Glenn Miller - At Last!

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3/26/09

Quick Tips - How to Identify Post and Pre War Hydra-Matics



If the original transmission is in the car, and shifting properly, perhaps best to leave well enough alone. But for how long, who knows? Now, possibly, the trans has been replaced with a postwar unit. There have been 60+ years for this to have happened!

The quickest giveaway is to peek down at the transmission from the drivers side of the engine. On the postwar models, there is a large Hexagonal plug-like thing at about 1:00 on the transmission as youre viewing it. Thats not there on the prewar transmissions. Its the pressure regulator valve.

On the controls, the selector detent is entirely in the steering column hub, where the indicator is on the prewar models. If the detents are light and not sharp, its a prewar box. The detent was moved to the control valve body inside the transmission after the war, and is very pronounced. Another of the many improvements made in 46 was the addition of a "reverse blocker piston" in the control valve body. This keeps you from yanking the lever into the reverse position until after the transmission engages. Even then, the rear wheels must be OUT OF MOTION before you can drop it into reverse. Whats happening here is that the drum of the reverse planetary must be stopped for a pawl to engage it and lock it. That wasnt there on the prewar jobs, and many a reverse planetary and reverse pawl were chewed up.

In order to make the transmission easier for rocking the car in snow or mud (Heaven forbid!) a new cone clutch reverse engager was incorporated in the transmissions from 1951 on. I have a 51 transmission in one of my 41s, and you can yank the selector into reverse as soon as the engine fires, with no problem. On that one, the 2-3 upshift is a little brutal, but I can live with it easily. The other two 41s have 49 Cadillac boxes in them, and Ive never driven a more enjoyable transmission.

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