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Old 10-19-2019, 12:55 PM   #1
Jim.Morris2831
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Default Model B crank in Model A

My Roadster engine is at Schwalms being rebuilt. They notified me yesterday that the crankshaft is out of a Model B! It's counterbalanced. Does anyone have any experience with having a Model B crank in a Model A engine? Are there disadvantages or advantages? I don't understand why anybody in about 1980 would have put a B crank in the Roadster except for thinking that a B crank would somehow perform better.
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Old 10-19-2019, 01:31 PM   #2
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Default Re: Model B crank in Model A

To use a B crank in a Model A engine, the journals have to be ground down to Model A specs. That is actually a good way to use a B crank that is worn beyond specs. At least one engine in our club has an engine so equipped.
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Old 10-19-2019, 01:46 PM   #3
Purdy Swoft
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Default Re: Model B crank in Model A

I think that the counterbalanced crankshaft was suppose to run smoother with less vibration.

The model B crankshaft has larger bearing journals .

Larger journals help make a stronger engine.

Model B crankshafts can have the main bearing journals reground to model A size .

Larger rod bearing journals allows for the use of model B rods .

The counterbalanced crankshaft will not cause the engine to run faster .

If anything a heavier crankshaft will reduce acceleration and throttle response.

If I used a model B crankshaft in a model A engine , I would prefer the unbalanced 1932 version .

I would get the larger rod bearing journals with the early B crank without the extra weight of the 33 and later counterbalanced B type cranks .

I've been told that the counterweights don't really take out the vibration

This would just be my choice and why , not a recommendation ...
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Old 10-19-2019, 01:51 PM   #4
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Default Re: Model B crank in Model A

My Father did this in the mid '70's, the idea was to get the benefit of a counterbalanced crank. The other thing to do was bolt on the weights to a Model A crank for smoother operation at high revs.
He did say it really made no great difference & probably was not worth the effort & cost
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Old 10-19-2019, 02:40 PM   #5
Purdy Swoft
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Default Re: Model B crank in Model A

Actually the early model A beaver tail crankshaft was the lightest .

Some prefer the lighter crankshaft for quicker revs and acceleration .

Truth be known, nothing is going to help all that much without first increasing the compression ratio .
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Old 10-19-2019, 03:56 PM   #6
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Default Re: Model B crank in Model A

Counterbalancing is important if you plan on revving your engine beyond 3500 rpm..so is crossdrilling the crankshaft and pressurizing the rods..racing its mandatory,street its not.
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Old 10-19-2019, 04:02 PM   #7
Purdy Swoft
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Default Re: Model B crank in Model A

I don't race . mine are just fun cars .
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Old 10-19-2019, 04:43 PM   #8
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Default Re: Model B crank in Model A

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Originally Posted by Purdy Swoft View Post
Actually the early model A beaver tail crankshaft was the lightest .

Some prefer the lighter crankshaft for quicker revs and acceleration .

Truth be known, nothing is going to help all that much without first increasing the compression ratio .

If you lighten the flywheel doesn't that even things up somewhat?
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Old 10-19-2019, 05:09 PM   #9
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Default Re: Model B crank in Model A

A lighter flywheel helps with faster acceleration and takes a load off the main bearings .

. I use a lighter model B flywheel .

The B flywheel helps some .

I can sometimes pick up a model B flywheel for way less money than having a model A flywheel lightened .

The model B flywheel was lightened at the factory .
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Old 10-19-2019, 05:15 PM   #10
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Default Re: Model B crank in Model A

Back in the 60's I fitted a Model B crank, cam and flywheel to my Model A Block, keeping the B rods and grinding down the mains to fit. It proved to be a very smooth and relatively powerful motor that would rev very freely. I don't recall getting it balanced. When I later fitted a hi comp head it proved very powerful.
Today I have a very improved motor in my current A but somehow it doesn't feel as good as that early motor despite fanatical balancing and other improvements. From memory the early motor had a Marvel Schebler carb.

Last edited by wensum; 10-31-2019 at 07:54 PM.
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Old 10-19-2019, 05:42 PM   #11
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Default Re: Model B crank in Model A

I remember when almost every model A that I would see used a marvel carburetor .

I have used Marvel carbs before and prefer them to a Tillitson any day .

Back when I was in my teens I knew nothing about the GAV .

I never changed the adjustment when I run the Marvel carbs and never knew the difference .

I've got one of Charley Yapps Vortex manifolds and I am thinking about installing two Marvels on my September 29 .
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Old 10-19-2019, 11:53 PM   #12
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Default Re: Model B crank in Model A

I've seen a couple of A engines with B cranks. The clear distance between the main bearing bolts (2") is the limiting factor for journal size. The main bearing journals on the B crank have to be reduced to less than that to fit. So that I can leave the journal as large as possible, I'd prefer to use insert bearings from where ever. It would be larger than the original A specs but much smaller than the B.
I'd be looking for a good touring engine, not a racer so cross drilling wouldn't be required.
If A con rods were used, there is also potential to grind the crank to give a longer stroke. That would give higher piston speeds but more compression and torque. How far do you go for a hobby car and how many $ do you have to spare?
I have never built such an engine but I wonder whether the A main bearing caps would be good enough or should they be remade? Any info on that?
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Old 10-20-2019, 09:21 AM   #13
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Default Re: Model B crank in Model A

