[Lynn Eades]

Thank you Michael R, for educating me on carrier engine numbers.
Malcolm, I am trying to get to the bottom of your low compressions (in the apparently good cylinders)

Ford did 3 different piston tops:
the flat top piston is for the early 21 stud engines.
There are two left:
The first is a sperical top. The profile when looked at from the side is a gentle curve going up one side over the center and back down.
The other is what Ford called a "Combination" When observed from the side it appears that the climb is a constant flat, coming to a point at the top and straight down the other side.
These pistons are not interchangeable.
The sperical one Part no 78C-6199-A
There are a couple of combination pistons.
If you look inside the piston skirt and you should find a number like 01T-6110-A

Cylinder heads. Some types fit both sides but mostly there is a left (6050) and a right (6049)
They may be hard to see, but as far as I know, they are all identified with part numbers
Those numbers above are the base number. All have prefixes and some have suffixes.
Your engine could/ should be wearing heads with:
81A-6049-A (or 6050)
81T-6049
C91A-60xx- B1

On compression ratios:
At sea level a comp. ratio of:
5.4:1 = 99psi
5.6:1 = 102psi
5.8:1 = 105psi
6.0:1 = 108psi
6.2:1 = 111psi
6.4:1 = 114psi
6.6:1 = 117psi

The above is all drawn from Ford service bulletins.
The whole point of my post is so that you can check that the combination of pistons and cylinder heads is a match.
In 42 the valve line changed. (design) Early heads could be relieved to fit the later blocks. If these heads were fitted to an early engine, they would have a lower than std. C.R.
Food for thought.