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interchangeability between 38-39 and 40-47 40 series transmission synchro drums

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For years there has been a lot of confusion surounding the use of 38-39 and 40 and later synchro drums in 40-50 series cars. Both styles are nearly impossible to find but a 38-39 owner will have a better chance of finding drums if 40 and later will work. The short answer is yes, 38-39 models can use 40 and later synchro drums. In the Master Chassis Parts Book 38-39 has 1 part number and 40 and later has another. The reason for that is there's a design change between 39 and 40. 38-39 synchro drums had the bronze inserts spin poured into the steel shell and then the bronze friction surface was machined out in a process similar to a babbit connecting rod. 40 and later had the bronze friction surface press fitted into the steel shell. Personally I prefer the 40 and later synchro drum in my 39 because the bronze friction surface in the earlier style has a tendency of delaminating from the steel shell and spreading bronze fragments in the transmission case. Thanks Leif

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My understanding part of the 1940 design change was a change of angle. Will search for the data.

One 39 owner I know machined a later drum to the 1939 angle


From an email with another 1939 owner (a very good mechanic-engineer)




The biggest lesson I learned from all this, the synchros MUST be hand tested with the gear that they are going on.  The '39 and '40 only vary by one degree, but I can tell you they will not work together (gear clashing).  This does not require any special equipment - for example 2nd gear.  Put the gear on a solid flat surface, put the synchro drum on, push down on the drum and try to rock it back and forth in several different positions.  If you get any rocking IT WILL NOT SHIFT PROPERLY!.  It must seat solidly, and when you try to rotate the drum, it should "lock" so you can not turn it.


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This is an important question and I appreciate any feed back from folks on the Prewar Buick Forum. I could be wrong and if I added inaccurate information on this forum I appologize. I am aware there is supposed to be a 1 degree difference between 38-39 and 40 synchros. I have nos for both types. I looked for the angle difference and couldn't find it. It is true  the drums must seat solidly or they will not shift properly.  I've had good luck running 40 synchos in my 39 case for over 30 years without a problem. At the time the drums were used so maybe they were worn enough  so that a difference in 1 degree angle didn't matter. Does anybody have access to a Muncie parts book? Apparently Muncie has the same part number listed for series 40 synchros 38-47, but since I no longer have access to a Muncie parts book  I can't prove it. Whoever is right really doesn't matter as long as members of this forum are provided correct information. Love you guys love the forum.             Thanks Leif  

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