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post #19 of (permalink) Old 10-13-2011
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Originally Posted by Sharad View Post
If you remove the stops, how do you avoid bending the shift forks? Bent shift forks are a pretty common problem, according to the transmission builders and racers I've heard from.
That is what we read on the websites. You would think it would be true...

Out of the kazillion T-5s, T-45, and TR3650s I've torn down, I can't say that I've EVER seen a bent fork...

They BREAK because they are brittle.
The pads melt down and the tabs wear off.
Sometimes they just crack.
Sometimes the linkage snaps.
Sometimes a roll-pin breaks, or comes out.

The movement of the shifter is limited by a built-in stop inside the transmission. Having an external stop only acts as a backup stop.
The installation instructions for aftermarket shifters say to adjust the stop so that it just barely NEVER hits. The internal stops are very solid and don't have a habit of failing. Tremec says if you have an aftermarket shifter, to adjust the stops way out of the way or remove them so that they can't interfere with the normal operation of the transmission.

When shifting a gear there is never anything firm that the fork actually hits because the linkage is designed to not allow that much movement. The fork slides a slider ring over a ring of teeth on the edge of the gear being selected. Each synchro hub has 3 "keys" with springs under them that press into detents in the inside of the slider. The tips of these keys are engaged in notches in the synchro ring.

Generally, when a fork breaks, these keys have become dislodged and have jammed, locking that slider so it can't move. At that point, the fork is now trying to move something it can't and it may break. Sometimes the driver is able to force the slider enough to shear off the part of the key that has become jammed and it will shift again. Unfortunately, with a sheared key, the synchro can no longer operate properly.

With a T-5 open on a workbench it is easy to replicate this by sliding the 3-4 slider as far as it will go. The keys will dislodge and jam right before your eyes. The reality is that had the forks and top cover been in place, the factory shifter stops would have prevented the slider from moving that far! I believe that someone observed this while trying to figure out why they keys jam and break and came up with the aftermarket shifter stops as a solution, but the reality is that it doesn't work...

It is my opinion that the main reason the keys pop out is once the bearings get some wear on them, there is slack in the main shaft and input shaft allowing too large of a gap between the synchro hub and the synchro rings. The rings move away from the hub, and even though the fork and slider never moved too far, the ends of the keys pop up and disengage from their notches in the rings. The ring can then rotate and the key has no place to go (no available notch) when the slider tries to return to its normal position. Now it is jammed! Had the bearings not been worn, and the endplay still set to the proper adjustment, this could not have happened.

The Billet Keys for third and fourth are shaped a little differently from the originals and don't seem to be able to pop up so easily and thus, upgrading to the billet keys is a worthy upgrade.

Keep your transmission's input shaft properly shimmed and you shouldn't have trouble with forks and keys!

Stan -
1965 5.0 Coupe /MAF/EFI/Astro A-5 T-5/B303 Roller Cam

Mustang Manual Transmission Specialist
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