how do i tell if a engine is a 400 or a 351?

 

the crankshaft number

 

you cant tell just by looking at it

 

no, they are the same except for pistons, crankshaft and balancer. well the balancer is thinner on a 351m. i think its like 1.5-2.5 inches on the 400. the only real way is the engine code if you know its the stock engine or the part num on the crankshaft.

if you have a 351m and want to stroke it out to a 400, just change the 3 parts and bang, its a 400.

 

 

i always just grabbed a pistons and measured the stroke 400=4''

351=3.5''

 

Get help! Have someone turn the engine over hand with a socket on the crank while you use a lo---ng screwdriver and measure the distance the piston goes down from TDC. 3.5" = 351 4.0" = 400.

 

[updated:LAST EDITED ON 04-Sep-02 AT 02:35 AM (EST)]if it's all stock and it will spin the tires, then it's a 400...

 

>how do i tell if a engine is a 400 or a 351?

The VIN number should be stamped on the back of the block, I believe above the starter. The fifth position in the VIN should be a letter that corresponds to the engine. I don't have the late 1970's codes, but an "S" is likely a 400. An "H" is likely a 351, but you can't tell whether it was a 351M or 351W since they used that code for both. I presume you are talking 1975 or later.

If you cannot find the VIN on the block, and you believe the engine is original, then go by the VIN on the vehicle. Also keep in mind that even if the VIN on an engine indicates 351M, someone could have "upgraded" it to a 400...