Well, I just called the machine shop to find out if my pistons were done yet, and got bad news. My engine is a 400M and not a 351M. He said the rods are from a 400 and the pistons I gave him are for a 351. I always thought they were the same engine and that everything was the same. My buddy tells me the only differnce is the bore/piston size. Is this correct? I'm hoping I can still use all the other things from my 351M rebuild kit? Any help is appreciated. Also, what is the CI of a 400 bored 0.030 over?

 

The difference between the 351M and 400 is the crankshaft, rods and pistons. They have the same bore size but the 400 has a longer stroke. Otherwise the engines are the same. You might post any other questions in the 351C/351M/400 Forum. Hope this helps.

Jason

1993 F-150 300 I-6 w/5-Spd.
1977 F-250 351M w/4-Spd.

 

Beav,
The distance between the wristpin and the top of the piston is much greater for the 351M than the 400. That means you'd either push the head off the top of the motor or you'd blow the crank out the bottom.
Now for your solution:!
Do you want to build a hot rod?
Find the distance from wristpin to top of piston for a stock 400M. Then find the distance from the top of the stock piston assembly to the top of the deck(block).
Now, review pistons that were sold for the 351C and configure a new rod/piston assembly that will give you a compression ratio of 9.5:1. This can give a 400 some serious potential power.
If you need more details please reply to me E-mail and I'll gather some details for you.
Good Luck,
KingFisher

 

So when you say the bore is the same, does that mean the 351M piston heads would fit? Or do I need piston heads for a 400M?

 

They are the same diameter.
4" bore


The wrist pins fit in a different area of the piston
which make it different that the 351M that's why the different rods.





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Doesn't sound like much of a setback to me to have a 400 instead of a 351.

I thought 351 & 400 had same block & rods, only the crank & pistons are different? The 351 pistons have a much shorter wristpin height than the 400 pistons?

 

All,
The 351M has a shorter stroke than the 400. This means that the piston must be taller so that it can reach the same height as the 400 piston to get the same compression.
The 400 has a longer stroke so the rod is shorter and so is the piston. The 400 crank will push the piston up higher. I can't make it anymore LAY than that.
By putting a 351M piston on a 400 rod, you would have about 20:1 compression ratio, or something like that, if it didn't ram the piston through the head.
The 400 piston is too short and doesn't develope enough compression. The trick is to find a 351C piston that does the job.
Good luck,
KingFisher

 

the rods in a 351m or 400 are identicle. The differences are in the crank and the pistons. That's why the 351m pistons are taller- to make up for the short 400 rods, in a shorter throw crank. If you want all the details, check out Bubba's M-block Workshop:
http://home.earthlink.net/~bubbaf250/
Anything you ever wanted to know about the M-block is here, along with a history of the engine, and performance mods that can be made.

Jared