My friend told me that the 400 was the same block as the 350 is this true or is my friend a dumb a**

 

Your friend is wrong. They are not the same block.

 

That is true for a Pontiac motor though, right? I heard that basically Pontiac used the same block for most of their motors and that's why the Pont. 350 weights more then a SBC 350.

Someone fill me in

 

Yep, the 350 and 400 pontiac blocks only differ in bore size. I know cause i swapped my 350 out for a 400 and changed NOTHING.... just the block.

 

What's going on here?!

Are the 350 and 400 SBC the same block --- well "yes" and "no".

NO, because:
- the bore of the 400 is 4.125" (4 - 1/8"), while the bore of the 350 is 4",
- the 400 block is clearanced for the 3.75" (3 - 3/4") crank, while the 350 block must be machined for a 3.75" crank (it's only clearanced for a 3.48" crank), and
- the main crank journals are larger on a 400 (2.65"? vs 2.45"?) although the 350 can be machined to accept the 400 crank anyways.

However, they ARE the same because:
- the "block" is the same physical shape and size, with mounts in the same place so one will fit exactly where the other fit (true for 265's, 267's, 283's, 302's, 305's, 307's, 327's, 350's, & 400's).
- 350 & 400 heads can be interchanged with the exception of the extra steam holes in the 400 heads to alleviate overheating problems they have because of the extra large bore (and subsequent cooling problems because of thin cylinder walls).
- water pumps, pulleys, intakes, headers, cams, etc., etc., etc., are all identical and can be interchanged as well.

So, are they exactly the same?? Well, NO, but they're damned close (a 400 is pretty much a bored out 350, with a few subtle differences).

 

There is more of a difference than just larger bore. The thinner cylinder walls are not from the block simply being bored larger. The cylinders in the 400 are "siamesed" as the 350 is not.

 

The smaller Pontiacs use a different main journal size than the bigger ones (400+).

 

The smaller pontiacs do have a smaller main journal size then the bigger pontiacs. But, the 400 pontiac is a "small" pontiac. It shares the same journal size as the 350 poncho.

 

from the outside, with the engine in the car, there is precious little to tell the two apart. some (not all) 400 blocks have three freeze plugs on each side (as opposed to two on a 350).
and of course, there's the casting number...

The one main difference in the block between a 350 and a 400 is the same as the difference between a 305 and a 350. Bore size, and the fact that you cannot bore one out to the other. A 350 block simply does not have enough cylinder wall thickness to bore out to a 400 bore size.

Now, you can swap a 400 in place of a 350 (or any other SBC) without making any other changes.

 

Let's not forget that a 400 is an externally balanced engine, while ALL other sbc's are internally balanced