Balancing Model T crank

[R.Brown]

Yeah, yeah... one of those "no rush, whenever you get time" jobs that I am sure we all get.. anyways, gotta get to it I guess.

If anybody has gone thru this deal, I'd like to hear about it.

This one has bolt on counterweights, and from the reading I have been doing on it, some like them some don't. Anyways here are my questions:

Do I need to worry about putting bobweights on the rod pins? I had not planned on it... I know the deal with a horizontally opposed not using bobweights, but did not know if something was different with a moveable bobweight.

Is the procedure to spinning up the crank and moving the bobweight around to get it close and then maybe some drill correction to fine tune it??? Is what I have to do here?

I have read different thoughts on whether these cranks should have a bolt on counterweight or no CW. This guy brought all the stuff in for it, so I am in unchartered waters here.

Any help is appreciated,

Ryan

[Unkl Ian]

Have you checked on the Secrets of Speed site ?

They are right into that stuff.

[BRENT FAY]

If my 2 brain cells remember right the Model T didnt come with counter weights - but you could purchase them and they would be in pairs that would clamp (bolted to each other) to the rod arm between 1&2 - 3&4 rod journals. I might have (many years ago) only balanced a couple of T cranks but quite a few A cranks. Did not bob weight them and would spin the crank and make needed corrections. Also would balance rod and pistons seperately. I also would machine the flywheel down from 75# to 50# to make it accell better and to help with wear on rear main and balance the flywheel seperate also. I will try and research this more tonite or tomorrow and let you know more about the counterweights. Deutz air cooled farm tractors had bolt on CW's that i had to remove before I could grind the crank. Then I would bolt back on in the position they were in and balance with no BW's (inline 4 cyl) Brent

[machine shop tom]

I have a customer who brought in several "A" cranks, etc. to be balanced. He had fabricated and installed counterweights on them. His counterweights were brazed on (!). Balancing the cranks was straightforward and I was pleasantly surprised at the consistency of his modifications. He also made his own aluminum rods for one engine and I was amazed at how well they were made, especially since he did them on manual machines. He also lightened his flywheel which was balanced separately, as mentioned above.

tom

I have edited this to say that the counterweights, not bobweights, were brazed on. Also, the cranks were balanced without bobweights. Sorry, I get goofy when I am hungry!

[SchmidtMotorWorks]

Do I need to worry about putting bobweights on the rod pins? I had not planned on it... I know the deal with a horizontally opposed not using bobweights, but did not know if something was different with a moveable bobweight.

No, you don't need to use bobweights, the need for bob weights is determined by a combination of the crank configuration and block configuration. An inline 4 with flat rod pin positions does not need bobweights. You could put them on but all that would happen is you would add the imperfection of the bob weights to the balance.

As a rule of thumb, if a crank balances the same with and without bobweights, you don't need them.

[MadBill]

An inline 4 with flat rod pin positions does not need bobweights. You could put them on but all that would happen is you would add the imperfection of the bob weights to the balance.
As a rule of thumb, if a crank balances the same with and without bobweights, you don't need them.

Theoretically a flat crank four doesn't need bob weights. However, I have heard a number of stories of them being static balanced per routine, then spun with bob weights and needing some additional work. Claims of smoother high RPM operation are common.
We recently did an Alfa Romeo 1750 cc and had the shop compare. His equipment can measure to 1/4 gram, and he did an obsessively accurate static job. He was interested to know too, so did the spin gratis. The difference was minute, a couple of grams more to remove from a factory drilled hole. The engine is very smooth running now, but who knows how it would have been without the spin?
One theory is that minute errors in stroke or indexing could affect the inherent 'natural balance'. My guess would be that a T crank might not measure up to modern designs in this regard, so a spin could be an informative final step.
One other factor: As I recall, that 'T' crank looks like a giant bent wire, no counterweights at all. Higher end four bangers (Subarus, some Hondas, etc.), go to the trouble of putting a nice little counterweight on each side of each throw (8 in all). This eliminates the bending loads that occur when the load is here, but the counterbalancing throw is way over there. Such loads will produce a strain that rises as the square of the speed, so it's possible that such a crank will distort enough as the RPM rises to become noticeably rough. Bob weighting and then spinning the shaft at max engine revs might be quite revealing. (I know Callies offers high speed balancing, don't know how widespread the capability might be.)