You lucked out. JMO
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Old 10-20-2019, 09:41 AM   #14
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Default Re: Model B crank in Model A

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Originally Posted by Synchro909 View Post
I've seen a couple of A engines with B cranks. The clear distance between the main bearing bolts (2") is the limiting factor for journal size. The main bearing journals on the B crank have to be reduced to less than that to fit. So that I can leave the journal as large as possible, I'd prefer to use insert bearings from where ever. It would be larger than the original A specs but much smaller than the B.
I'd be looking for a good touring engine, not a racer so cross drilling wouldn't be required.
If A con rods were used, there is also potential to grind the crank to give a longer stroke. That would give higher piston speeds but more compression and torque. How far do you go for a hobby car and how many $ do you have to spare?
I have never built such an engine but I wonder whether the A main bearing caps would be good enough or should they be remade? Any info on that?
A touring upgrade? use the B crankshaft,cut the main journals to 1.600.Use B rods for the added bearing support(bigger journal). Cut ten pounds off the A flywheel to compensate for the added weight of the crankshaft.Flywheel mass stores kinetic energy,provides smooth shifting and reduces driveline vibration.You can cut another 10lbs if you want for 20 total.If using a stock A trans the lighter flywheel allows you to shift faster (rev up and down quicker).

People end up paying big money for things they dont need,and fail to buy the things they do need.A well built stock model A lower end will provide years of service,take the money and put it where it really helps,compression,fuel and cam.
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Old 10-20-2019, 12:05 PM   #15
Jim Brierley
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Default Re: Model B crank in Model A

A counterweighted crank will probably not feel any smoother, but the main brgs will likely last longer. I did that in my last engine, runs great, but the 4-port Riley on top helps a little too. If I did it again I would turn the main to 1.700" and use available inserts. This would fit between the A main bolts, which is exactly 2". You could probably fit a B or C crank between the bolts if it was a little undersize, .010" or .020"?
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Old 10-20-2019, 03:56 PM   #16
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Default Re: Model B crank in Model A

The last "B" crank I put into a "A" block ground mains to 1.850 dia and used rod bearings from a saturn motor. The line bore just cleared the bolts.

Grinding the crank to "A" main dimensions is just asking for trouble, especially if cross drilled and the proper corner radius is not achieved.

If I remember correctly, the "B" crank flange will also probably need to be turned down to fit in the cap or block.

This is a logical conversion if going to turn up the power/compression and breathing.
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Old 10-22-2019, 04:03 AM   #17
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Default Re: Model B crank in Model A

Hmmm, too B or not to B, that is the question.
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Old 10-22-2019, 05:12 PM   #18
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Default Re: Model B crank in Model A

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Originally Posted by johnneilson View Post
The last "B" crank I put into a "A" block ground mains to 1.850 dia and used rod bearings from a saturn motor. The line bore just cleared the bolts.

Grinding the crank to "A" main dimensions is just asking for trouble, especially if cross drilled and the proper corner radius is not achieved.

If I remember correctly, the "B" crank flange will also probably need to be turned down to fit in the cap or block.

This is a logical conversion if going to turn up the power/compression and breathing.
Agree 110%
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Old 10-22-2019, 05:17 PM   #19
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Default Re: Model B crank in Model A

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Originally Posted by Railcarmover View Post
A touring upgrade? use the B crankshaft,cut the main journals to 1.600.Use B rods for the added bearing support(bigger journal). Cut ten pounds off the A flywheel to compensate for the added weight of the crankshaft.Flywheel mass stores kinetic energy,provides smooth shifting and reduces driveline vibration.You can cut another 10lbs if you want for 20 total.If using a stock A trans the lighter flywheel allows you to shift faster (rev up and down quicker).

People end up paying big money for things they dont need,and fail to buy the things they do need.A well built stock model A lower end will provide years of service,take the money and put it where it really helps,compression,fuel and cam.

Thanks, good info.


Slightly off topic but what's the cost of a new SCAT crank these days?
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Old 10-22-2019, 05:20 PM   #20
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Default Re: Model B crank in Model A

Quote:
Originally Posted by Railcarmover View Post
A touring upgrade? use the B crankshaft,cut the main journals to 1.600.Use B rods for the added bearing support(bigger journal). Cut ten pounds off the A flywheel to compensate for the added weight of the crankshaft.Flywheel mass stores kinetic energy,provides smooth shifting and reduces driveline vibration.You can cut another 10lbs if you want for 20 total.If using a stock A trans the lighter flywheel allows you to shift faster (rev up and down quicker).

People end up paying big money for things they dont need,and fail to buy the things they do need.A well built stock model A lower end will provide years of service,take the money and put it where it really helps,compression,fuel and cam.
Having caught a cracked crank shaft on a Modle A motor JUST short of breaking, I like the idea of something sturdier. Another solution is to fit one of those new counterweighted A cranks (Burlington or Scat).
Otherwise, I can't fault your last paragraph. Better performance always comes down to the three Cs. Carb, Cam and Compression. Better durability is another thing!
I think it should be pointed out (no one has yet) that if the main bearing journals are turned down as much as would be required to fit a B or C crank in an A block, they will have to be hardened again afterwards.
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