Ol’ Cotton: Trading Five Gears For A Tremec Magnum

If you’ve come this far with us, you’ve seen the parts list for the transmission swap, the old NV3500 5-speed get ceremoniously thrown in the local landfill, and you’re finally ready to hear how we’re going to go from five gears to six in our 2004 Silverado named Ol’ Cotton with a Tremec Magnum 6-speed swap.

The Flywheel

It’s good practice to do a mid-job cleanup and reorganize your tools after you finish the disassembly part of the swap. Disassembly is usually easier than the actual Tremec Magnum swap, but it’s messier with dirt, oil, and crud falling on the ground. This is also a good time to fix any engine oil leaks like the oil pan, the rear cover, or the rear main seal. An engine oil leak can easily get to the clutch disk and ruin it, and trust us, you don’t want to remove and reinstall the Tremec Magnum more than you have to.

https://youtu.be/QN2JJ083HWA

With the old pilot bearing out, it’s time to tap the new one in. The pilot bearing used for this swap is the GM small pilot bearing and can be done carefully with a small hammer and a socket that is the same diameter as the bearing. It is possible to damage the pilot bearing during install, so don’t go full Hulk with the hammer and only tap it in far enough to be flush with the surface.

Before you install the flywheel, do a quick check that the back of the flywheel and the crank mounting surface are clean. Wipe or brush off any surface rust to make sure the flywheel sits flat and true. The flywheel is held on with six bolts that are torqued down in three stages in a star pattern with a 15mm socket. The first torque is 15 ft/lbs, the third step is 37 ft/lbs, and the last is 74 ft/lbs. If you are using ARP flywheel bolts, they will use a different socket size and include instructions for a slightly different torque spec.

You will need a flywheel holding tool to help you torque these bolts; otherwise, the engine will spin over before you reach the required torque. There are plenty of different tools out there for this job that work great. We used a sturdy piece of plate steel and two bolts to lock the flywheel in place with a pressure plate bolt and bellhousing bolt.

C5 Z06 Clutch Kit

Once the flywheel is torqued down and you remove the holding tool, you will need to clean off the clutch surface of the flywheel and pressure plate before installing the clutch. It is very important to wipe off any oils or rust-protectant chemicals that the clutch kit was shipped with, as these will inhibit clutch break-in and life. A good brake cleaner and a clean, lint-free towel is recommended. 

Using the clutch alignment tool, place the clutch disc against the flywheel, and take careful note of which side is facing in or out. The clutch disc will be marked with a “flywheel side” and/or “pressure plate side” to tell you how to orient it.

The pressure plate is installed over the clutch disc, and the six pressure plate bolts are installed hand-tight, at first, with a 13mm socket. As you carefully tighten down each bolt, you check to make sure that the clutch alignment tool can still easily slide in and out. If not, you will not be able to slide the Tremec Magnum in all the way. The torque spec for the pressure plate bolts is 52 ft/lbs.

Modifying the Slave Cylinder For The Silverado

Before the bellhousing and transmission are bolted together, the slave cylinder needs to be bolted to the faceplate of the Tremec Magnum. The slave cylinder is a genuine GM 98-02 F-body part with a couple modifications to make it work for Ol’ Cotton’s swap.

The first mod is the Tick remote bleeder valve, which is as simple as unscrewing the stock shorty bleeder and installing the Tick. While this mod isn’t required, it almost should be. This makes bleeding the clutch way easier, and it is very affordable.

The second change is swapping out the clutch lines. Our clutch pedal, clutch master cylinder, and clutch master cylinder hydraulic line are all stock GM pieces that came with our factory 5-speed Silverado. The clutch master cylinder is the correct size to match up with the 98-02 F-body slave cylinder that we’re using with our Tremec Magnum swap, but the 98-02 F-body slave cylinder that we have to use doesn’t have a clutch line like the 99-06 Silverado slave cylinder.

A few gentle taps with a punch to the retaining pin on the Silverado and F-body slave cylinders will push the pin all the way through, allowing you to remove the clutch lines from the slave cylinders. Luckily for us, GM kept a lot of these dimensions similar between different applications, and you will notice that both use the same style end and O-ring seal. So it is as simple as swapping the Silverado slave cylinder line over to the F-body slave cylinder, and reinstalling the retaining pin with a few gentle taps from the hammer, and you’re done with slave cylinder mods.

When bolting up the freshly modified slave cylinder, it centers on the machined face of the faceplate on the Tremec Magnum and is held in place with just two small bolts. These bolts are torqued to 71 in/lbs (or about 6 ft/lbs) with a 10mm socket. Be careful not to over-tighten these two small bolts.

Installing The Bellhousing

You have the option to mount the bellhousing to the Tremec Magnum and install it as one unit, or install the bellhousing separately to the block before installing the transmission. We tend to mount the bellhousing to the block first instead of the transmission.

