Garrett's gearbox - The Autopsy

[Aussie Mike]

Hi All,

At our recent Easter bash at Winton raceway Garrett's T56 Gearbox cried enough.

Feeling somewhat responsible since I was at the wheel when it broke I've stripped it to find out what's wrong. Garrett is currently using my T56 to keep him mobile.


I've been playing around with these boxes for a little while with the shifter relocation I worked out. I've been keen to learn more about these boxes so this is a good oportunity.

If anyone has had a look at the inside of a T56 they are one beefy box. All of the gears are about an inch wide and and everything looks like it's on steroids compared to a T5. However it looks like bits of it aren't beefy enough for the 550 Rear wheel HP Garrett is torturing it with. Garrett's gear box needs some help.

I stripped the box to find everything looked good internally. The gears were fine showing little wear as were the bearings. The synchros were showing a little wear on the teeth of the blocker rings but overall pretty good. These boxes use carbon fiber facings on the synchros.

As I striped the syncros appart I found the cause of the problem. Garrett's box had locked itself in 4th gear and it turns out this is a common failing with the T56. It's caused by the keys that the synchroniser sleeves engage with. They serve a dual purpose in that they act as a detent to lock hold the syncroniser sleve in it's neutral position and they lock the blocker rings in position in relation to the hub.

The factory keys are pressed steel and under hard shifting can break or distort causing the synchroniser hub and sleeve to jam. The ends of the 3rd-4th keys are distorted causing them to catch and hang up in the hub. The 5th-6th keys are completely broken with their ends sheared off.

After doing a fair bit of research and talking to people on both sides of the Pacific I've found the fix is in 2 parts.

1. Replace the pressed steel keys with billet steel items.
2. Avoid flat changing.

So Garrett's gearbox is getting a nice new set of syncros and billet keys in 3rd-4th and 5th-6th. It's also getting a steel selector fork for 3rd-4th as the stock aluminium one is known to cause problems with 3rd gear shifts. The steel fork was developed for the LeMans racing Dodge Vipers but is now standard fitment in newer boxes.

Here's a pic of the offending keys and the new billet ones.


Cheers