Ol’ Cotton: Wrapping Up The Transmission Swap

Nick Adams August 21, 2026 8 min read 0

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When you last left us, we had just about wrapped things up with the Tremec Magnum transmission swap in project Ol’ Cotton; our 2004 RCSB Silverado that was getting a much-needed 6-speed upgrade from the old NV3500 5-speed. But before we could climb underneath one last time to hook up the exhaust, we noticed a new massive fluid leak coming from the bellhousing area that stopped our progress, and our motivation, dead in its tracks.

The fluid wasn’t golden brown like engine oil, or bright red like the Dex III ATF the Tremec Magnum requires; this fluid was clear. That could only mean one thing: Hydraulic clutch fluid. And since it was coming from the bellhousing area, that means it was either the slave cylinder, clutch lines, or both, and we would need to undo all the hard work we did with the transmission swap to properly find and repair the leak.

The Tremec Magnum Has to Come Back Out

So after a lot of heavy sighing, face rubbing, and several choice words, we climbed back underneath the truck to pull the Tremec back out. The handy transmission jack was placed under the Magnum to support it as the transmission mount, cross member, and the transmission-to-bellhousing bolts were removed. The shifter base was removed, and for the driveshaft, we undid the connection at the rear axle, wrapped up the U-joint caps in a generous wrapping of wide masking tape to keep them from falling off, but left the yoke in the back of the tailshaft housing to keep the transmission sealed from losing the new ATF we just poured in.

A Loose Fitting Causes a Hydraulic Leak

We were heavily betting on the chance that we could do the necessary diagnosis and repair without completely removing the heavy and bulky transmission. Shifting the Tremec Magnum about six inches rearward exposed the internal clutch slave cylinder and lines so we could get a better look, and our luck paid off. The AN fitting for the Tick remote bleeder was only hand-tight, allowing a clutch master cylinder’s worth of DOT 3 to simply flow through the entire system by gravity and land on the shop floor. This is great news for our transmission swap, because it means that no parts are defective or damaged, just a simple case of “I swear I tightened that”, when in reality, we missed it.

The fitting was tightened, and the clutch master cylinder was refilled with more DOT3 fluid before we re-mate the transmission to the bellhousing. This allowed us to see if the leak was fixed before tackling the part of a transmission swap that always seems to fight you: aligning the input shaft with the clutch and pilot bearing to reinstall the Tremec Magnum.

Back in Place and Ready for Exhaust

But with the help of the tranny jack, the transmission was back in place in no time, and the crossmember, mount, driveshaft, and shifter were all reinstalled. A thorough bleeding of the clutch system verified that our leak was fixed and everything was functioning properly.

Now we could carry on with installing the rest of the exhaust system. Since the Silverado never came with the bigger and bulkier T56 from the factory (even though it totally could have and should have), we were warned that our aftermarket exhaust, specifically the Y-pipe, might need to be persuaded with a hammer to get it to clear the transmission swap and fit properly.

The headers and Y-pipe on Ol’ Cotton are 1-7/8″ long tubes with a 3.0-inch Y-pipe. It was an eBay purchase many years ago for a questionably-cheap price, but they have held up well over the years and 50k miles for a no-name brand. But, as we were warned, the crossover tube on the Y-pipe hits the underside of the Tremec Magnum and required heavy beating to squish the 3.0-inch pipe into a 2.25-inch oval to clear it.

Making Room for the Tremec Magnum

Once the Y-pipe was reconnected to the headers, we found the next issue: the Y-pipe outlet. If you remember from the previous article, the transmission crossmember works with our Tremec transmission swap with zero modifications, but it did mount the transmission higher than the NV3500 5-speed, which worked out well for our driveshaft angles. But this means that the Y-pipe outlet, which was already very close to the cab’s floorboard, was now hitting.

This made it impossible to connect the rest of the cat-back exhaust system. But, for the time being, we left it open for the first test drive around the industrial complex where our shop is located. We wanted to verify that the clutch hydraulics were working properly, the Tremec Magnum shifted through the gears, there were no new leaks, and there weren’t any new unwanted driveline vibrations.

With open headers and a gaping hole in the floor for the new shifter position, this was the loudest vehicle we’ve ever test-driven. There was no way we would be able to put any considerable miles on our transmission swap to break in the clutch and transmission; we would end up deaf with a pocket full of tickets from the local PD for disturbing the peace.

Getting the Exhaust and Floor Finished

Before we jump the gun and test drive our fresh Tremec transmission swap, we needed to address the open exhaust and the hole in the floor. We decided that a temporary fix for the exhaust would work in the meantime, as we had already planned a new exhaust system in the very near future. A universal 3.0-inch flex pipe from Walker Exhaust was used to connect the rear half of the exhaust system to the Y-pipe. We didn’t have high hopes for a quick budget repair, but it proved to work out great with very few exhaust leaks.

The massive new hole in the floor of Ol’ Cotton was necessary. The Tremec Magnum offers a few different shifter position options, and we didn’t know which position we would want to use until the project was up and running and we could test drive each option. Oddly enough, we landed on the furthest rearward position, so now it was time for some new sheetmetal and shift boot.

Fabricating patch panels requires careful planning. As we said in last week’s video, we plan on having this truck for life and don’t want it to be a hack job. The tried and true method is to use some spare cardboard and tape to make templates that we can then transfer to sheet metal. Once the panels were transferred from cardboard to steel, we used rubberized sealant and sheetmetal screws to mate them to the existing floor panels and keep the weather out. We topped it off with a cheap rubber shift boot from Spectre to completely seal the floor. Once we decide on which replacement flooring we’re going with, rubber or carpet, we will swap the flooring out with a higher-quality boot.

Clutch hydraulics fixed, floor sealed, exhaust exhausting all the way out the back; it was time to put some miles on our Tremec Magnum transmission swap and break in the 6-speed and clutch.

Breaking In the New Six-Speed

Breaking in a new clutch and transmission is simple, but difficult because you want to enjoy the new performance from your transmission swap. For the first 400-500 miles, you need to do normal street driving. No hard launches or quick shifting. This ensures that the transmission’s gears and syncros are properly broken in and the clutch disc properly mates to the flywheel and pressure plate surfaces. If it is a little disappointing to read that you can’t immediately hop in and enjoy your new Tremec Magnum, we absolutely understand, as it was the ultimate test in self-control for the first 500 miles for us.

This isn’t the end of the story for Ol’ Cotton’s Tremec transmission swap. It’s one thing to do a 5-speed to 6-speed swap on a Silverado, but this truck isn’t just for burnouts and getting the groceries; Ol’ Cotton regularly ferried us all over the country for work, racing events, road trips, and the occasional project car. So stay tuned for a road trip with the new Tremec Magnum!

TABLE OF CONTENTS:

Episode 1: Our Modern Muscle Truck Project

Episode 2: Tremec Swap Parts List

Episode 3: The NV3500 Goes To The Scrap Pile

Episode 4: Trading Five Gears For A Tremec Magnum

Episode 5: Wrapping Up The Transmission Swap (you’re currently on this page)

Episode 6: GRAND FINALE COMING SOON!