How big can a 390 be bored to?

[briankmagby]

I was just curious as to how big a 390 can safely be bored to? How many thousandth’s of an inch? Can it be bored to a 406 or bigger?

[ted]

How much you can bore a 390 block will depend largely upon what year block you're dealing with. Typically, the newer the block, the thinner the cylinder walls. Some of the earlier blocks could actually get away with 0.080 over and make a standard 406 or 428 block but I've seen the later blocks actually hit water by way of casting flaws or pin holes in the cylinders at 0.030 over.

I would recommend getting the block sonic checked for both cylinder wall integrity and core shift as then you'd know for sure. Knowing which direction the core shift resides is invaluable when trying to bore one of these engines to the max as the better machine shops can offset bore the cylinder into the thicker portion of the shift and subsequently recenter the bore so the cylinder walls are more equal in all directions.

[briankmagby]

Thanks Ted, to be safe, I will probably just go with a minimum necessary boring to clean it up and get it ready.
Thanks.

[A Snake]

I agree completely with Ted. However, there are some blocks which can often furnish good raw material for that .080" overbore. Look for blocks with ribs cast into the side. They also show up as 360 CID pickup engines.

If you want cubes and still play it safe, find a 410/428 crankshaft for your 390 block. The only thing that really changes is the wrist pin placement in the piston. You and your builder MUST specify 410 CID (came in Mercurys)+overbore.

Al

[Michael C Henry]

A 390 is 4.050" .075" over makes it 4.125" that is stock fot 406 and 428. I bought a 406 in 68 and rebuilt it with a.040" over bore and new pistons.I have had 428+.040" same pistons in two different blocks. Weak walls and ring failure were never a problem.

[briankmagby]

Thanks Michael.

[stengun]

Howdy,

Originally posted by Michael C Henry

A 390 is 4.050" .075" over makes it 4.125" that is stock fot 406 and 428.

Standard bore on a 406/428 is 4.130" not 4.125". I've seen alot of 65-70 390 blocks go 4.13" and some go 4.15". I even seen a couple of 428-4v blocks go 4.223", which is std 427 bore. I saw 1 of these yesterday at the machine shop. Pretty neat. The machine says he has never had any problems boring a 428 block to std 427 size.

Paul