Garrett's gearbox - The Autopsy
 

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.
http://www.clubcobra.com/photopost/d...11409keys1.jpg

Cheers

Flat changing? Help this poor Yankee out, WHAT is that? :D

Hey Ernie, that is when you change gears without taking your foot of the accelerator pedal, Holding it flat.

Mike,
Since the T56 is a production car gearbox I cannot imagine that it was designed to be flat changed to begin with, especially with 550rwhp. That's not something I would consider doing unless the box was designed to do it like a V8 Supercar box. I'm not surprised the syncros gave up. You were just unluckly that the box gave up when you were driving it.

edit: Assuming that the box was flat changed.

top man
 

Well fellers what can i say but what a great bloke to have on your side ( expecially on your side of town )

I was really sad that after the flogging i gave the car all day Mike's turn resulted in a little bit of gear box problem ( recon i timed it just about right for Mike to be in the car !).

I really didnt want mike to spend the time in fixing the box as i know his project needs to keep rolling, but his keen style saw him wanting to take on the project, i think really so he could make sure his box was ok, and think its going to get a upgrade as well.

Looks like its another part of my car thats going to have a little "MM was here" stamped on it.

Thanks again Mike.

Mike's first official pushing pit crew member, Garrett.
Think i'll be water boy too.
Tyre pressure checker,
Windscreen de-bugger,
Rim Brake dust remover,
Undie washer !!!!??




%/

Haha, Garrett!

Great to see the car is on its way back to its best...

Great work Mike.

Todd

I don't think Garrett was flat changing but he has a take no prisoners driving style. and he's making obscene amounts of torque and HP for a box rated at 450 ft/lb

Yep Boxhead is right "Flat Changing" is banging it into the next gear without lifting the throttle or pressing the clutch. It's a pretty destructive technique on more than just the keys. A lot of the guys drag racing LS1 Vettes and Camaros in the US are guilty of this technique whilst trying to save a few tenths.

Going by the dammage to Garrett's keys I reckon the billet ones will be just the ticket. The standard keys aren't especialy distorted just the ends dammaged.

Cheers

Mike, this is a common problem in tremecs also. I had those little rectangles break the tips off too. It is from downshifting in my case, not the upshift. Where did you get the replacement parts? Are they custome made or does somebody sell them? Scott

"Flat changing?" You guys sure talk funny Down There sometimes. I used to do that way back when we called it "Power shifting" in Californy. But when I put my new 11" slicks on my old Stingray (327") my first try blew out the spider gears in my 4.11 Positraction. I don't do that no mo.' :p Rich

Flat changing does require a clutch... drag racers us this technique as generally they dont use anything but full throttle the whole way down the track and use the clutch to limit wheelspin, get the car back straight, etc.

Flat changing can have a tendancy to launch your car into a spin if you do it on an unpreped track. (I think that happened at Wakefield last year in the drags on Saturday yeah?)

I heard a rumor of a certain someone bashing the gears and having abit of a spin when his car came on boost from the lights...

If you run a management system that can monitor throttle, clutch position, and relative wheel rotation speeds, you can set it up to assist flat changes. MoTec does this.... I'm yet to see if it works...

Mike, if it doesn't, I'll be calling for a set of key's... at least :rolleyes:

With an air shifter (drag racing) the rev limiter is set to momentarily cut the ignition at the moment of the shift. No throttle or clutch action is required. I've heard this actually less severe on the drive train than using the clutch in high horse power applications.

Street cars with a standard 'h-pattern' gearbox still have to engage the clutch though.

I am aware of the Motec system that is used in V8 Supercars and the Lenco Airshift that is used in drag racing.

My brother in law used to have a 427 bb chev in a HK Monaro that had a 'clutched' auto, no converter slip with that baby!!!!
Easy to flat change too.....

Ive got one of those Clutched Turbo 400 boxes in the back of the shed. They run a Ford long style pressure plate inside a lakewood steel bellhousing. The pump on the aut is driven by an impeller that engages between the fingers on the Long clutch plate.

The box has a full manual valve body and a reversed shift pattern so instead of being P-R-N-3-2-1 its P-R-N-1-2-3. This is so you can shift up like a normal manual box using a ratchet shifter ie neutral 1-2-3 and then not accidentally knock it into neutral. These boxes were popular in drag racing befor the Lenco became readily available. They have also been made obsolete for the spotsman classes by the extensive devopment done on the power glide.

I still like the idea of a Clutched Auto and plan to use it in a nostalgia drag car at some time in the future.

Cheers

Hi Scott,

I got the parts through THE GEAR BOX They also do parts for the 5 speeds. Heres a link to their web page: The Gear Box

Cheers

My flat changing experience resulted in a little more than broken steel bits in my gearbox.....