[SchmidtMotorWorks]

I agree, static balancing will produce a different result than dynamic balancing. That said dynamic balancing with bob weights is pointless unless the index is way off. Even then, I'm not sure that would cure anything.

As far as "high speed balancing" pure nonsense. It is all a matter of sensetivity of the vibration sensors, balance is balance and doesn't change at different speeds.

Spinning a crank of the balancer at high speed with weights seemsto duplicate bending is probably not worth much either considering how small the bending is and it would certainly be much different in the engine than on the balancer.

[MadBill]

...As far as "high speed balancing" pure nonsense. It is all a matter of sensetivity of the vibration sensors, balance is balance and doesn't change at different speeds.....

So then, Callies (allegedly) and this outfit (in print) are just catering to people's fantasies?: http://www.testdevices.com/balancing_services.htm (See "At Speed Balancing" article.)

[machine shop tom]

In highly-stressed or high-rpm applications, it may be beneficial to use bobweights on a flat crank. But as the wheels fall of a Model A at aout 60 MPH, it's probably a moot point.

tom

[SchmidtMotorWorks]

So then, Callies (allegedly) and this outfit (in print) are just catering to people's fantasies?:

Like the article said, it applies to cases where the shape of the object being balanced changes at speed. Even if the crank did change shape at speed, the change in shape would be different in the balaning rig than in the engine anyhow.

I saw a balancing rig made in the 60s that would balance a complete assembled engine. They would rev up the engine with an electric motor and drill holes in the crank from below, then bolt the pan on.

[SchmidtMotorWorks]

In highly-stressed or high-rpm applications, it may be beneficial to use bobweights on a flat crank.

Why? Assuming the index is close a perfect set of bob weights will have zero imbalance. So, the only change you could make is including the imperfection of your bobweights (and positioning) into the crank.

[R.Brown]

Thanks to everyone who has helped out. I talked to a couple of "T" specialty shops and the concensus is to leave the bolt on counterweights out. After talking to the customer, it was just kind of a monkey see, monkey do deal, so he is fine leaving them off. He has another "T" that he drives and it runs fine without counterweights.

The only thing holding us up now is the .015 bend in the crank!

Ryan

[MadBill]

[quote="SchmidtMotorWorks...I saw a balancing rig made in the 60s that would balance a complete assembled engine. They would rev up the engine with an electric motor and drill holes in the crank from below, then bolt the pan on.[/quote]
As is often the case, this is drifting a bit from the original question (sorry Ryan, but you know every pearl starts as a grain of sand...), but to your point re the effect of firing loads being imposed on top of inertia forces, wouldn't it be great to strain gauge the main studs, run the engine up at WOT, do a high frequency sampling of the loads and come up with some magic all-encompassing balance factors?

[OldSStroker]

Thanks to everyone who has helped out. I talked to a couple of "T" specialty shops and the concensus is to leave the bolt on counterweights out. After talking to the customer, it was just kind of a monkey see, monkey do deal, so he is fine leaving them off. He has another "T" that he drives and it runs fine without counterweights.

The only thing holding us up now is the .015 bend in the crank!

Ryan

T cranks have become a problem, and about the only company making new ones has had some troubles.

There are 2 basic T cranks, I'm told. The "diamond" crank used thru 1925 has a roughly diamond shaped webs between the throws. The '26-'27 crank has a more rectangular section and it's beefier than the diamond. My T-guru said throw out a diamond and find a '26-'27 if you are building a driver/tourer.

I'd have all of them that I could find Magnaflux inspected, choose the best one (with the smallest cracks ) and straighten it. From my T-restoring buddies, .015 is probably what it bends in operation! Wasn't there a time, early in the life of the T that there was no upper bearing (babbitt) in the center main? Remember it's a 3-main engine.

My $.02

[machine shop tom]

In highly-stressed or high-rpm applications, it may be beneficial to use bobweights on a flat crank.

Why? Assuming the index is close a perfect set of bob weights will have zero imbalance. So, the only change you could make is including the imperfection of your bobweights (and positioning) into the crank.

Good point. No sense in going to the trouble of adding bobweights if it only puts a fly in the ointment.

tom