Installing just the bellhousing on the block makes it easier to reach and torque the upper bellhousing bolts. It also makes installing the Tremec Magnum easier because it will be less bulky and slightly lighter. The bellhousing is held into place by eight bolts that are torqued to 36 ft/lbs with a 15mm socket. Make sure any wiring and hoses are out of the way so they don’t get damaged and the bellhousing mounts flush.

The Tremec Magnum Finally Lands In Ol’ Cotton

This is the moment we’ve all been waiting for, mounting the Tremec Magnum in its final resting place in Ol’ Cotton. This bulletproof 6-speed will put the ‘man’ in our man-pedal equipped Silverado that Ol’ Cotton deserves… and it’s long overdue.

You will need to remove the shifter base before installing and cover the hole with masking tape to keep any dirt or dust from falling into the transmission. Also, leave the tailshaft plug from shipping in place, just to be safe while handling it. Then place the Tremec Magnum on a proper transmission jack. If you watch our full install video on YouTube, you will see that I learned the hard way about trying to muscle a 147lb transmission by myself and how impossible and unsafe it really is. Whether you have a two-post lift or are doing it alone on the floor like us, you will need a transmission jack.

The reason for a transmission jack isn’t because you need help with force, sliding the Tremec Magnum in place is just done by hand, it is to give you a tool that can help you hold that bulky weight while you align the input shaft to get it to slide without much resistance into the clutch disc and pilot bearing, and that can take some wiggling, giggling, and most importantly: patience.

Trying to force the Tremec Magnum can easily result in damage to the clutch disc, pilot bearing, slave cylinder, or worse. It is very important to never try to use the bellhousing bolts or transmission bolts to forcibly install the Tremec Magnum. This can result in a broken engine block, bellhousing, or transmission case. Once you finally get the 6-speed mounted, by hand, you can install the eight bolts and torque them to 36 ft/lbs.

The Crossmember And The Angles

If you have a GMT800 truck that was equipped with an NV3500 5-speed like Ol’ Cotton was, you can install the transmission crossmember, unmodified, with a new transmission mount, to complete mounting the Tremec Magnum in our project truck. The four large crossmember bolts and nuts are torqued to 52 ft/lbs with a wrench and socket that measures 18mm or 21mm, depending on the year of your truck.

We spoke in our previous articles about the importance of using cribbing, or, in our case, a spare set of wheels and tires, to lift Ol’ Cotton off the ground, but still under its own weight. The reason is so we can properly measure the driveshaft and the driveline angles, which has to be done as the truck sits.

You have to make sure that the transmission output shaft and the axle pinion are sitting at the proper angles so they are parallel to each other. This keeps driveline vibrations at bay, which can cause serious damage over time. In our case, the rear axle pinion is pointing up at 2.5-degrees and the freshly installed Tremec Magnum is pointing down at around 6.0-degrees. This isn’t too far off, so spacing the transmission up using some spare metal between the crossmember and mount will bring it close to the same 2.5-degree measurement we want.

Measuring For The Custom Driveshaft

How you take measurements for your new driveshaft will be determined by whoever you choose to build it for you and your Tremec Magnum swap. Ours was ordered by Precision Shaft Technologies, and the instructions were simple: measure the total distance in inches from the tip of the output shaft to the centerline of the rear U-joint on the axle.

It is very important to get this measurement right, as a new custom-made driveshaft doesn’t take up a big chunk of the Tremec Magnum swap budget, and there is zero return policy on a custom driveshaft. If you mess this up, you have to own your mistake and buy another. So, with our gut full of anxiety, we measured thrice and ordered once.

The measurement came to 58-5/8 inches, and due to PST’s popularity from their growing reputation, it took almost four full weeks for the custom, heavy-duty, 4-inch diameter aluminum driveshaft to arrive. And our measurement was proven to be perfect as we slid the yoke into the tailshaft housing and bolted the U-joint straps on the axle into place. The four small bolts on the rear yoke are torqued to 19 ft/lbs with an 11mm socket.

Making Room For The Shifter

If you measure the Tremec Magnum and NV3500’s shifter positions and compare, you will notice that the Magnum’s most rearward possible shifter position is around eleven inches further back.

This usually isn’t a problem; just take some careful measurements, cut a new rectangular hole in the floor, and cover up the old hole from the NV3500’s further-forward shifter hole. But we were unsure of which possible shifter position we were going to choose from all the possibilities with the Tremec Magnum.

So, with a heavy heart, we took a fresh cutoff wheel to the tunnel of Ol’ Cotton, and cut a massive new hole. This will allow us to test each possible shifter position configuration and then cover the hole back up with custom patchwork and new flooring when we’re done.

A Fluid Leak Brings Ol’ Cotton Back Apart

At this moment, we would’ve liked to tell you about reinstalling the exhaust, firing up Ol’ Cotton for the first time in two full months, and ripping down the road, shifting our new Tremec Magnum 6-speed with joy. But our luck has run short on this swap. Before we could set off, our project Silverado started puking fluid on the ground. We couldn’t identify where it was coming from, which only meant one thing: we had to undo all of our hard work to get the 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 (you’re currently on this page)

Episode 5: COMING SOON!