Don’t let your transmission slip away

Unlike many teen drivers, I can only remember deliberately “popping a clutch” once — in a 1949 Ford pickup — just to have the experience. My reluctance to abuse clutches is probably due to growing up working in my dad’s scrap yard, where I drove our 1946 Reo (civilian) 2-ton truck and operated our 1937 Northwest crawler crane, which was all friction clutches and brakes. I also helped my dad reline the crane’s friction brands and install a new clutch in our Reo. These machines were friends and put food on our table, so it seemed idiotic to abuse them.

My respect for clutches and machines carried over when I bought my first jeep, an M38. I had many adventures with it. And, since the jeep and I were friends, it seemed equally idiotic to abuse it.

Has this clutch care philosophy paid off? I installed a rebuilt NAPA clutch disk, pressure plate, and new release bearing in my 1965 Nissan Patrol (which uses an 11 inch International Harvester truck clutch) in 1997. The vehicle is my daily-driver and has been on many long trips since then, both on and off-road, and the clutch finally failed two months ago. Plenty of disk left, but one of the pressure plate fingers fatigued and broke. That’s 24 years service from a clutch, so you be the judge.

A clutch in an automotive vehicle, whether in a jeep, MUTT, M37, M35, or in any other historic military vehicle (HMV) or civilian vehicle with a manual transmission, provides a way to connect or disconnect the engine from the power-train. Since, unlike steam engines or electric motors, internal-combustion engines don’t develop a high starting torque, they have to be disconnected from the power-train and allowed to startup and run without a load until the RPMs are sufficient to give them enough torque to overcome the static inertia of a vehicle at rest.

The application of engine power to the load (letting out the clutch) should be smooth and gradual to lessen the stress on the drive-train while putting the vehicle in motion... something that some drivers never seem to learn no matter how old they are!

After engagement, the clutch assembly must transmit all the engine power to the drive-train without slipping. In addition, the clutch is used to disconnect the engine from the drive-train while the gears in the transmission are shifted.

The transfer of engine power to the drive-train through the clutch is accomplished by bringing one or more rotating friction plates into contact with the engine’s flywheel and the clutch’s “pressure plate.” Again, contact should be smooth and relatively gradual.

Contact of the friction plate to the flywheel and pressure plate is established and maintained by spring tension within the clutch “cover” and controlled by the vehicle’s driver through a foot pedal and linkage. As the spring tension increases (when letting out the clutch) so does the friction between the friction disk, engine flywheel and pressure plate, and the vehicle begins to move.

When the clutch is fully engaged, there should be no slippage between these three components. This results in a “solid” connection between the engine, transmission, drive shaft(s), driving axle(s), and ultimately the driving wheels as the vehicle moves down the road.

The three main parts of most automotive clutch assemblies consist of the DRIVING PARTS, which are attached to the engine’s flywheel and always rotate with it, the DRIVEN PARTS, which is the friction disk (or disks) attached to the transmission’s input shaft and always rotating with it, and the OPERATING PARTS, which include the tension springs within the cover assembly that apply and release the pressure between the driving parts. The OPERATING PARTS also include the release bearing — often called the “throw-out bearing” — as well as the linkages to the clutch pedal and the pedal itself.

The DRIVING PARTS usually consist of two cast-iron plates with contact surfaces machines to a smooth finish. One of these plates is generally the rear face of engine’s flywheel, and the other is a comparatively heavy disk in the clutch cover assembly. This disk is fitted into the steel cover, which also contains some of the OPERATING PARTS... the tension springs.

By the way, cast-iron is one of the most desirable materials for an engine flywheel and pressure plate disk because it contains enough graphite to provide some lubrication as the friction plate slips during engagement.

The DRIVEN PARTS usually consist of a friction disk (or disks on large heavy-duty clutches) with a splined hub that is free to slide back and forth on the transmission’s input shaft and also drives the shaft when the clutch is engaged.

