[Bored&Stroked]

Originally Posted by AZ Chris

Rather than start a new thread, I figured this would be a better place.

Recently got this block and trying to ID it. From everything I can find it appears to be a 239 from a 39-41 (closer to 41?) truck based on the stamp on the left rear of the block 99T-638628 and the lower water passage.

From what I can gather this is a good block but one question is whether parts for a "59" block will fit properly.

That block is usually a 41-42 Merc type block - or a block that was used in WWII for a variety of uses. There is a bunch of flathead lore about this 'keystone' block being thicker and the cat's ass for a big bore stroker motor. Truth be told, they are not really any thicker or better than other years - just another perpetuated myth. I just built this same block/engine for my 32 Cabriolet - only because I wanted a pre-war block and this is the best one. Mine was essentially a NOS block - came in a create from WWII - STD bore, had just been broken (the engine) then not ran/used.

I have my own sonic tester and have tested a lot of different blocks - can find no consistent correlation between block year/type/nomenclature and being thicker than others.

As some noted, it should have the earlier valve angles (32-42) - but they are only slightly different from the 46-53 engines. There is some debate whether or not the 39-42 Mercuries used the LATE valve angle or the early one - I've never been able to confirm this. (To move the valves further away from the bigger 3 3/16 bore).

Valve angle: 32-42 = 101.45 degrees, 46-53 = 100.026 degrees


It is always wise to check for valve to head clearances - especially if you're running the early (small chamber) heads and/or an unknown cam. You can do this with a gasket and some modeling clay - you'll want .060+ minimum over the valves at the top of the pockets in the heads (which is where they would hit if they are too close).