[bluegoat65]

Looking for opinions on max safe overbore. I have 74 400 block cast number 481988. Just picked it up from the shop after cleaning, magging and an sonic check. My goal is to go 4.250 bore if possible. Mind you I plan to full fill the block with Embecco 885 and just cool the heads. The thrust and non thrust sides of the cylinder on average are at .240 wall thickness. The front and backs of the cylinders average about .150.. The shop that did the work claims I should have no worries with the front to back dimensions coming down to about .085 with the block fully filled and being strictly an race motor. Just hoping you all can help make me feel warm fuzzy with there response.

[grnmnstr]

I wouldn't completely fill the block. A little water around the cylinders is good.

[Jack Ferris]

I've been using a 4.25 bore for many years, but on a 455 block. Filled about 1" from the deck. Not sure about boring a 400 block to 4.25.

[bluegoat65]

Jerry did you have that block sonic checked or did you just wing it on a prayer you wouldn t encounter any problems.

[455Grandville]

Originally Posted by bluegoat65

Jerry did you have that block sonic checked or did you just wing it on a prayer you wouldn t encounter any problems.

He mentioned .240 thickness on the non thrust and thrust sides.

x 2 for what you're saying, though - various block thickness with core shift and internal

corrosion.

[WARPed]

What are your plans for this build? NA, NOx or boost? going the whole nine yards like adding 4 bolt caps, lifter brace and the like? If so strongly consider an aftermarket block cuz when you are all said and done doing a factory block you will be about 2/3 to 3/4 away from an IA-II/KRE block that will probably out last you. Sadly that is rarely the case with factory blocks.
JMO