crankshafts

a-bone newby

a-bone newby

Senior Member
a lot of talk of countered cranks, is it nessary for an A that motored around town as I am adding new babbited rods and a 6.1 head not a touring car just my baby for local drives. my car has 82000 miles and doesnt burn any oil YET so all tips is helpful. the snow is almost gon and out she comes. PS thanks for the patience as a new owner not the same as all the racing I did in the 60s
 
The Model A crank shaft was balanced to <1 gms, todays modern crank shafts are balanced to <4 gms . How are counter balances going to help? I would not do it.
 
Ha.
well now ya wouldn't wanna put anything too good in your A now would you. You may be interested to know that a good machine shop can bring your crank out to 1/2 gm or less. You just have to be motivated. And no why would you wanna run a counterweighted crank, what good would that do? On my modern cars I drop the pan and with a plasma cutter I chop all the counterweights off my cranks. who needs'em
 
Last edited:
Balanceing a crankshaft and counterweighting a crankshaft serve two different purposes.

You can take a perfect balanced tire and add a two ounce weight on the outside top of the rim and also add a two ounce weight on the inside bottom of the rim. The tire is still in prefect static balance, but way off in dynamic balance, as you would find out by spinning the tire. That same principle applies to the crankshaft. The original crank has two heavy weights near the center of the crank wanting to pull the crank one direction, while the two heavy weights on the ends are trying to pull the crank the opposite direction. These bending forces are what wears out babbit. Sure the Model A crank is heavier than the T crank, but the forces are still there trying to bend the crank. If you add weight directly opposite the crank throws, this will do much to counter the pull trying to bend the crank. I would add counterweights any time the engine is opened up and the crank is out. Besides, once it runs so much better you may want to take longer trips than just around town.
 
The factory A would run 50,000 or more miles on the original babbitt.

The factory A was also tightly balanced.

The rods were built to +-1 gram at each end, pistons within a couple of grams and the assembled units in any one engine would be within 4 grams.

The crank would be machined with all the mains on center and the flywheel would be less then .001" off center. The crank and flywheel were also balanced.

The engines were run hard in harsh conditions and did not blow through babbitt. Not babied like most guys today. Keep in mind the factory cars were designed to run 60 MPH and they were run that hard even on the crappy roads of the day. Read the stories of that era and you will have a new respect for that car people baby today. They really can be run hard and put away wet and not be hurt.

Today we hear of many issues from people not building the engines to factory specs. Many have this attitude that is is just a simple old car. The truth is, when you get the prints out and start restoring the engine to factory specs you are building to race car tolerances. You have to put a lot of effort and thinking into making the engine right.
Today the rods are not balanced end to end and typical numbers are within 5 grams total weight. I have measured and aftermarket boxed set of pistons with 50 grams difference in the pistons (not typical). We know the cranks are being ground with the mains not on center and the distance of the throws not the same. The rear location of the flywheel we have seen off by as much as .008". Add in an unevenly worn cam with improper clearances and a distributer that is off a bit and you get an engine that behaves in odd manners.
As for babbitt failing. I would put more money on the babbitt (or inserts) failing from the guy messing something up. Good babbitt will take a lot of abuse. Babbitt that is a bit loose (very common) or babbitt poured not following the rules will have good reason to fail early.

While counter balancing would positive improvement, it really is not a needed expense for most cars. Of all the expenses on the engine, you want to pay for the highest quality attention to factory details. You need to truely restore the engine back to factory tolerances. That gives you a car that will run 60 MPH all day long and give you 50,000 miles (or more) of reliable life.
If the engine is not built right in the first place, the HC head, counter balanced crank and what ever else you throw in there won't help.
 
other than maching ect shopwork I do my own when building my race chev engines I balanced comonents myself with a .ooo scale with good results
 
I guess not so "Newby" is asking for tips regarding crankshafts. My tip: leave it alone until there are more convincing symptoms that may show problems in the declared use, if that is truly the limit of the performance needs. Years ago I drove a Model A regularly, almost daily. The many miles were driven with cheap almost discarded engines that were patched up to go a few extra miles. Trouble most often was at the center main and I knew the sound well. In this real world, such strategy probably would not work as well excep perhaps in the imagination of one who does not want to do an engine in the style that shops practice these days. There may well be as many good used engines available in BC as we have here in Los Angeles which could shift the choices toward used, which could include counterweighted crank. Good Luck: Fred A
 
You must log in or register to reply here.
Back
Top