Older style synchro

HDowse

HDowse

Senior Member
I’ve got the transmission out of the 37 pickup and find that all the gears look good along with the older style synchro. The synchro teeth are still full size and only show some minor wear. I'm wondering if I need to replace the 6 balls and springs since it 'clicks' into the gears OK or so it seems. Do I replace them on general principle just to be safe? Also, should I replace the inner synchro hub just to be sure? Here’s a picture of the synchro, front and back. Any advice is most appreciated and thanks the Mac VP for his book!
 

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Hunt,
Take the main shaft and 2nd gear and engage the tapers on each into the respective brass shoes on the syncro. With moderate effort each gear will be grabbed by the shoes and resist turning. Ideally the full taper on each gear should fully engage
the shoe but not go deeper. Springs and balls OK as long as there is no sludge buildup
in the spring pockets. Do not ignore the second gear bushing. 2nd gear spins on the bushing anytime the car is in motion EXCEPT when the trans is in 2nd. Among other reasons 2nd gear pop can be traced in many cases to wear on this bushing.

Charlie ny
 
Charlie,
The reason for pulling the ****** was twofold: it popped out of second and the synchro didn’t work into 2nd or 3rd. I agree on the replacing the 2nd gear bushing and found it with a small amount of slop. Also the cluster has .020 motion front to back. I’m hoping those two were the reason for the 2nd gear problem that I understand is common on worn transmissions. The new bushing and cluster thrusts should tighten things up.
But the synchro problem still has me concerned to the point it might make sense to change up to the newer style synchro and necessary gear set changes. Thanks for the help and I look forward to seeing your work for my '40 Fordor when everything comes together.
 
to fit a later style syncro you will need to replace the shifter fork to a 91 (1939) fork. your syncro looks good but according to your problem list, I would at least replace the wear parts and install new bearings, thrust washers, etc. look for wear on the existing shifter handle short snub that does the 2/3 shifting these tend to wear on the sides and causes the drop out of second and in the future do not back down the car in second or third-use the brakes, brakes are easier to repair!
 
Good advice. All new wear items are being ordered, including the thin shim for the cluster if it’s needed. Could you explain what the short snub piece is? I’ve taken the shifter apart and the round piece that contacts the fork is within the spec that Mac BP suggests in his book, just under .5” by .002. Is that the part you mentioned? Thanks, Hunt
 
Here’s the link to our webpage covering the wear issues on the shifter levers:

http://www.vanpeltsales.com/FH_web/flathead_transID_3speed-Problems-levers.htm

The six little springs inside the synchro hub are a crucial part of the design…..they need to apply a certain level of tension to the six balls. When assembled and in your hands, the synchro assembly should be nearly impossible to press the hub out of its neutral position with just your fingers. Tired springs, missing springs, broken springs, or the incorrect springs will all reduce this important degree of pressure. We often see these early synchros put back together with the wrong springs (ie the ones used with the 1939-44 three ball type synchronizer). This part of my discussion has to do with the poor synchronization during the shift into second or high gears (resulting in the hated crunch of gear teeth). It has nothing to do with the popping out of second gear question.

There are other factors that affect the popping out of second gear, including the bushing inside second gear as Charlie stated, worn out or too thin snap rings on the two ball bearings, worn thrust washers at second gear, and so on.
 
The early syncro hub has to move on the main shaft, when shifting to 2nd or 3rd there is room for the hub to move, on the shaft before the syncro sleeve moves and as Mac has mentioned above the springs are important for this to happen. If either of the brass rings move in the hub the hub needs to be replaced. The rings have dimples and they should all be deep and present. As Charlie says the input shaft and the intermediate gear syncro tapers should not bottom out in the hub. I do question the appearance of one side of your hub brass ring, the outward edge looks worn, but pictures can be deceiving.
 
If you don't have it, purchase Mac's book and follow it to a T. Be prepared to take the trans apart a few times in order to get the clearances correct. It all part of making sure your clearances are correct.

It's a crappy job, but crucial to getting it right.
 
Thanks Mac, Terry and Tim for your help. I'll disassemble the synchro hub and order the new springs and ball set. Mac, I got the second gear bushing out using your tutorial and was happy to see that it's pretty easy to do once figured out. To test clearances, I reassembled the cluster gear with the existing thrust washers, the rear one being pretty worn, and got .022 slop front to back. The new thrusts ought to take care of that problem but if not I'll add the 8M-7128 shim to the rear. All new thrusts, bushings, bearings throughout should tighten up the tolerances. Time will tell if the rebuild makes shifts smooth and second stays in gear. I'll post more questions as they come up and will follow up after a test ride. Thanks again, Hunt.
 
To reassemble the synchro, I used popsicle sticks and a hose clamp to depress the balls and springs.
 
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