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In the "old days" folks claimed that the early 39 - 42 Merc blocks and the various WWII variants were the best blocks to Hotrod. We always called these "keystone" blocks - given the style of the three center water ports on the deck.

The Ford blocks were 221 cubic inches and had different water passages (the center one was a trapezoid, not round like the Mercs). While there are many myths associated with these blocks, I've not seen any reason to believe they are better than a 59x style block.

As noted above, they moved the valves in the 59x series and potentially this causes fewer cracking issues in the transfer area (better cooling in this specific area).

Bottom line - if you have ANY good 239 cubic inch block from 39 - 48, you're in great shape for a good flathead build!

Side Note: The 39 - 42 blocks that I've had that were factory relieved usually came from trucks, commercial or wartime applications. But with Henry Ford - anything is possible!
 
It has been my understanding by a number of reports that the early 239 blocks had a generous amount of cast iron in the cylinder walls making them thicker than later 239 engines. Somewhere in late 1940 or 1941 Ford and other motor companies started to conserve on raw materials to include iron products. This is evident in the 1941 engine block design change that reduced the intake deck and did away with the pan rail casting sand clean out ports but I don't have enough info to get any definitive proof of a wall thickness change. I have heard folks mention that those later 239 blocks don't have any thicker walls than the post war engines.
 
They had 99T heads available around mid to late 1939. The 81T heads were used to start out. The 239 truck engine went through changes just like the Mercury car engine. In 1939, Ford used stuff that was available at the time until more appropriate parts were available.

Trucks had the compression reduced to prevent warranty problems due to the loads they had to carry. Ford didn't want to chance problems with detonation so they relieved the blocks and used reduced compression heads.
 
rotorwrench said:
They had 99T heads available around mid to late 1939. The 81T heads were used to start out. The 239 truck engine went through changes just like the Mercury car engine. In 1939, Ford used stuff that was available at the time until more appropriate parts were available.

Trucks had the compression reduced to prevent warranty problems due to the loads they had to carry. Ford didn't want to chance problems with detonation so they relieved the blocks and used reduced compression heads.[/QUOTE

Thank you
Click to expand...
 
I believe another factor for the relief was. To lower the chance of cracking between valve seats and bore on big stuff. Sure I read in Ford paperwork back along.
Martin.
 
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