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Tim Ayers · 05-11-2021, 07:10 PM

A picture would help, but is the top water hole is a like a squat triangle and the lower is a trapezoid and the bore is for sure 3 3/16", sounds like you have hit the flathead lottery with 99A Mercury blocks. Rumor is they are the thickest of all blocks cast.

Another home test is what's called the pencil test. If a pencil rests on the "ledge" above the cam cavity and where the distributor sits, then it's a safe bet with all of the other indicators that they are 99A blocks. I have a 59L block that passes this test, so it's not fool proof.

If so, then yes, they have the early 101.5 degrees. I'll leave it to others to say if it will make a difference. The 77B is a super tame cam. Some say worse then stock actually.

I'll let the cam experts chime in about that one, but if these blocks are crack free, you did well.
For some reason, these blocks like to crack at one of the center intake valve seats into the bores. Look them over carefully. If pass a visual inspection, have the cleaned, mag'd and pressure tested to be 100% sure they are good.

By chance, does either block have what's referred as a raised intake deck? Meaning the sides where the intake side are lower.

05-11-2021, 07:10 PM	#2
Tim Ayers Senior Member Join Date: May 2010 Location: NJ Posts: 7,228	Re: Pre 1945 Block Cams A picture would help, but is the top water hole is a like a squat triangle and the lower is a trapezoid and the bore is for sure 3 3/16", sounds like you have hit the flathead lottery with 99A Mercury blocks. Rumor is they are the thickest of all blocks cast. Another home test is what's called the pencil test. If a pencil rests on the "ledge" above the cam cavity and where the distributor sits, then it's a safe bet with all of the other indicators that they are 99A blocks. I have a 59L block that passes this test, so it's not fool proof. If so, then yes, they have the early 101.5 degrees. I'll leave it to others to say if it will make a difference. The 77B is a super tame cam. Some say worse then stock actually. I'll let the cam experts chime in about that one, but if these blocks are crack free, you did well. For some reason, these blocks like to crack at one of the center intake valve seats into the bores. Look them over carefully. If pass a visual inspection, have the cleaned, mag'd and pressure tested to be 100% sure they are good. By chance, does either block have what's referred as a raised intake deck? Meaning the sides where the intake side are lower.