351M or 400??
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Senior User
Joined: Mar 2003
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From: up state N.Y.
351M or 400??
Bought a 79 Bronco with a reman mill,and I know it's a 335series mill,but how do I tell if it's a 5.8L or 6.6L?The OEM valve cover's say 6.6L but vin# say's 5.8L?
Cargo Master
Joined: Dec 2004
Posts: 2,232
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From: NE Texas
The VIN is only indicating which engine was originally in the vehicle. It is difficult to tell the difference in the two without disassembly or pumping a cylinder. The difference is that the 400 has a 1/2 inch longer stroke. If it is really mean then the odds are that it is a 400.
The ONLY difference between the 351M and 400 is the crankshaft and pistons. I think that the harmonic balancer from the factory is different, but they are interchangable, so even if you could identify which balancer is on the engine, it doesn't really prove anything.
One who knows what to look for can identify a 400 vs. a 351 crankshaft, but you would have to pull the pan to do this. Fortunately on the 4X4 chassis the pan comes off very easily if you want to go to that much trouble and get one of the gurus to send you some pictures of what to look for.
Good luck,
The ONLY difference between the 351M and 400 is the crankshaft and pistons. I think that the harmonic balancer from the factory is different, but they are interchangable, so even if you could identify which balancer is on the engine, it doesn't really prove anything.
One who knows what to look for can identify a 400 vs. a 351 crankshaft, but you would have to pull the pan to do this. Fortunately on the 4X4 chassis the pan comes off very easily if you want to go to that much trouble and get one of the gurus to send you some pictures of what to look for.
Good luck,
I'll have the Roast Duck
Joined: Jul 2002
Posts: 9,622
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From: Northshore, MN
If the vin says S=400, or H=351M, then you have an idea what to assume. So looking at the engine itself, you can try to find the stroke, by placing a wire down a spark plug hole, 4"=400, 3.5"=351M. But it's not really accurate if not done right, IMO. You could drop the oil pan and look for a 5M or 5MB stamped on the first throw of the crankshaft. Finding a K means it's a 351M. Or you could attempt to read the pn on the harmonic balancer. I was given a reman, too, that had a 1974 400 casting number on the block (boy was I excited), 78 and 79 heads, a 351M crank(then I was let down), and .030 over pistons, with the bores rusted.
sorry, Doc....simultaneous posting.
sorry, Doc....simultaneous posting.
Last edited by havi; Sep 30, 2007 at 08:46 AM.
New User
Joined: Nov 2004
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If you are planning to open the engine to see the numbers of the crankshaft you may consider putting a new aftermarket timing chain with a tree index timing to eliminate the retarded timing of the oem chain. Hope it’s a 400 
