Small Block 400 Question

Question

Can you use both a 400 head, and a 350 head on a Small Block 400.

One of the 400 heads was cracked and the machine shop said we could use a 350 head on that side. Can I?

Also will it mess my performance up on it.

Answer 1

The compression is likely to be different. This would cause a power imbalance and the engine would seem to run rough. The 350 heads have different cooling passages, there are minor differences, it will bolt on but performance and cooling will be effected.

Answer 2

350 heads have more "meat" between the bores than 400 heads, so I don't recommend that solution. They just don't fit right. Look for another 400 head (they are out there) or but a set of TrickFlow heads. A set of 350 heads could be modified to the 400 bore size (4.125) but the cost would be high.

Answer 3

It will fit, but you'll be asking for trouble.  With all the different types of heads GM made (valve sizes, chamber sizes, ex..) you could end up with to uneven compression and/or power on each side of the motor.  Then lets not forget there are differences in the water passages that need to be addressed.  If the machinist doesn't do it right you could end up with a cracked block.  I've seen "reputable" machinist s***w simple things like this up.  Good luck.