[Scarebird]

A friend of mine has a 68 'bird with 350 and ST300, 2.56 open. The original block developed freeze cracks. We replaced it with another 350 but it is worn and smokes badly. I hauled down a 400 from a 68 GTO from the Seattle area, but it has pitting in one cylinder bore that most likely will not clear up with a 60 over bore - the block is 30 over now. 60 over seems to be it for pistons for a 400; anything more is custom territory and $$$.

My friend who sold us the 400 is willing to refund if we cannot use it but I was running some numbers and came up with this. A lot of guys run 455 cranks in 400s, but what if you just run the 455 pistons and kept the 400 crank?

The difference in stroke is 0.46, which halved is 0.23". Add this to the stock Pontiac 6.625" connecting rod distance on you get 6.855". If using BBC 6.8 rods you can offset grind the crank to get to 6.805", which should not peek much above the deck and use stock overbore 455 pistons (read: much cheaper) and coupled with #96 heads should make a good road engine.

Am I missing something here? I tried the search feature - nothing but 455 cranks into 400 blocks and I do not recall anybody doing this.

[Tom Vaught]

Been done before on a 400 block with even less stroke, longer rods, and shorter compression height pistons.

3.5" Steel Forged Pontiac 366 cid crank done by Moldex. Stroke increased to 3.5" with raw forging, stock stroke was 3.375"
7.080" Carrillo rods
1.375" compression height Wiseco Pistons

What you are asking to do should be a non issue. Some say the 6.8" rod would be better vs the 7.080 rod. 7.080" Carrillo rod is better for me vs a 6.8" Eagle rod for example. It also allows a 1.385" compression height piston (light) with no ring support required.

Tom V.

[johnta1]

Use this calculator and see which setup would work.

Rod Length Calculator

Use the actual comp. height of your piston, your block deck height (or around 10.24")

Your anticipated deck clearance (piston top to deck - .020"?)

If you have to order rods anyway, you probably can get some with a length close to what you need ( like 6.855" or so)

[AG]

You can offset grind your 400 crank to 3.79", with a 455 piston with a CH of 1.500" and a 6.800" rod you get a deck height of 10.195". Should be no problem. The caveat is a CH of 1.500", a TRW forged piston is usually 1.480". You will end up paying extra somewhere, it's your decision where.

[Scarebird]

Originally Posted by AG

...You will end up paying extra somewhere, it's your decision where.

Cheapest way to do this would be to find 80 over 400 pistons - don't think they exist.

[t money]

To take a 400 over 0.60 you should sonic the cylinder wall thickness and maybe hard block it Can’t you just sleeve the bad cylinder and rebore to 0.40 or 0.60. Speed pro still have cheap pistons in that size.

[242177P]

If the remaining bores clean up OK, just one sleeve (.030) is the cheapest approach.

[steve25]

X2 with the above way to go by far!

[scott70]

Originally Posted by AG

You can offset grind your 400 crank to 3.79", with a 455 piston with a CH of 1.500" and a 6.800" rod you get a deck height of 10.195". Should be no problem. The caveat is a CH of 1.500", a TRW forged piston is usually 1.480". You will end up paying extra somewhere, it's your decision where.

I thought I remember seeing.you can get trw shelf pistons at 1.497 CH.

[AG]

The last set of TRWs I measured were 1.480", I think the older ones had a CH closer to 1.500".

[chiphead]

Sleeve the block and keep your wallet and sanity. The franken-motor looks good on paper but good luck getting all the small bits work together. 6.8 BBC rods use floater pistons, not pressed-pin stock type pistons. You're back to custom pistons anyways unless there are cheap pistons that will take the BBC .990 pin.

[steve25]

No to mention the unneeded huge Rod side clearance spraying unnecessary oil everywhere and creating piston rock which reduces ring seal at high rev's!

[Skip Fix]

I was thinking one sleeve also.

[Scarebird]

...and that is why I asked.

[hurryinhoosier62]

One sleeve is a simple, cost effective solution.