Crankshaft Grinding Notes

 

1/2010

My brother has been buying equipment to start up his own engine machining shop. During his research on crankgrinders before he bought one he learned some stuff that should scare you. There is a reason why so many engines have vibrations (I am being inclusive to all engines). Crankgrinding is not an automatic process. It requires a considerable amount of operator expertise and a willingness to maintain a high level of quality.
The crank needs to be properly centered on the grinder and then properly indexed to get each throw at the proper angle. Each pair of throws for the rods would need the crank to be move a precision distance from the center line of the crank.
Now imaging a guy who looked at our little simple old crank and decided it was just an old car. So they throw it on the machine and loosely get it in shape. When they are done the throws are only off by a few thousands of an inch. Redoing the grinding wheel radius wastes the wheel so the one they use for a modern chevy will be fine. Oh, to save material the guy decided to center on the wear on each main. The center main wears 108* from the front and rear so its center line will be way off from the front and rear. The rods are done the same way so each throw has a unique distance. The flywheel is only off the center line by .006".
Oh wait, you clamp the crank in a block with striaght mains. Now the crank bends because the center of the center main is off from the ends. The causes the front and rear mains to wobble some and now the flywheel is off center and out of the plane of rotation.
No problem, after all this is just a little old car.

As with all parts of a restoration one first must ask what was the original part and why. The Model A crank was a precision engineered part. Ford require .001" or more accuracy. The rear flange located the flywheel to less than .001" from center. If you take the A crank in to a grinder and ask them to maintain factory precision they will charge you a lot more and tell you that you want a racing grind. If they can even meet the original factory specifications. A lot of guys just do not know how to grind a any crank that accurately. My brother does Mercedes engines for a living and he has yet to get a local shop do one right.

The next time you think you have a bent crank, think harder. First you have to check the wear. The front and rear mains wear 180* degrees out from the center main. This will give the impression that you have a bent crank from the start. If you have the crank ground then the probablity is high the guy worried more about saving material then grinding the crank on center. If the crank was ground in the past then it could be really wack out of shape.
As soon as you bend the crank you have to make sure the front an rear mains journal surfaces are inline. If you have introduced a wobble then you will make a taper on the babbitt and not have full bearing contact. A much smaller bearing surface will be holding the crank which increases wear and the probablity of a failure. You also need to make sure the rear flange does not wobble as that flywheel is a lot of mass to swing off plane.
Check for a proper radius. The grinder has to waste some of his wheel to do a new radius. Too sharp a radius will cause the crank to crack I can show you a Model A crank with this problem.
Check you throws. Nothing more annoying than having one piston hit the head.

Balance is also important. Ford dynamically balanced the crank to 1/4 oz. Everything that was attached to the crank was balanced.

Now lets throw another thought into the crank equation. The more you grind off the crank the thicker the babbitt you have in the block. This is not good. The thicker the babbitt the more it is able to be squished as it is softer than the steel behind it. There is a sweet spot for the the thickness of babbitt. Too thick and it can deform too much and break. To thin and the babbitt will not have enough structure to hold together (on the non-tinned block and cap). The tinned parts can be much thinner as the babbitt is tightly bonded to the steel.

Nothing is easy when dealing with the Model A. Build the engine like you are building a race engine and you will have a sweet running trouble free original car. If you take the short route and do not check everything then you are destined for a problem car.
I hope this opens your eyes some.

 

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