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South Bend Clutch
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Technical Bulletins


Technical Bulletin From LazarSmith
OEM manual transmission clutches are designed to minimize NVH (noise, vibration and harmonics) to meet the demands of the marketplace, but this nod to a plush driving experience in “heavy-duty" pickup trucks carries a high cost in terms of durability and performance.

Retrofitting one of these factory DMFs (Dual Mass Flywheel) clutches with a true heavy-duty solid flywheel aftermarket clutch (South Bend Clutch is the prime example) should be performed with the following in mind:


Diesel ignition pulses (the Cummins generates a particularly strong harmonic environment) can no longer damage or be dampened by the DMF, soft organic clutch lining or concave-apply pressure plate; so expect to hear gear rollover from the transmission and possibly rattle from the clutch disc(s) and/ or center plate. Though harmless and totally normal, gear rollover (allowed by the tooth mesh's required backlash) is drastically reduced by refilling the transmission case with a quality gear oil - be certain it is both compatible and effective with the transmission internals). For example the Mercedes G56 – installed behind every HD Ram's manually-shifted Cummins since 2005 – is filled with ATF (Automatic Transmission Fluid) by Dodge… not the best choice in a manual gearbox. TIP: Overfilling the transmission case with GL-6 gear oil is the best way to reduce gear tooth friction, wear and noise.


Since the OEM DMF masks noise indicative of transmission condition and problems, inspecting the input shaft for end play is necessary; although some end play is normal on a used transmission, (small amounts do not present a functional issue except for high-power high-load applications) – remember greater end play can increase clutch and transmission operating noise, and if large enough will cause clutch engagement and disengagement issues. The G56 input shaft, for instance, should not touch the throw out bearing retainer/ tube/ sleeve when laterally wiggled (it's too difficult to measure axial end play without the correct tools), nor should wiggling the input shaft make noise - or slower shifting from delayed synchronizer lockup can occur. Zero shaft end play is ideal and should be expected from a rebuilt or remanufactured transmission (countershaft, main shaft and input shaft tolerances are all adjusted from inside the transmission case). TIP: G56 disassembly and adjustment should only be attempted by a trained, experienced professional.

Engine Operating Range

Clutch and transmission durability are maximized by shifting at higher engine RPM – drive train stresses caused by high torque at low RPM are very hard on parts longevity - 500 HP at 3000 RPM is far easier on the clutch and transmission than 500 HP at 1500 RPM. Case in point: there's no good reason to short-shift a Cummins common rail when loaded – even in stock form, the engine ECM will fuel through 3200 RPM, and the G56's gear splits keep the engine well away from damaging low-RPM peak torque. Growling vibrations audible when heavily loading an aftermarket clutch with a CTD result from operating the engine close to the 9th order of torsional harmonic for that length crankshaft – not at all desirable, and an indication to downshift or reduce throttle position. TIP: The OEM DMF LUK clutch can be compared to a fusible link for the drive train to help move the truck through the warranty period.


Since turbo diesel common rails use pilot injection to help control engine combustion noise, unmasking the ignition pulses by deleting the DMF often makes those combustion events more audible – especially in the case of worn OEM fuel injectors or upgraded aftermarket injectors with higher-flowing tips; obviously, those stronger ignition pulses translate to more clutch and transmission noise. ECM tuning – the engine control module's software that senses and dictates all engine operating parameters – has a significant effect on the severity of ignition pulses (combustion events) and therefore a large impact on how much noise is allowed by an aftermarket clutch. TIP: Custom ECM tuning (such as EFI Live) has many benefits besides reduced engine and drive train noise.

High HP Applications

Mile Lazar 2.26.13

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