This disk is often called the clutch plate. It is usually made of spring steel segments attached to a splined hub. Friction materials (clutch lining) are attached to both sides of this disk, generally by rivets... just as many brake shoes have their friction materials (linings) riveted on.

Naturally the friction material must he highly resistant to heat and wear. Most OEM friction plates on vintage HMVs and civilian vehicles had linings made of cotton and asbestos fibers woven or molded together and impregnated with resins as bonding agents, though “safer” materials than asbestos are often used today. In addition, copper wires are often pressed into the lining material to give it more strength and resist cracking or breaking,

To make clutch engagement as smooth as possible and resist “chattering,” several methods are used to give the clutch plate some flexibility. One method is to “dish” the plate so that the inner and and other edges of the friction material make contact with the driving parts first and the rest of the material makes contact gradually as the spring tension increases (letting out the clutch) and the “dished” disk flattens out.

Another method is to make the spring steel segments slightly twisted, which also causes the friction material to make a more gradual contact as the disk flattens out under spring tension.

The friction disk is usually fitted with a flexible center around the splined hub to absorb the torsional vibration of the engine’s crankshaft, which would otherwise be transmitted to the vehicle’s power-train. This flexible center usually consists of compression springs mounted between the hub and the disk. These springs allow the disk to rotate slightly in relation to the hub, effectively dampening the crankshaft’s torsional vibrations.

The OPERATING PARTS of most clutch assemblies consist of many small coil springs located behind and around the outer portion of the pressure plate within the clutch cover, or a one-piece diaphragm. In spring type clutch assemblies, levers that pivot on the cover force the pressure plate away from the driven (friction) disk whenever the clutch release (throw-out) bearing moves forward against the levers... as when stepping on the clutch pedal. This frees the driven disk between the engine’s flywheel and the pressure plate, and the clutch is disengaged.

This is sometimes called a “finger-type” clutch and was used in most jeeps from WWII into the 1970s, with the exception of V-6 powered civilian models which usually used a diaphragm type. “Finger-type clutches” were also used in most other vintage U.S. MVs, such as Dodge WCs and M37s, and Reo M35s, with the exception of vehicles built by General Motors.

In diaphragam-type clutches, such as used on the G.M.C. CCKW and DUKW, and the Chevrolet G-506, the dish-shaped diaphragm performs the function of the “fingers.”

Of course, another OPERATING PART of a clutch assembly is the release or throw-out bearing. This is usually a sealed ball-thrust bearing mounted on a cast-iron sleeve which is free to move back and forth on a tubular housing on the front of the transmission that surrounds the input shaft.

This bearing is connected through a linkage to the clutch pedal, which when depressed (stepping on the clutch pedal) engages either the fingers on the pressure plate or the diaphragm, moving the pressure plate back and freeing the friction disk from spring tension between the engine’s flywheel and the pressure plate.

Now we know the basics of a clutch assembly, what its parts consist of, and how they operate. Another basic fact about clutches is that how long they last depends mostly on YOU, the vehicle’s driver. How long should an average clutch last? Again, that depends mostly on you. I replaced the clutch assembly in my Nissan Patrol L60 13 years ago, have over 100,000 miles on the truck, both on and off-road, and the clutch is still fine. On the other hand, I know several people who have to replace clutches in their “extreme off-road” jeeps at least once a year.

Of course, one could say the same thing about engines, brakes, and entire vehicles; misuse or abuse them and one will wear them out a lot faster than if one treated them with care and respect. (The same could be said about friends.) However, except maybe for brakes, the clutch is often one of the MOST abused components of any vehicle, and sometimes this abuse is caused by ignorance and poor driving habits rather than by deliberate intent such as adolescent “clutch popping.”

Let’s get that out of the way first: “popping the clutch” is probably the worst thing one can do; and unless one is just learning how to drive, or one’s foot accidentally slips off the clutch pedal, it is always deliberate. There is never any reason to do it except to indulge in adolescent behavior, and if you disagree that’s tough toast.

