I've read here that there are differences in the balancers of the two engines. Is it just that the timing marks are different or are there other problems. I'm switching to a 400 crank and pistons and if all I have to do is mark tdc in a new spot on the dampner its not such a big deal, but is that the only difference?

 

the balancers are different ( i think the 400's was a little heavier and balanced a little differently) im not sure if they are different enough to keep you from using them interchangibly, but if your rebuilding id buy a new 400 balancer just to be sure

 

400 balancer is heavier but still balanced the same as 351m.

 

It will let the motor spin up faster. It is a good idle to get it balanced, it well make it last longer.

 

My first post since rejoining (my old name doesn't work) ...

... The 400 has a little longer balance weight on the backside as I recall. I can look at my 400 setup, but the 351 is out in the truck, snow piled up, etc ...

...but I do believe there is sufficient difference to render interchange of them unsatisfactory.

I also have a theory that it is a balance problem that causes 351Ms to wear mains out at 75,000 in many cases.

 

Excuse my ignorance, but why are the timing marks different? Wouldn't tdc be the same on both cranks? I am going from a 351 to a 400, and my local ford parts guy told me my balancer would work. I'd like to know if he was wrong so I can build this right from the start.

 

The harmonic balancer serves two functions:

1. External balancing (correct the imbalance of the crankshaft)

2. Damping harmonic vibrations (i.e., torsional harmonic oscillations)

Both 351M and 400 engines have the same crankshaft imbalance as other pre-'82 small block Fords (28 oz/in). Therefore, a harmonic balancer for either a 351M or 400 will balance correctly on either crankshaft.

Harmonic damping is tuned to the specific crankshaft. It requires a specific relationship between the weight of the inner and outer rings, and a specific flexibility (durometer) of the elastomer between the rings to work effectively. Therefore, the harmonic balancer from a 351M will not properly damp the torsional harmonic oscillations of a 400 crankshaft, and vice versa.

Inadequate harmonic damping causes excessive main bearing wear, and it will reduce the engine's overall longevity.

In a low-rpm application that is driven modestly (never running over 3-3.5K rpm), with close to factory-stock power levels, you can get away with interchanging 351M and 400 harmonic balancers.

If you have any ambitions for more-than-stock performance, I recommend that you at least use the correct balancer for the crankshaft.

If you have ambitions for substantial performance improvement (more than 20% over the stock torque and hp), I strongly recommend that you invest in a fluid-type damper and get the complete rotating/reciprocating assembly professionally balanced. The upfront costs will be repaid many times over in better performance, reliability, and longevity.

 

Well, the one on the truck (351m) is buried behind pulleys and stuff, so I can't help with timing marks.

I suspect the timing marks between a 351m and a 400 are the same ...
... and I suspect the counter weight on the two are different as the motors are externally balanced and a 400 has a 1/2" longer stroke which swings the rod big ends out an extra 1/4" all way round. Such action has to effect balance. It seems to me that I read somewhere that the weight is bigger on 400 dampers than on 351m dampers...
... but I can't make the comparison for reasons stated at top ...

 

The external weight on the balancer is partially determined by stroke and counterweight size but let's not forget about piston and rod weight. The 351M crank has a shorter stroke but the pistons are much heavier than the 400. The counterweights are essentially the same.

Hipo 289s and 428SCJs came with an additional counterweight to offset the heavier rods. Most FE engines are internally balanced except for the 410 and 428s, because of the longer stroke.

All 289, 302 & 351W prior to '82, 351C/M and 400 are externally balanced. The cranks vary by 20+lbs and the strokes vary from 2.87" to 4.00" yet they use the same 28oz weight for the H balancer.

 

FYI, I aquired a new harmonic balancer for my 400 today. The timing marks are identical to the ones on the 351m I am replacing.

 

80 dump ... were the weghts the same size on the back side and located the same in reguards to the keyway?

I mean, while you have them both off side by side and etc .....

Thanks

 

It seems from what I have been able to find now, that the balancers are the same in size and weight, however the weight distribution is different. The only place where I have seen the 351m and the 400 use the same part# balancer is on either internally balanced engines where you need a zero balance damper anyway, or the Fluidampr where it seems the internal silicone filled damper is able to compensate itself correctly for any of the Ford engines in the small block and 335 series.

 

Very intersesting thread. Are any of you guys using the Fluid Damper or something like it?

 

I can't see any visual difference between the 400 and 351 dampers at all, but I'm sure there is something different.

 

An update... on further inspection, the 400 damper has a series of 1/4" holes drilled partially through the outer ring beside the weights, while the 351 damper had the same holes 180 degrees away.