Diesel trucks equipped with manual transmissions can’t be beat when it comes down to towing. Many diesel owners prefer the stick on the floor over a shifter in the column, even though there are many excellent automatic transmissions. Manual transmissions have existed for many years, but the good news is that the technology has continued to improve, both on the transmissions as well as the crucial link between them. There’s a lot of new technology in clutches targeted towards the diesel truck market that address the concerns of both the diehard stick crowd and performance enthusiasts, but it all boils down to how to best get the power to the ground.
With the big power that modern diesels are putting down these days, especially once a few aftermarket modifications are added to the mix, it’s easy to understand why most stock clutches aren’t up to the task. Cummins’ dual-mass flywheels have reliability issues, but when you put a lot of weight on a slightly warmed-up Cummins engine, clutch problems are inevitable. Multi-disc systems are popular because they can handle extra power and heavier loads. But many multi-disc systems have issues that make them difficult to drive. They may cause noise, or rough engagement. Centerforce offers the Diesel Twin multi-disc solution. It provides superior holding power up to 1,700 lbft and everyday driving ability of a one-disc.
We traveled to H&H Diesel Performance in Dewey, Arizona, where we had the opportunity to watch a 2005 Ram get the ultimate solution to rough shifting problems and a major upgrade in clutch holding power all at the same time. We didn’t have the opportunity to witness the drivability problems the truck had when it came in, but we did drive the truck once the repairs were complete. We can honestly say the clutch “feel” of the Centerforce twin-disc system was exactly as it should have been: it felt no different than a stock clutch when it came to shifting, engagement or anything else that would indicate that the truck was now equipped with a multi-disc clutch. Find out how to fix the shifting issues of this Ram truck. DW
1 The subject of our clutch upgrade was a 2005 Ram 3500 4×4 dually. The truck’s owner uses it for hauling equipment and heavy things in its bed. This truck has a few mild performance upgrades. The truck’s shifting problems gradually became worse. The owner wisely chose to take the truck to H&H Diesel Performance in Dewey, Arizona, one of the better-known diesel shops in the region. H&H handled the diagnosis and wrenching duties for this article.2 Truck owner Mike Reidhead reported difficulty in shifting gears. Initially thinking the problem was in the clutch, the techs at H&H tore down the truck and discovered that the clutch, while showing some wear, was in good shape. The six-speed G56 had problems, as was revealed by a closer inspection. The tranny was sent out to Red’s Transmissions in Buckeye, Arizona, for a rebuild and came back a few days later looking better than new. The exact cause of transmission problems could not be determined, but the synchros had been heavily worn. There were also signs that the tranny may have run out of fluid at one point. H&H recommends checking the input shaft carefully for excessive play, which is indicative of front transmission bearing problems.3 The truck was equipped with an aftermarket clutch because of previous problems with the dual mass factory flywheel. It had been used for a considerable amount of time. Though it’s always a tough call to replace a clutch that’s within specifications, the owner wisely chose to go ahead and replace it. Bill Hughes from H&H handled final removal of the clutch and flywheel assembly, as well as the rest of the wrench-spinning on the project.4 The multi-disc Diesel Twin clutch from Centerforce is a pretty serious unit that’s capable of handling up to 1,700 lb-ft of torque. The system includes a one-piece flywheel to replace the dual-mass flywheel. It also includes a pair clutch discs, as well as a floater and pressure plate. Multi-disc clutches have a great reputation, but also for being noisy and requiring a lot of pedal force. Centerforce has overcome the negative traits of a traditional multiple-disc clutch to create a clutch which is both powerful and works like a normal clutch.The drive floater can make noise when it is engaged. It tends to ratchet against the pins or studs of the flywheel. Centerforce’s solution is to use a spring-loaded bushing on the floater disk that engages the four pins of the flywheel. (The small pin in between the two larger pins at the center of the photo.) This bushing removes the small amount slack from the pins, resulting in a smooth engagement without the usual rattle. Not only that, but the grooves and the holes drilled into the floater help to dissipate any heat.Centerforce also uses a dual spring hub on the clutch discs to reduce noise and rough engagement. Two lighter springs can be seen in the disc’s hub, as well as four heavier and thicker ones. This dual spring system helps reduce the amount of noise generated by the G56 transmission. Centerforce also manufactures the clutch material in-house. This and other tricks result in a clutch with the holding power of multi-disc systems, but the driveability of more traditional clutches.7 No internal transmission modifications are necessary to handle the increased thickness of the Centerforce Diesel Twin system, but there’s one small modification needed for proper clutch fork geometry. Centerforce recommends removing a flat washer from behind the fork pivot and installing the new throw-out on the original clutch. A new throw-out bearing should be a part of any clutch replacement, whether it’s supplied with the new clutch or not. Attention: Pay attention to the instructions when installing the throw-out. A poor installation can result in a