I'm putting a cam in my 1965 F250 and I was cleaning the tops of the pistons when I saw that they were stamped "40". I measured the bore and it is 4.040 inches. The stroke is still 3.50 inches. What does this make my engine now? I wish my dad would let me take it out and bore and stroke it more to make it a 390, but he won't

 

You only have to stroke it to get a 390.

To figure out displacement, it just takes simple math.

Take the radius of the bore, square it and multiply it by pi (~3.14). Take the product and multiply it by the stroke and then the number of cylinders.

 

The factory 360 cubic inch truck engine which came out in 1968 used the 390's bore (4.05) and the 352's stroke (3.50). So your engine is just under 360 cubic inches.

 

358.93 to be exact. (can't be any more exact due to significant digits)

 

a .040 overbore is fairly common to find in the older engines . just about every engine I have had reconditioned for Mustangs needed boring to .040 over stock to clean up the taper and out of round in the cylinders .

 

There is nothing wrong with an .040 overbore. It is my belief that the 352 of the era had the same cylinder castings as a 390, 410 but were "underbored". The real problem with the design of your 352 (and mine) is the compression ratio. My solution will be a 428/410 crank and rods and 410 pistons. The CR will be higher but the motor will still be cheaper to run, even on 93 gas. And it will have an extra bunch of juice!
Eric

 

The 300 horse 390's from the factory had 10.1:1 compression and they loved 100+ leaded fuel. Others had up to 11:1. and you could pump them up to 13:1 with domed pistons.
Can't you bore them 80+ over and still have some meat left? There is all kind of ways to build an FE.

 

I agree the 40 is not a Ford number but rather a over bore size.


John

 

352: 4.00" bore, 3.50" stroke.

360: 4.05" bore, 3.50" stroke.

390: 4.05" bore, 3.78" stroke.