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351m to 400 conversion

I have a 351M that I converting to a 400 by swaping the crank and pistons. My question is, are the harmonic balencers the same on the 351M and 400?
 

LEB Ben

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Bubba's M Block is showing different part numbers between the 351M and 400 balancers:

http://web.archive.org/web/20070402125656/home.earthlink.net/~bubbaf250/parts/parts04.html


That being said...I'd be inclined to say they're different.


What project Bronco has to say:


The 351M uses a different harmonic balancer than the 400 -- It is best to buy a new 400 harmonic balancer to go along with a 400 crankshaft. You can use a 351M balancer, however, you will have to rebalance the 400 crankshaft by adjusting the counter weights to bring the assembly into balance. Also, the timing marks on the 351M balancer will be nearly useless on a 400 crank


FYI on the flywheels as well:

Note: Both 351M and 400 engines have the same 28 oz-in crankshaft imbalance, so all M-block flywheels have the same balancing counterweight, and they can be interchanged without vibration or balance problems.

After M-block production ended in 1982, Ford re-engineered the M-block flywheel to accommodate both engines with one part. The current M-block flywheel service part (E5TZ-6375-H) has holes drilled and tapped for both 351M and 400 pressure plates.
 
Last edited:
Thanks for the reply and the link. I'll be bookmarking it for sure. An excerpt from that link is:

Note: Both 351M and 400 engines have the same 28 oz-in crankshaft imbalance, so all M-block flywheels have the same balancing counterweight, and they can be interchanged without vibration or balance problems.

After M-block production ended in 1982, Ford re-engineered the M-block flywheel to accommodate both engines with one part. The current M-block flywheel service part (E5TZ-6375-H) has holes drilled and tapped for both 351M and 400 pressure plates.

When the 351M was introduced in 1975, it used the same flexplate as the 400. When M-block engines were introduced in trucks, the truck engines equipped with automatic transmissions used the same flexplate as the car engines.

From that note, I'm not worried about the flexplate. The harmonic balencer does have different part numbers so I may have to replace it.
 

LEB Ben

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Since it's externally balanced, just get everything balanced together.
 
Are you saying that as long as the flexplate and balencer came off the same crank, they should work together on the new crank?
 

LEB Ben

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No...I'm saying the flex plate will work regardless and the 400 crank will need to be rebalanced to the 351M balancer...which will be rendered useless because the timing marks won't line up then.
 
You lost me a little on that one. I had assumed that the timing marks would be the same since the timing set (chain and gears) is the same on the 351C, 351M, and the 400 and the only difference I have found between the 351M and 400 is the crank and pistons (from various web sites). I'm getting a zero degree timing set for the rebuild instead of the retarded one the factory used.
I knew the flex plate was interchangable from the site you refered me to so I wasn't worried about that.
Guess I'll get a 400 harmonic balencer rather than try to modify a new crank.
 

mtflat

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You lost me a little on that one. ........Guess I'll get a 400 harmonic balencer rather than try to modify a new crank.

That's the other side of what Ben was trying to say. Much easier to go your route with a new balancer.
 

DNFXDLI

The Token Canadian
Staff member
Or, just call Tim and get a stroker :D
 
NPD and Summit both list the same part number as the replacement for both motors. I wasn't going to replace it but it does have a good groove so I guess I will. Thanks for the info.
 
O'Reilly Auto Parts list P/N PB1082N as the replacement harmonic balancer for vertually all 351M and 400 from 71-79 in both cars and trucks. Hope that info helps anyone with the same question I had. It would appear that even tho they had different factory part numbers, they are interchangeable.
 

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