T56 Rebuild

Drivetrain Intro  Street Twin  T56 to TH400 conversion

Spring '02 update:  Click here for the Borg Warner T56 parts book, which should help you identify the correct part numbers for your T56 rebuild.

After Boom #3, I knew I had some serious downtime, which meant the T56 could get rebuilt to replace the hammered input shaft.  The pic below left, yet another blatantly ripped off from Joe Gervais' site, shows the damage caused by the two Street Twin disks to the input shaft splines.

The problem with replacing the input shaft is that it is exactly the last piece you get to (almost) during the disassembly procedure.  Newer Helm manuals, such as my '96 version, lack a detailed T56 rebuild section.  Ever-helpful (and resourceful!) local f-bud Alan Blaine came through with a copies taken from his '94 version.  The only super-precision steps are setting the main shaft and counter shaft clearances, and this must be done only if the bearings are replaced.  Well, replacing the input shaft means replacing at least one of the main shaft bearings, so it's clearance had to be set.  The manual calls out some pricey Kent-Moore fixtures for measuring the clearance, but Alan and I pooled our tools and talents to come up with the rig shown below right.

Not shown is the 25 year old Craftsman 1-ton bottle jack we used to apply force to the end of the input shaft.  Or the upside down paint can with the trashed rotor on it that we used to raise the bottle jack off the floor.  Kludgy it may have been, but it worked perfectly.  I had ordered an assortment of shims from preferred GM parts supplier Dal Slabaugh.  Alan and I measured the play with no shims, then with about half the shim thickness we calculated would provide the specified preload, and finally with enough shims to create 0.002" of play.   All measurements matched our calculations, so we installed the final shim stack that created 0.002" of preload (preload means there is X.002" of stuff between walls that are X.000" apart).

The only other tricky part was aligning the shift forks before sliding the main case in place so that everything fit together right.  After a couple of test fits, we nailed the secret, so final assembly was cake.

Since putting this page together I had to pull the T56 apart again, this second time for a twisted mainshaft (see Boom #?), so I took a bunch of pix showing the correct shift selector orientation from several different perspectives.  My hope is that they'll help someone avoid the panic I felt the first time it didn't want to go together.

This tranny had 36k miles and a few dozen runs at the strip on it, but all the bearings and synchros looked fine.  As a followup on the hammered input shaft, I took it to a heat treating company to have the surface hardness checked.  It turned out to be Rockwell 62C on the splines, which is plenty hard.  George at McLeod assured me that the hubs they use on the Street Twin disks are spec'ed at 38-42.  In theory, the softer hubs should have worn, not the harder input shaft  But there ya go.

Each "stock" Street Twin disk hub has 3/8" (roughly) of spline contact length (for a total of 3/4"), versus a full inch for the hub from a stock GM disk.  So the total spline contact for the Street Twin disks is less than stock.  When I told George that, he said to return my disks to McLeod so he could replace the hubs with parts having 1/2" of spline contact so I could have at least the same contact length as stock.  We'll see if it helped.

The pic below left is the main shaft (left), counter shaft (right), 1-2 and 3-4 synchro sliders (shiny pulley-shaped things between main shaft gears), and the beefy tapered roller bearings.  The middle pic is all the T56 guts spread out on my makeshift work table.  The right hand pic shows the 1-2 and 3-4 shift forks, and the 1-2-3-4 and 5-6-reverse shift rails.  Everybody needs to pull one of these things apart, if only to finally see how a synchronizer really works.

The pic below left shows how long the main shaft is, extending from just behind the front of the case all the way to the end of the tail housing.  The right hand pic shows that Borg/Warner turned a 4-speed into a 6-speed by grafting another housing onto the end of the main case to hold 5th, 6th, and reverse.  The end of the main shaft is at the top of the pic.   These are the splines that engage (and twist!) the yoke on the front of the driveshaft.  The black plastic ring is what the driveshaft yoke runs into when you push it into the back of the tranny.  Below that is the speedo "gear", and a bit below that is the rear main shaft roller bearing