I'm getting a brand new 400 crank and was wondering if all I had to do was to get it machined to fit? Would my pistons hit the head or could I use them with the rods I already have. My 350 is pretty much stock right now and the mods listed below. Would I be better off to stay where I'm at or should I go for the 400 crank option. I'm getting the crank for free, but if it's not worth it I'll just tell him to keep it. Thanks.

Brad...

 

If you put that in there with the rods & pistons you now have, the pistons will come out of the block just about 1/8".

The block is 9.025" tall from crank centerline to deck surface, give or take a few .001"s. The sum of: ½ of the stroke, plus the rod length, plus the "compression height" of the piston from the centerline of the pin to the top surface, plus the deck clearance of .025" nominal, must equal 9.025". If it's less, the pistons will be too far down in the bore at TDC; if it's greater, they'll come out the top of the block.

So: for a 350, that's ½ of 3.48", which is 1.74"; plus 5.7"; plus the "compression height", which is 1.5625"; plus .025", which gives you the magic number. For a 400, the stroke increases to 3.75", so ½ of that is 1.875"; the compression height and deck clearance are the same; so the stock 400 rod is shortened to 5.565".

You will have to change the rods (to stock 400 rods), the pistons (to custom nut readily available ones), or both (to something even more custom).

In addition, the 400 crank's main journals are larger than the 350 block's bearing bores. You have to have the mains turned down.

Make sense?

 

The journals have to go from 2.45 inches to 2.60 to acomidate a 400's mains.

Also, a 400 crank is EXTERNALLY BALANCED!!! I cannot stress this enough. This means that i must be custom balanced at a machine shop to the rods and pistons, with the harmonic balancer and the flywheel! This is such a common mistake, but so crucial! This is as opposed to the 350's internally balanced crank.

 

Right, the 400 main journals are bigger than a 350's, but you don't (usually) align-bore the block to the 400 journal size, instead you turn the 400 crank down to the 350 size. You could do it the other way I suppose, but it isn't the common practice.

Green is right about balance; in my 400 I simply took a stock 83-85 305 HO (lightweight) flywheel to the balance shop and had them "unbalance" it to the stock 400 balance spec.

 

Well after reading all this, I think I'll pass on the 400 crank idea. I need to get my car back together soon. I guess if I want a 383 I'll just buy the kit for it. Thanks for the help.

Brad...