How to Tell the Difference Between a 350 Chevy Motor and a 400 Motor

by Don Bowman
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A small block 350 and a 400 have an identical block design. Most accessories will fit either engine. The major difference is found in the casting numbers and the way they are balanced. Most 350 cubic inch engines are internally balanced whereas the 400 engines are not. The 400 cubic inch engine is externally balanced with the flywheel--or flexplate in the case of an automatic. The Chevy 350 engine will turn at a much higher RPM than its counterpart. The 400 cubic inch will turn a maximum of 6,500 RPM.

Step 1

Inspect the casting numbers on the engine block. The Chevy 400 cubic inch engine has only three distinct numbers. These numbers are 330817, 3951509 and 3951511. The 1511 casting numbers were mostly four-bolt mains. The interesting point is that the 0817 and 1509 two-bolt mains are the most popular. The reasoning behind this is that the two-bolt mains have much more nickel in the webbing, which causes the block to be much stronger than the four-bolt main.

Step 2

Check the flywheel or flexplate. The 400 has a staggered bolt pattern and is externally weighted. Large weights can be seen on the plate between the engine and plate. The plate will have 168 teeth on all the 400 engines, requiring a starter made for this engine.

Step 3

Check the bore on the engine if the heads are off. The Chevy 350 cubic inch engine has a 4-inch bore whereas a 400 cubic inch engine has a 4.125-inch bore. The 350 has a 3.48-inch stroke, and the 400 has a 3.75-inch stroke. The 350 rods are 5.7 inches whereas the 400 has 5.565 inch rods. This is how an engine is stroked—by lengthening the crankshaft stroke and shortening the rod by the same amount.

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