Model B vs. Model A Crankshafts

J

John R

Well-known member
The Model B and Model A crankshafts and main caps are different. What is the difference? Can either of these Model A parts be used on a Model B and vice-versa?
 
Answer to part of my question: both rod and main bearings are larger. But can any of the parts be interchanged between the A and B.
 
The larger mains can be turned down to the smaller A size and use
B rods. I think you have to dimple the A pan for the bigger rod to not hit.
 
I've heard different accounts of whether it's a good idea to grind B crankshaft journals down to A size. Setting aside the tragedy of wasting so much quality material, the main question was whether the new surface of the journal would be softer than expected, because the hardening treatment applied when it was manufactured would not have penetrated to that depth. This could cause premature wear. I don't recall where we ended up on it, though.
 
You can turn down the B crank mains to Model A and offset grind the rod journals .125 and have a stroke of 4 3/8. Bore it .125 over and you have 230 cu. in. just say'n
 
Model B cranks have an advantage in that about half the cranks extant are "counterweighted" with forged-on counterweights. That is not to say that a non-counterweight B or even an A crank can't have "weights" applied. Some here have gotten VERY good at applying and weld-staking applied weights.

Of course the usual thought is to reduce your rotating mass by cutting down the flywheel afterwards - something which the Model A can benefit from since the stock flywheel is a little "overweighted" anyway.

Reducing the rotating mass MAY make the car drive more like a "modern car" (if that is your objective) with a slight tendency to increase clashing of transmission gears. The whole Ford thought between flywheel weight and drivability was one of how to best balance the two downsides in shifting: spin down and clash.

Joe K
 
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The main bearings on a B crankshaft are 2" in diameter. The clearance between the main bearing bolts in an A engine is 2". Bearing inserts generally come in increments of 0.010" so even at the first undersize inserts, there will be 0.005" clearance between the journal and the bolts. I have wondered about reducing the main bearing journals to the next insert size and boring the block to take them and use B rods also altered to take inserts. The main bearing inserts would be the same ones that Terry used in his new motor. (but undersize)
I have a B engine running on inserts The inserts I described above are used in the main bearings and the rod bearings are from a (I think) Toyota fork lift engine. I have the numbers if anyone is interested. Both the mains and rod bearings are 0.020" undersized.
As has been mentioned, with all of that extra weight in the crankshaft, lightening the flywheel is recommended.My only reservation is the As gravity oil feed to the main bearings. If the motor goes a lot better that an A engine, it will be driven faster and harder so pressure feeding the mains becomes more important.
At the end of all of this, I decided to not try to reinvent this particular wheel so I bought one of Terry's engines and left all the head scratching to him.
 
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