Flathead 21a block bore

We have a flathead 21a 1942 truck block which measures 3.1875 ,3 & 3/16 or close to. Is this standard bore or is 3 & 1/16 std for these or has it been previously bored out to 3&3/16 STD? It has the raised inlet face. The reconditioning plate says STD 3.1875 bore dated 1955.
We want to take it to 3 & 3/16 + .060 over.

 

1941-42 239 block, it sounds like, not a 21A

 

Truck and Merc blocks had the larger bore those years. You should be fine with 60 over. I have a '41 Merc block sitting in my garage and its cylinder walls are THICK. They're not just an bored out 221, the cylinders are cast thicker from the factory.

 

If the bottom center coolant hole is keystone shaped and the middle one is round, you have a 3 3/16 Merc block

 

Its at a buddys shed , but I think the water holes were from top to bottom triangle ,round ,t****zoid. Obviously we want it to be a 239 motor. But need to be sure B4 we start. As have previously sonic tested a block that was 3 & 3/16 STD but only had 140 thou cyl walls so must have been a small motor bored to 239 .

 

221 has 2 t****zoids, visibly verydamnthick walls
Look in through the keystone...wall thickness visible on a prewar 239 is startling.

 

I don't want to diverge too much from the OP original question but I got to ask, will these pre-war 239 blocks take a bigger bore than the post-war ones? I've heard conflicting stories regarding this.

I've saved my merc block for my plans of building a monster flathead someday, but I'm no longer sure if it’s any better than a 59 series block.

 

They look thicker there, and I believe the 99's were the engine of choice for killer dragster and Bonneville engines...but that may be deceptive. I think JWL reported that he had sawn up multiple blocks and all 239's were actually about the same.
On my '39 Merc block, I was actually able to caliper the wall of a cylinder where it approached the t****zoid...but of couse I have NO IDEA if thickness there is similar to thickness anywhere else up-down-around, and MANY engines of all brands have an cored-in area of thinness and weakness at very top or bottom of cylinders due to core pattern where the vertical meets the horizontal.
The spot I could measure semi-accurately was ridiculously thick...if it were like that all over, that cylinder could have gone to 3 1/2" without the trace of a worry.
The general lack of flatheads bored that far suggests that the thickness is not uniform!
And of couse going to the outer limits on anything that has had 70 years of direct exposure to water is seriously risky no matter what was in there in 1939.