Is there a way to externally distinguish between a 351M and 400? I'm looking for a 400 to replace the 351M in my 79 F-250 and want to be sure I'm getting what I'm paying for.

 

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.

 

Where exactly are the casting numbers located? Also, where would the information on which numbers apply to which engine be found?

 

there's different numbers on the crank...if you go back in the forums deeper you will find them...also the balancer is different but i believe you still can use it with little or no problems.

 

Are the crank numbers externally visible? I'll most likely be dealing with complete engines. Also, what exactly is different about the balancers?

 

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).

 

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.

 

Thanks for the info. JBradley500, I did dig a little deeper and found the casting numbers you were talking about. thanks.

 

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.

 

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.

 

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

 

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.