Autoblog Maintenance 301: Transmission R&R, Part IV
Follow along as we walk through the process of building a six-speed manual trans, and hopefully you'll realize that it's a fairly straightforward task when armed with the proper service documentation and a handful of tools. To start off, we take what Tremec refers to as the midplate (it's actually the front cover) and place it face-down on a pair of sawhorses (alternatively, a workbench with a hole cut in it to accommodate the input shaft can be used).
The first step is to set the mainshaft endplay and countershaft preload. This adjustment is established using shims under the bearing races; to determine the proper shim clearance, we start by dropping in the races without shims.
The input shaft is set into place.
We then drop in the mainshaft assembly (left) and countershaft (right).
The case is then set into place and secured with a few bolts. We then clamp a dial indicator to the proper fixture (a 10mm bolt with a foot-long piece of 5/16" rod welded to the end) to measure the shaft endplay. As stated by George Kreppein from Rockland Standard Gear, "Endplay tolerances are critical to the life span of these transmissions. Keep all clearances on the tight side, and you'll solve most of the performance issues of the T56."
We then start by shimming the countershaft to achieve the desired amount of preload. It had an endplay of 0.032" without any shims in place, so a shim of 0.032-0.034" would produce the recommended 0.000-0.002" of preload. We decided to run a bit of extra preload to keep things in place under extreme load. The downside to this would be an increased risk of premature bearing wear, but that's far less of a concern than the gear tooth damage that occurs when the countershaft and input shaft are allowed to move apart from each other. We picked a 0.036" shim from our parts bin to yield 0.004" of preload.
With the countershaft now preloaded, we turn our attention to the mainshaft/input shaft stack. Tremec's specs call for an endplay of 0.000-0.002", but we went the other way and selected a shim to obtain 0.002" of preload. This is done with the shimmed countershaft in place, as leaving it out while setting the mainshaft endplay will cause incorrect readings.
We performed the shimming procedure in the opposite order of that recommended in the Tremec service manual, but this wasn't simple ignorance - shimming the mainshaft before the countershaft can lead towards excessive mainshaft endplay. We believe that this is ultimately what killed our last gearbox. Time will tell if it solves our problem this time around. Those building up a gearbox for less severe applications are likely better served by sticking to Tremec's recommended endplay/preload specs.
With the shimming process complete, we drive in a fresh input shaft seal using a driver from our Kent-Moore tool kit. If the correct driver is not available, a socket of the proper diameter can be used instead, but be careful not to damage the seal.
After installing the seals, we drop the shims and bearing races into place.
As an assembly lube, we use contact solution. Actually, this bottle happens to be a convenient way to dispense automatic transmission fluid.
The input shaft is set into place for the last time, with a squirt of ATF applied to the seal and bearing.
Next, we gently set into place the countershaft.
A second set of hands then comes in handy to set the mainshaft into place, and the main shift rail along with the 1-2 and 3-4 shift forks needs to be installed at the same time. A bit of jostling is required to seat everything into place.
After this task was completed, we then installed the secondary shift rail, which handles the 5-6 and Reverse shift forks.
One more surprise - the SSR detent plate (right) is different than that used in the LT1 trans (left). We stuck with the LT1 detent plate, and added a 0.060" shim to the detent spring to add a bit more firmness to the shift feel. Some would call it "notchy", we think of it as being more mechanical; either way, it won't be mistaken for a stock Honda.
Sealant is required between the midplate and case, and so we applied Permatex's expensive - and effective - Right Stuff RTV.
We then dropped the case into place, while at the same time also maneuvering the detent lever into place along the shift rail.
The case bolts were coated with thread sealer and tightened to the recommended 26 lb-ft.
Applying a bit of threadlock to the shift rail bolts ensures that they won't come out without a fight. These get installed and torqued to 20 lb-ft.
At this point, we noticed that the Reverse light switch was different, so that was swapped out for the part from our old transmission.
With the case in place, it's time to start assembling the 5-6 and Reverse gearsets.
The 5-6 drive gears, synchro assembly, and 5-6 shift fork are the first to go into place.
The large snap rings that hold the gears into place on the countershaft extension provide ample opportunity for pinched fingertips.
We drive the 5-6 driven gear onto the mainshaft using a soft brass punch (a piece of properly-sized black pipe, slipped over the mainshaft, also works well for this task).
It's then time to drop the Reverse shift fork and synchro assembly onto the mainshaft and shift rail.
The Reverse driven gear gets installed onto the mainshaft and secured with a snap ring.
The last part to go onto the mainshaft is the speedometer reluctor "gear", with a couple of snap rings to keep it in place.
RTV sealant is applied to the tailshaft housing, and it is installed with properly-torqued bolts.
The last part to go on before the transmission is installed is the shift fork and pivot. Applying a bit of threadlock to this bolt is a great idea.
That's it - the trans is back together after a few hours of work, and it's time to crawl back under the Impala to complete this job. We'll wrap up the project in the next installment, and we're damn anxious to get back to rowing through the gears.
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