Besides putting severe stress on all the components of a clutch assembly, this silly behavior also stresses a vehicle’s transmission, transfer case, drive shafts, universal joints and axle shafts, as well as the engine, transmission and transfer case mounts.

In the clutch assembly, such stress can break the friction plate’s torsional-dampening springs, twist the pressure plate’s tension springs, and/or rip the friction plate’s lining material off its mounting rivets. And the heat generated between the friction material, the engine’s flywheel and the pressure plate disk can warp both the disk and the flywheel.

In addition, and despite what some may think, the strength of components such as clutch assemblies, drive shafts, universal joints and axle shafts is not always in proportion to the size of a vehicle. In other words, one may get away with childish clutch-popping in a jeep for quite a while, but doing it once in an M35 might be the last time one does it... until extensive and costly repairs are made. Mass DOES matter, and there’s a big difference between trying to peel out in a small vehicle versus trying to do the same with ten tons of steel.

I realize this warning will be laughed at or dismissed by those who still want to act like they’re 16, but I assume that these people either like replacing parts and repairing their vehicles a lot, or are fortunate enough to be able to afford to have someone else do it for them.

Other bad things one may unknowingly be doing to their clutch assemblies are excessive slipping, such as when holding a vehicle at a stop light on an incline by slipping the clutch instead of putting it all the way “in” and holding the vehicle in place with the brakes. What is happening if one does this is the friction plate is slipping between the flywheel and pressure plate and rapidly building up heat. This not only wears out the friction material but may also be warping and/or scoring the flywheel and pressure plate. In cases like this, the release (throw-out) bearing is also turning, which wears it out.

Even if not slipping the clutch on a hill, one should always put the transmission in neutral when waiting at a traffic light (or anywhere else) rather than just stepping on the clutch pedal.

Another bad habit is resting one’s foot on the clutch pedal while driving. Even if one isn’t putting enough pressure on the pedal to slip the friction disk, it’s still likely that the throw-out bearing is spinning and wearing out.

Don’t try to change a vehicle’s direction (from forward to reverse) with the clutch. If you’re rolling backward, for example, use the brakes to stop the vehicle before letting out the clutch to go forward. This is especially important for HMVs such at the CCKW and its variants, as well as the DUKW, because they have relatively small clutches in proportion to their size and weight. And anyone who has ever replaced a clutch on a DUKW knows how much fun that is!

What are the some of kindest things one can do for their clutches?

Always engage (“let out”) the clutch gently. When starting from a stop, use only the minimum amount of engine RPMs needed to put the vehicle in motion.

Try to “match revs” as much as possible when shifting gears, and engage (“let in”) the clutch gently. This especially applies when downshifting.

Avoid holding the clutch pedal down for long periods of time, such as when stopped somewhere with the engine running: shift to neutral instead.

Disengage the clutch when starting the engine. While leaving the clutch engaged when starting won’t hurt the clutch itself, one is putting more stress on the starter, especially in cold weather, because it must also turn the transmission’s main shaft, and in cold weather the transmission’s oil is thick which causes a lot of resistance.

Those who own vintage HMVs with six-volt electrical systems and “crash-box” transmissions will especially benefit by disengaging the clutch when starting the engine.

All clutches will eventually wear out — at least in vehicles that are driven — so what are some of the signs that a clutch is wearing out or at least needs to be “adjusted?” These signs should also be looked for if one is checking out a vehicle prior to purchase.

(By the way, “adjusting the clutch” is not really an accurate description because, except for large heavy-duty vehicles or machinery, one cannot adjust the clutch itself but rather only the clutch linkage (OPERATING PARTS) for clutch pedal free play.)

Noisy release (throw out bearing): If you hear a whine or squeal when stepping on the clutch pedal it usually indicates that the release bearing is wearing out due to depleted lubrication. Some large vehicles and machinery have a fitting to lube these bearings, but most common HMVs do not. I have had success by drilling a small hole in the case of sealed release bearings and using a needle attachment on a grease gun to lube them; but unless you want to try this your only other option is to replace the bearing.

