Ok:
I have been looking for a 400 engine for my truck.
I'm talking to a guy that says he has a 1977 351m 400?
Is this a 400 engine?

He says it is a 351m w/ 400 heads,etc.

I've never really undrestood the difference between the two.

Is there an easy way to tell them apart? If it is a 400?
Are there casting #s somewhere that identify an engine as a 400?

Thanks all. Eric

 

No, the difference between a 400 and a 351M are the crankshaft stroke and the pistons. You have to remove the oil pan to check the crankshaft #s to tell. You can also tell by measuring the stroke through the sparkplug hole. The 351M has a stroke of 3.5", while a 400's stroke is 4.0".

 

There is no way to tell them apart without measuring the stroke, or getting the crankshaft casting number. From the outside they are identical, including the heads.