which is better/cheaper i have a stock model a crankshaft, and im debating whether i should keep it and have the counter weights machined on. or if i should spead the 500+ on a 32' b crank, and still use extra money to clean and ground it down. which is cheaper, which one is better?
Seems no one wants to answer this. All I know is, a shop nearby has done Model A cranks forever. They cut out steel counter weights, and weld them to the cranks. They have a pattern they use for the correct size and clearance. Then they balance the crank, pistons and rods. Makes the Model run really smooth.
I don't know the particulars of a Model A crank but I do know a few basic principles on counterweight. First, the added weight will be able to smooth out the transitions between combustion torque and inertial torque. Additionally, the added weight counters the offset forces of the connection rod attached to the rod journal. Mostly I'm just flappin my jaws to keep the topic at the top, but if I was building one and had no prior knowledge, I would elect for counterbalancing the A crank, since I already had it in my possession. That's just me though.
If running babbitt bearings, the thinner the babbitt the more durable it can be. An advantage to the B is that its larger diameter lets a worn crank be cut down to standard A. I think some people even add some main diameter above that when using them in A blocks to get a little of the extra stiffness. Most A cranks of course are a considerably undersized by now and require thick babbitt.
Most modern engines have more bearing journal size than they need. I believe an early Ford may be an exception to this. I think Hank bent those cranks out of coat hangers. I saw a couple of 'em at the last shop I worked at; they were pretty light. I haven't seen a B crank recently enough to remember what one looks like. Do they have counterweights anywhere but the ends? If so, I might look into running one. But, I think you can also get brand new A cranks w/ weights. BTTT for a worthy question. -Bill
bruce's advice is right on. if you go with the b crank you need b rods and dimple the oilpan sides for clearance. i have counterweighted and dynamically balanced a cranks that use sbc rear seals. they do help them run somewhat smoother but there is no magic bullet . also...be sure to run a lightened flywheel to compensate for the added crank weight
Why not spend the money and buy a new counter weighted crank. Instead of spending all that money on a 80 year old B crank that might break at the flywheel flange.
Something interesting off of speedtalk: http://speedtalk.com/forum/viewtopic.php?f=1&t=22059&start=0 -Bill
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