You should also pull the clutch cover (or “bell housing”) inspection plate, and with the engine idling in neutral, check to see that the release bearing only turns when the clutch pedal is stepped on. If it’s turning all the time, one will have to adjust the pedal linkage free play.

Slipping clutch: This is often noticeable when driving in top gear on a highway. The engine speeds up but the vehicle doesn’t. This usually occurs when one starts up an incline, and this is when one usually first notices it. Shifting to a lower gear may stop the slippage, but the clutch should be replaced as soon as possible.

A hot clutch that one can smell, even though the vehicle is only cruising, is another indication that the clutch is slipping.

Chattering or shuddering: If the clutch chatters and/or the vehicle always shudders when starting from a stop, it usually indicates that the friction material (clutch lining) is badly worn. However, it may also indicate that the flywheel and/or pressure plate disk is warped (from excessive overheating). Shuddering may also be due to loose or broken engine, transmission, and/or transfer case mounts.

How do you replace a clutch, and can it be done by most average HMVers with basic mechanical skills and tools?

In most cases yes, though the process differs widely from vehicle to vehicle; or simply said, it’s a lot easier to replace a clutch on some common HMVs than on others; and sometimes it’s not the size of a vehicle that makes the job more difficult.

For example, most vintage jeeps have a one-piece bell housing. This means that unlike, say, an M37 or a CCKW, that have openings on the bottoms of their bell housings through which, after removing a cover and taking out the transmission, one may remove the clutch assembly and engine flywheel, on most vintage jeeps one will likely have to pull the engine to replace a clutch assembly.

Other vehicles, such as older Land Rovers, have a bizarre arrangement of engine and/or transmission mountings and frame cross-members that can turn what should have been a relatively simple job into a mechanic’s nightmare.

Then there’s the DUKW, which, if you haven’t been there and done that, you’re probably better off not!

Of course, like any smart HMVer, you have a service manual for your vehicle so you can see what’s involved in replacing its clutch and decide if you feel up to the task. For most common U.S. HMVs, in addition to a good basic tool set you will also need a transmission jack. These can usually be rented.

I’ve installed many clutches on many different vehicles from small to huge without a transmission jack, but in most cases the job would have been much easier with one.

Obviously, we don’t have the space to go into detail about replacing clutches on all common HMVs — again, that’s what manuals are for — but you don’t always need every special tool that a manual may say is required. Here are a few tips:

I’ve had good luck removing pilot bushings or bearings by packing the cavity behind them with grease and then inserting a socket or bolt that just fits the bearing and tapping it with a heavy hammer. The hydraulic pressure will usually push the bearing out, Likewise, the new bearing may usually be tapped into place with a socket and hammer rather than requiring a special installer tool. It’s always a smart idea to replace the pilot bushing or bearing whenever one installs a new clutch.

A “pilot shaft” to align the friction disk is handy but not always necessary. Align the friction disk as best as you can by sight, and leave the clutch cover mounting bolts loose. Gently wiggle the transmission input shaft into place. Turning the flywheel will help. Once the transmission is bolted to the bell housing, tighten the clutch cover bolts.

Always carefully inspect the engine flywheel for roughness, burning, warping or cracks ... especially if the vehicle was shuddering. Unless you are sure that a flywheel is true and in good condition, it should be taken to a shop to be checked and resurfaced. At the very least, carefully sand the flywheel to remove any glaze.

As mentioned earlier, most common HMV clutches cannot be “adjusted.” What you are actually adjusting is the clutch control linkage for proper clutch pedal free play so the release bearing doesn’t turn except when one fully depresses the clutch pedal. While this adjustment varies from vehicle to vehicle (read your manual), a basic setting is about 1.5 inches.

Hopefully, this article has helped in the understanding of clutches and provided some basic information about using and caring for them. Once again, everything specific to your own vehicle’s clutch is covered in its appropriate service manual. Like any other vehicular component — indeed, like any vehicle (or friend) — treat it with proper care and respect and it will last you a long time. 

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