How to tell a 350 block from a 400?

I heard they have a different number of freeze plugs. Anyoneknowfor sure? Also, does anyone know if there's a weight difference? Thanks.

No weigh difference you can easily tell. A lot of 400's have 3 freeze plugs on the side vs 2 for 350's, but not all have the 3. I have 2 #509 400 blocks and both are missing the middle plug. But they do have kind of a raised area where the plug would be. 400's do have bulges on the block sides for added room where the water can flow past the outside of the larger cylinders in the water jacket.

Best thing to do if you want to make sure you are looking at a 400 block is to go to mortec.com and look at the small block casting #'s for a 400.

pull the heads and look for the steam holes in between the bores, you cant go wrong, if it has the two steam holes its a 400

Also the balancer has some material removed from the inside of it. It is visible if you turn the balancer just right.

there were some 4 bolt main 400 blocks also built from 1970-1972 only.

Thanks for the info guys. It is a bare block so I couldn't go by the balancer. It seems like the best way would be the steam hole method. Are you saying that between each and every cylinder there are two holes?

I'm not interested in a 4 bolt block since they are weaker than the 2 bolt 400s. Thanks again.

Thanks for the info guys. It is a bare block so I couldn't go by the balancer. It seems like the best way would be the steam hole method. Are you saying that between each and every cylinder there are two holes?

I'm not interested in a 4 bolt block since they are weaker than the 2 bolt 400s. Thanks again.


Yup I jsut went out and looked at mine to be sure, in between each of the bores there are two steam holes about 1/4" in diameter.

Yup I jsut went out and looked at mine to be sure, in between each of the bores there are two steam holes about 1/4" in diameter.
I just looked at a few on ebay just for reference. On some it seems like it's either hard to tell or maybe on of those steam holes had been welded up. :confused: But I did see what you're talking about on a couple.

check your casting number is the best way to identify what you have, below is copied from Mor tec

block # Cubes year main bolts
3951509....400...70-71...4
3951509....400...74-80...2
3951511....400...70-73...4...Some replacement blocks had 2-bolt main caps

casting number is at the back of the block just above the transmission drivers side.