Murray Fahnestock, Model T Ford Dealer and Owner, July, 1925 "A Training School for Ford Mechanics".  Reprinted on p. 497 of "Ford Service Bulletin Essentials"

Many interesting discoveries were made with regard to vibration in Ford engines - as some of the rebuilt engines had considerable vibration. In some engines this was due to the fact that magnets had been removed and replaced with others, and then not afterwards balanced. On other engines, the flywheel ring gears had been removed and new ones put on; which, of course, sometimes loosened the magnets and changed their balance.

But most of the trouble was found (to the surprise of all) to be due to a cause which no one had suspected - that of the transmission shaft being out of alignment. The only attention that heretofore has been paid to the transmission shaft was to see that the transmission shaft was not worn excessively, or that the spot on which the clutch drum fastens was not too loose. If these two parts were all right, it was always assumed that the transmission shaft was all right, and no attention was paid to this rather important point and, of course, no dynamometer tests.

As a result of the rebuilding, some wonderful engines were secured. But with some of the engines, it was impossible to get up a speed of more than twelve to fifteen hundred at the most, even when running light. While the vibration in some engines seemed enough to break off the crank case arms. So the engines were pulled out and dismantled again, in attempts to locate the trouble. By the process of elimination, after having checked everything else, there was nothing left but the transmission, so the transmission shafts were checked and found to be out!

The transmission shaft was replaced with a new one, and when the engine was assembled, the vibration was decreased more than one-half. But not being satisfied until all of the vibration had been overcome, the engine was pulled down again. Of course, being short-sighted on common sense (as is usual when we get mad), the driving plate bushing was neglected. Now this driving plate bushing is really an extension of the transmission shaft itself, and is just like joining up two sections of a fishing pole, because they telescope one another.

One of the driving plate bushings was replaced by a mechanic using a regular 15/16 inch reamer but, when it was put together (after having been reamed in this manner), it did not line up at all. In fact, being a good fit on the transmission shaft prevented it lining up at the flange, on the extreme diameter of the driving plate where it bolts onto the transmission brake drum.

This proved that it had not been reamed true but, not having any other special fixture for the purpose, a special reamer was made by grinding down an old brake drum reamer (the reamer blades to 15/16 inch and the arbor or shank to .999 inch). Then, by using a brand new brake drum for a fixture and bolting the driving plate to the brake drum, and using the arbor through the-two bushings of the brake drum, this gave perfect alignment to ream the bushings.

In this same position, the driving plate was removed and turned around, one bolt hole at a time, thus proving that it had been reamed correctly and at the center.

The driving plate bushing was now replaced, the engine assembled, and put back on the dynamometer. To the surprise and amazement of all, the very same engine could be speeded up to 2500 and then even to 3,000 revolutions per minute, with not enough vibration in it to tip over a lead pencil placed on top of a cylinder head bolt. This same engine would throttle down so prettily with a standard Ford carburetor that it was almost unbelievable.

Of course, after making these discoveries, every other engine was then checked up on these same points, and each engine ran from 2,500 to 2,800 r. p. m. the first time.