Variation on a Theme by Tremec - The T5 Pocket Bearing Conversion

Last time we were here at the blog, I had just triumphantly finished my interior and taken my boy for his first ride in ol' Grace. Things were looking good here on the blog. But just like the flawless feed of an Instagram influencer, in reality, things were not flawless. Just one day after I got the interior together I experienced extreme crunchiness in the gearbox.

This was not a good crunch, like the crunch of a tortilla chip, or a deep fried hundred dollar bill. No, this was like the crunch of dead spiders, and it was present in all gears except 4th. I limped back home not particularly thrilled at the prospect of spending more time and money on a transmission that had no more than a couple hundred miles on it. In any case I knew I'd have to tear down the transmission and inspect the dead spiders within. So out it came...

Once I got it out, the teardown very quickly showed the issue. In the T5, the input shaft and main shaft are co-axial, and the front of the main shaft (91 below) is supported by a bearing (63 below) in the back of the input shaft (62 below). Any time the engine is running and the transmission is not in 4th gear (4th gear simply locks the input shaft and main shaft together) these two shafts run at a different  speeds, so when I saw the state of these bearing rollers, it became abundantly clear why there was so much noise in every gear except 4th.

T5 World Class pocket bearing as the design engineers intended

T5 World Class pocket bearing as mangled by me

The death of a main shaft

Fuzzy magnets are never a good sign, especially when accompanied by free-range roller bearings

Many little bits of mangled bearing passed between these gear teeth

I pulled the transmission apart and began to ponder what to do next. I guess I'm getting old because as I looked at the junkyard T5 option, I found myself muttering and griping about how back in my day, we could get a T5 at a junkyard for $150 and now it costs $300 for an already broken transmission. Old person or not, I already had a broken T5. Paying someone else to rebuild the transmission was going to cost me $700 or more depending on parts, and I'm a cheapskate who doesn't trust people to work on his car. I then talked to the guy who built this transmission six years ago, and he said he could sell me a new input and main shaft for $350, and then I would need new bearings and a counter shaft in addition. None of these options were really doing it for me.

A friend had mentioned that I could look into modifying my transmission to use a Cobra T5 pocket bearing. The T5 that was used in the 1993 Mustang Cobra used a tapered roller bearing in the input shaft pocket, as opposed to the loose cylindrical rollers and thrust bearing used in the regular T5. It may provide a little extra stability and strength to the transmission, but I have no evidence either way. It turns out both the input and main shaft can be machined on a lathe so that they can accept the Cobra bearing. I have a friend who recently bought a lathe who was willing to try this modification. I couldn't find any detailed drawings on to show exactly what needed to be done, so I made my own. You can find those below.

Input and main shaft stripped down and ready for machining

Cobra T5 pocket bearing

Cobra T5 pocket bearing modification

Cobra T5 pocket bearing input shaft modification detail

Cobra T5 pocket bearing main shaft modification detail

At this point I took the parts to my friend and we found his lathe to be too small for these parts. Next I turned to my trusty assistant, Craigslist, where I found an ad for a guy doing small machining jobs out of his garage. For $130, he modified my input and main shaft to meet the drawings I provided.

As I inspected the fit of the bearing into the input shaft pocket, I noticed the slot in the side of the pocket which allows lubrication to the pocket bearing was mostly blocked by the new bearing race. Out came the die grinder to open up a path for fluid flow into the pocket.

pocket lubrication access slot

precision machining

modified lubrication access slot with bearing race installed

One other (totally unnecessary) modification I made to this transmission while it was out was to add provision to read input shaft speed via hall effect sensor. There isn't really a good location to read input shaft gear teeth directly but any time the input shaft is spinning, the counter shaft is spinning as well, and at a fixed speed ratio. In my specific transmission the gearing between input shaft and counter shaft is 23/34, or about 68%. The sensor port is at the third gear location which has 29 teeth, leading to a final count of 19.61 teeth per revolution of the input shaft.

Why do this? First off, because I'm a mega-nerd. Second, recording input shaft speed would allow me to accurately understand clutch slip during take off and during gear changes. Third, an input shaft speed sensor is necessary for the implementation of auto rev-matching. I don't have immediate plans to add that feature, but I may want to play around with it in the future.

Thick pad on passenger's side

Tapped to 3/8 NPT

View into the case at 3rd gear teeth on counter shaft

 Now it was time to begin re-assembly. With over four months between teardonwn and re-assebly, also not having any experience with the internals of a transmission I was a little unsure of myself. Thankfully there are some really good YouTube videos on T5 rebuilding. I watched them over and over prior to and during the build. This turned out to be an effective preparation and the build went smoothly.

