[voodoohalfbreed]

I recently took my 421 block to the machine shop, was told I need bored .030 over. In searching for 421 pistons, I can buy cheap overseas junk for about 400 bucks. I called butler, about 900 for custom made pistons. I believe the 421 bore at .030 over is 4.123, I found a set of good 389 pistons .060 over, which is 4.122. The piston pin for 421 is 3.25 long, for 389 is 3 long, i assume length is based on piston. With proper valve reliefs in pistons would the 389 pistons work??? Not sure of any issues, balancing issues etc???
Thanks
Scott

[tom s]

389s wont work with a stock rod.Check with Paul K on this site and see what a set of Autotec custom would cost.I have bought the last 3 engine pistons from him.Tom

[voodoohalfbreed]

Originally Posted by tom s

389s wont work with a stock rod.Check with Paul K on this site and see what a set of Autotec custom would cost.I have bought the last 3 engine pistons from him.Tom

What is different?

[tom s]

The wrist pin location.You would need a diff length rod.Tom

[voodoohalfbreed]

Originally Posted by tom s

The wrist pin location.You would need a diff length rod.Tom

I thought the rods were the same from 65-79???
I have stock 421 rods, crank etc...

[b-man]

The rods are all the same.

Your problem is the 389 stroke is shorter than the 421 stroke, 3.75” vs 4.00”.

Hence the difference in piston pin location between 389 and 421.

So 389 pistons won’t work unless you use a shorter rod.

[voodoohalfbreed]

Originally Posted by b-man

The rods are all the same.

Your problem is the 389 stroke is shorter than the 421 stroke, 3.75” vs 4.00”.

Hence the difference in piston pin location between 389 and 421.

The crank determine the stroke, or possibly the height of the piston, the pin location would not change???

[voodoohalfbreed]

Originally Posted by b-man

The rods are all the same.

Your problem is the 389 stroke is shorter than the 421 stroke, 3.75” vs 4.00”.

Hence the difference in piston pin location between 389 and 421.

So 389 pistons won’t work unless you use a shorter rod.

Are you saying the height of the pistons are different or that the pin is in a different position between the 2???

[b-man]

The compression height (pin location in reference to the piston top) is different between the two engines.

389 pistons won’t work, they’ll end up popping above the block deck by roughly 1/8”.

[voodoohalfbreed]

Originally Posted by b-man

The compression height (pin location in reference to the piston top) is different between the two engines.

389 pistons won’t work, they’ll end up popping above the block deck by roughly 1/8”.

Thanks, that's what I wanted to know!!!

[voodoohalfbreed]

What about 428 pistons??? The stroke is the same. Are the valve reliefs in different locations?

[b-man]

Yes, you are correct. Same stroke, reliefs in different locations.

With a low-lift factory performance cam it might not matter, but always check for clearance.

You pretty much need to resign yourself to throwing down for a set of custom pistons when building a 421. I did exactly that when I built my 421 HO.

[voodoohalfbreed]

Currently I have a stock 428 engine in my 2+2. It has the 093 heads and tri-power and does just fine. I am not sure what the stock 428 cam is, might be the same as 66 421 ,I dont know, but it would seem I could use 428 pistons and cam and only thing that would be changed is the bore.. opinions???
Thanks
Scott