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I have a1952 Mercury M1 with 239 flathead and I understand the 3 speed Trans is out of a 1950 or earlier truck. I have replaced the synchro, 2nd gear and the input shaft, the trans shifts okay from first to second but grinds badly when downshifting from 3rd to 2nd . I have tried 90 w gear oil 85 140 gear oi,l adding Lucas oil stabilizer and have also tried adding friction modifier but grinding continues. The reason I replaced the input shaft is the synchro ring and input shaft both showed some discoloration ( very dark ) when I disassemble it last time . It almost seems like it is not completely disengaging from 3rd as it tries to go into 2nd . Hoping someone has run into this before or may have an idea. Thanks
Specified lube is straight 140 grade GL-3 or -4, but I doubt changing to that will make a difference. Unless you are using GL-5 lubes, then it is possible the lubes are attacking the bronze blocking rings, but that usually is a long-term problem. Do you assume it's an earlier trans because it's a floor shift? StaLube makes a good GL-4 140-wt and it is available at NAPA and on Amazon:
With the top off the trans (assuming it's a floor shift), you can measure the gap between the blocking ring and the synchro hub. The gap should be about 0.060" for a new ring. If the gap is OK, you have a different problem, maybe excessive end-play. Photo courtesy VanPelt Sales, a great source for trans parts and also a 3-sp rebuild manual (vanpeltsales.com)
Thanks for your info, The gaps between the teeth are within specs. I was also wondering about end play but thinking maybe it is to tight,
There is 1 shim behind the rear bearing and that has not changed but I have removed the front and rear gaskets and only using silicone which I guess could tighten it up . Also wondering if I have main clutch maybe hanging up but I have no issues with any other shifts.
Did you replace the 3 struts in the synchro assembly (7A044 below)? If so, I found the modern replacements have ends are a little wider than they should be. I dressed them with a file and noticed an improvement.
The struts' ends were too tight a fit in the recesses of the blocker rings, and didn't move smoothly in the synchro hub. If you have your old struts, look at how they are made (assuming they are repro's too). They are basically folded down to create the sides and ends, The folded parts forming the ends had sharp edges. I believe the OEM struts were seamless.
So that's strange, my Synchro has much narrower struts and they are cast . It was purchased from Early Ford and everything I read is there is only 1 synchro for this trans but maybe it is different because it has a different manufacturer...not sure or maybe that my problem.
Are you comparing them to the exploded view above? That makes them look wide,they really aren't. They should fit close in the slots in the hub, a "running fit". Are they cast bronze? Seems to me my originals were. My replacements were from VanPelt's.
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