351M vs. 400
Senior User
Joined: Oct 2005
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Pull the casting numbers - they are different...However, a 351M and 400 share just about everything. I have never noticed any external difference. For that matter, the internal differences are very slight too. You can make any 351M a 400 with a little work.
Post Fiend
Joined: Mar 2005
Posts: 8,542
Likes: 5
From: Louisiana
Hopefully what ever engine you see will have the stock valve covers on them, and usually with alittle cleaning you can see a emissions sticker that states if the engine is a 5.8L 351M or 6.6L 400. Other than that if they are still in the original truck, you can look on the vin number on the driverside door jam(it would be a certain number/letter in a certain spot in the vin so you can tell which motor it is).
Posting Guru
Joined: Jul 2006
Posts: 1,354
Likes: 1
From: America
eaiest way to distinguish...get something thats a few inches long and take out a spark plug, hold this object on the piston top and crank the engine over..if the object goes up 4 inches, its a 400. if it goes up 3.5 inches, its a 351.
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Senior User
Joined: Oct 2005
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Thanks for the info. JBradley500, I did dig a little deeper and found the casting numbers you were talking about. thanks.
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Posting Legend
Joined: Sep 2001
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This site has the crank ID info:
(TMI)
Due to the configuration of the 351M/400 it is very difficult to measure the stroke thru the plug hole. The casting numbers and differences are all internal. The sticker or the VIN is a good way to determine the engine type. The 4th letter in the 77-79 VIN code designates the engine, H is a 351M, S is a 400.
(TMI)
Due to the configuration of the 351M/400 it is very difficult to measure the stroke thru the plug hole. The casting numbers and differences are all internal. The sticker or the VIN is a good way to determine the engine type. The 4th letter in the 77-79 VIN code designates the engine, H is a 351M, S is a 400.
Senior User
Joined: Jan 2004
Posts: 367
Likes: 1
From: denver
In my opinion, the only way to tell is to check for the casting on the crank or directly measure the crank. I have a 78 F-250 that I bought that was a 400 truck, started the rebuild, just to find that a P.O. installed a 351M crank. Basically, if you cant see or measure the crank, you won't know 100% sure that you have a 400. Since you have a 351M, you could track down a 400 crank and balancer, and build your 351M as a 400 (what I did, found an unturned crank for $50), or insist that the engine is verified prior to purchacing. TMI site should provide the correct casting numbers.
New User
Joined: Jan 2008
Posts: 0
Likes: 24
There are a couple of ways to tell in the casting numbers. But one needs to know the ALL the numbers. This includes the seperate date code.
But, yes the definitive way is to ID the crank.
Look for a 71-72 ,400
Casting D1AE-A
But, yes the definitive way is to ID the crank.
Look for a 71-72 ,400
Casting D1AE-A
Junior User
Joined: Dec 2003
Posts: 82
Likes: 0
From: Utah
I just bought a 400 ( I thought ) at a junk yard. Sticker on truck said it was a 400. took it home while cleaning it up I found a rebuilder tag on block behind starter said it was rebuilt 351m. So i am going to run it untill i get my 400 rebuilt.
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