growing noise when you depress the clutch pedal.As was mentioned previously, the truck already had an aftermarket flywheel installed to replace the problematic factory dual-mass clutch. Hughes still had to remove his old flywheel, as the Centerforce kit included a new one which must be used in conjunction with the Diesel Twin Assembly. Keep a hand on the flywheel during flywheel removal, as it’s heavy enough to do some serious damage to feet and/or concrete if it falls off!Hughes advises that you inspect the rear main seal carefully once the flywheel has been removed. They have been known to get pushed out if the harmonic balancer is worn out, and if there’s any sign of leakage, the time to address it is NOW. A new rear main now won’t add much to the repair bill, and a leaky rear main will destroy a new clutch assembly. Fortunately this truck’s rear main was perfect, even with more than 100,000 miles on the engine.The Centerforce flywheel is preassembled, with all the pins and spacers already installed. It’s ready to use. All of the hardware is included and it’s all genuine ARP fasteners. Hughes torqued the Centerforce flywheel to 105 ft lb after installing it on the crank. He used a special tool to hold the flywheel, although a second assistant holding the crank would be sufficient if you were doing it at home.11 Everything is pre-assembled with the Centerforce kit, but there’s some assembly required when installing it in the truck. The clutch discs must be installed in the correct position. They are not just side-specific. To eliminate confusion, Centerforce labels all parts.12 When assembling, pay attention to the labels on the clutch disks and floater discs. Hughes’ work was double checked at every stage. Also note the clutch alignment device that was included with the Diesel Twin. This is a great touch as you don’t have to go out and find one.The pressure plate is installed after the floater disk has been positioned between the two clutch discs, with the correct sides facing each other. It’s critical that the pressure plate be torqued properly to 35 ft-lb using the supplied fasteners so that the clutch will work properly. It’s quite an assembly, so be sure that the pilot tool is easy to remove when the clutch is torqued to spec.14 Check the alignment of the paint marks on the pressure plate, the floater disk, and the flywheel as a last check. Centerforce balances and marks each component individually after balancing the clutch assembly. This step is another way to eliminate noise and problematic engagement.15 It’s impossible to show all of the tricks that Centerforce incorporates into the pressure plate to further counteract the traditional complaints of a multi-disc clutch system, but there are several. They have used their 30 years’ experience in designing this system, from the innovative use of ball bearings that enhances both clamping force and smooth engagement to the more conventional (for Centerforce), weights placed on the fingers of pressure plate. The weights are placed further away from the finger tips than on a conventional gas engine. This is to compensate the slower rpms of a Cummins engine. It is enough to say that a lot technology was put into the Diesel Twin clutch system, knowing that diesel drivers would push it to the limit.The Centerforce system requires two other minor modifications. Both are included in the kit: a spacer for starters and a clutch hydraulic replacement system. The starter spacer can be bolted in. Due to the thicker assembly, and the different timing for the engagement of the twin-disc systems, it is necessary to use the clutch master and slave. Do not disassemble the clutch hydraulics for installation; it’s pre-bled and must be installed as one unit. Don’t worry about replacement parts, though; everything is genuine Mopar.17 To demonstrate just how easy the hydraulics swap is, the clutch master cylinder installation doesn’t even require a wrench. After disconnecting the master cylinder from the pedal, turn it counterclockwise by a quarter-turn. Install the new master cylinder using the same mounting plate on the firewall. Use caution when fishing the slave cylinder with the plastic line down the driver’s side fenderwell. The slave cylinder installation on the tranny is also an R&R procedure. Normal circumstances should not require the hydraulic system to be bled.18 With the clutch installation done, it’s a simple matter of bolting everything back up in the reverse order of removal. If you have the right equipment and are in a tight spot, it might be best to separate the transmission and the transfer case, and install each separately. Assembling them together can be awkward. Hughes recommends installing trannys and t cases individually. The tranny was difficult to install against the back of engine even when installed by itself.In the course of researching this article, a small controversy was discovered. Centerforce says. Dodge recommends using full synthetic ATF+4 for this application. Mercedes, the manufacturer of the transmission, recommends gear oil 75w90. Plus, using the fill plug position for the transmission can cause the tranny to be “filled” half a quart below the recommended capacity for both companies. For this project, ATF+4 in the OE-specified amount was used. The truck for this article was in need of a rebuild after 120,000 km despite having been regularly serviced by the owner. He purchased the truck used.20 Reassembling the switch tower and shifter is the final step in the installation. Installing the shift tower and stick is simple as long as your transmission is in neutral. Hughes spent more time on the console’s re-assembly than the actual shifter. After installation, a test drive revealed that the truck shifted like a normal clutch. Engagement was progressive, easy, and there weren’t any funny noises.