Synchronizer assemblies and assorted other parts

Main shaft during assembly

Assembled main shaft

Top: old counter shaft   Bottom: new counter shaft

Setting pre-load with laminated shims. Peel to achieve correct thickness,

Countershaft retainer intalled

Main shaft installed

Rear view of trans case, showing overdrive gears and shift fork

almost fully assembled

Measuring original shim thickness

Setting input shaft pre-load with laminated shims

All together now!

I finally had the transmission fully assembled, or so I thought. Then in the wee hours of the night as I lay awake thanks to my newborn son... oh wait, I guess I should address that. We had another baby boy! #2 was born just over a week ago. He's super cute and has quickly learned the first thing that humans have to learn: to eat with one's mouth. He's working on the second lesson: the difference between night and day. I will say that he's actually doing quite well with this as well. He still wakes up every couple hours to eat and poop, but I'm probably getting almost six hours of sleep each night. And as veteran parents know, that's actually pretty good! So he's awesome and mom is healing up so we're pretty happy with all of that.  You might wonder how I'm able to work on the car at all with a new baby and a toddler. The answer is a very kind mother-in-law who has come to stay with us and has been incredible. 

Anyway... where were we? Oh yes, I was just saying that in the middle of the night I remembered that there is a small plastic part called an oil funnel which is inserted into the back of the countershaft. When I swapped in the new countershaft I had neglected to swap the oil funnel as well. I'm not sure exactly what this piece does, but I assume it has something to do with funneling oil, and oil tends to be important in transmissions. So I popped the tailshaft housing back off and installed it. Nice and easy, didn't take more than 20 minutes.

Ok so now the transmission was all put back together for real. No extra parts on the bench and no missing parts that I could find. I'm not 100% sure why this transmission disintegrated the pocket bearing in the first place, but I suspect it was one of two things. First, I think it was a little low on trans fluid. Second, the input shaft was probably not properly supported by the pilot bearing. When I originally drove it into the pocket in the rear of the crankshaft, I drove it a little deeper than should have, so now I needed to get it back out to the right depth. 

Pilot bearing in crankshaft pocket after being driven back out to correct depth.
A witness mark on the ID indicates the input shaft may have only engaged half the bearing.

Pilot bearings can be a bit tricky to remove. I think the "right" way to do it is with a slide hammer, but that has potential to damage the bearing. I tried a method I'd heard about but never tried, and that was to pack the crankshaft pocket with wet shop towel and then use the metal clutch alignment tool and a hammer to create pressure behind the bearing an drive it out. This actually ended up being a pretty good solution and I was able to drive it back out a little more than 1/4". I'm now confident that the input shaft is fully supported.

The magic of shop towels and hydraulic pressure.

The heavy lifting came next. Getting a transmission in place is always a bit of a challenge, especially when working alone, but with a little creativity, a ratchet strap, some wood blocks, and my floor jack, I was able to get it bolted up during a lunch break while working from home. One day I'll stop fooling around and just make a transmission adapter for my floor jack and make this job easy. 

Later that night I took care of all the loose ends like bolting up the slave cylinder, exhaust pipes, and driveshaft. I also filled the transmission with fluid. The World Class T5 uses ATF (the older non-World Class T5 uses gear oil), and in the past I've typically used some sort of synthetic ATF. I'd been doing some reading on the topic of which ATF is best and decided to move away from synthetic, and actually start using Type F ATF. As long as I've owned this transmission, shifting has felt a little too notchy and slow. It always required more force than I felt was comfortable. My theory is that the synthetic ATF is too slick, and the synchronizers didn't have enough friction to shift well,. Type F ATF has some friction modifiers in the fluid to further increase that kind of friction. 

I used another trick I'd heard of but never used to fill the transmission. I drilled a two holes in the cap and inserted a rubber hose through one hole down to the bottom of the bottle. The  other end went into the transmission. I then applied about 5-10 psi into the bottle via the other hole and the pressure quickly pushed the fluid into the transmission with very little mess. I highly recommend this method.

At this point I could check to see if I was successful, but it was already 10 PM and I actually like my neighbors. Well, let me be clear: I like my neighbors so much that I only let the engine run for 30 seconds. Three feet forward, three feet in reverse and I was satisfied that nothing was horribly wrong. Then the next morning I took it for a drive. It drives as good as it ever has. The engine fires right up without any trouble, and the shifts are nice and smooth (for a T5). No unexpected noises were heard, and no terrible smells were smelled. 

That freshly rebuilt transmission feeling

Well folks, this is the Centennial post! We've got 100 posts spanning eight years. If you've been reading along the entire time I am honestly impressed. Come to my house and I'll give you a popsicle. And if you really do exist, dear reader, let me know what you want to see from the future of the blog.