The clutch is the most problem-prone part of the drive train in cars with manual transmissions. That’s because it must take the heat and pressure for all of your gear change decisions. The gearbox and rear end just go along for the ride

How it works: the flywheel

Bolted to the flange of the crankshaft at the rear of the engine is the flywheel, which is the forward-most part of the clutch assembly. The ring gear for the starter is attached to it as well, but the main reason for the flywheel is to store energy.

The flywheel is the clutch’s driving surface, with the clutch disc being its driven surface. Because the clutch disc slips against the flywheel when the clutch is being engaged and disengaged, a lot of heat is generated. The flywheel also acts as a heat sink to prevent incinerating your clutch lining. As a result, flywheels get scorched, cracked and warped because of the tremendous temperatures they endure; and after long use they must be resurfaced, and in extreme cases, replaced.

The clutch disc

The clutch disc rides on the splines of the transmission’s clutch shaft and is made up of two layers of organic lining similar to brake lining. Sandwiched between these two layers is a set of springs and levers, or a wavy segmented metal disk that helps smooth the clutching action.

At the centre of the clutch disk is the hub assembly. It has springs and pin stops built into it that allow more flexing during shifting. The clutch disk slides on the transmission’s clutch shaft during shifting, but it doesn’t slide far. Even though the clutch pedal swings in a big arc when you depress it, your clutch disk only moves about 0.77mm to do its job.

The pressure plate assembly

In order for the clutch disk to clamp against the flywheel to drive the car, a set of strong springs, pushing against a cast iron ring called the pressure ring, are employed. A set of levers that look like pie slices — in the case of a diaphragm clutch — are moved by the clutch release bearing, and are used to pull the pressure ring away from the clutch disk when shifting gears. The clutch disk and pressure ring are contained in the clutch cover, which is a stamped metal pan.

Diaphragm clutches are  easy to spot because their sheet metal actuating diaphragm (called a belleville spring) looks like a sliced pie. A belleville spring works the same as when you press on the top of a biscuit tin with your thumb. Pressure increases until the lid pops — or goes past centre — after which little pressure is required to deform it further.

Replacing a clutch

A clutches isn’t hard to deal with once you have it out of the car. The hardest part is removal — because it is sandwiched between the engine and the transmission, one of these items must come out in order to get at it. Check you service manual for the best way to do it on your vehicle.

Jack your car up and put it on the sturdy jack stands. Put long pins in place of the top two bolts holding the transmission to the bell housing so you can slide the transmission straight back to avoid damaging your clutch disk. Make the pins from long bolts by cutting their heads off with a hacksaw. You will also need these pins to install your transmission later.

Mark the flywheel in relation to the crankshaft flange, and the pressure plate assembly in relation to the flywheel. These must go back on oriented the same way as when they were removed, in order to preserve engine balance. Save any shims you find between the bell housing and the engine or between the bell housing and the transmission, and put them back in the same places when you button things up, so your transmission will line up properly with your clutch. Evenly loosen the bolts holding the pressure plate assembly onto the flywheel a half turn at a time, so the springs inside it don’t warp the cover.

Inspection

Check your flywheel for discoloration or cracks. Also inspect it for warpage by placing a steel ruler across it edge-wise. If you see light under your ruler at any point, your flywheel is warped and will need to be resurfaced at a machine shop.

A flywheel with deep cracks that cannot be ground-out during resurfacing should always be replaced. A defective flywheel can explode like a hand grenade, with devastating results. After the flywheel is properly surfaced, a new or rebuilt clutch assembly should be attached to it and the whole thing balanced to prevent vibration during acceleration when you drive your car.

It is a good idea to replace your pilot bushing when doing the clutch. Fill its cavity with grease, then tap a drift the same diameter as your clutch shaft into the hole. Hydraulic action should pop out the old bushing. Install the new one using a soft brass drift of the correct diameter, being careful not to distort the bushing or create a burr on it.

Clutch and pressure plate assembly

Examine the clutch disc for oil contamination, glazing and wear. If the friction surfaces are worn thin, replace the clutch disk. If your clutch disk is only a little glazed, use sand paper to roughen it. If it is burned or discoloured, replace it. Never touch the lining surface with your fingers, and if you do create a smudge, remove it with alcohol-based brake lining cleaner.

Give the pressure ring a once-over. If it is burned, cracked or warped, the pressure plate assembly will need to be replaced. If the release diaphragm is worn, the verdict is the same.

Installation

If you removed it during disassembly, bolt the bell housing back onto the engine, making sure you install any shims in the same locations. Bolt the flywheel onto the crankshaft flange with new bolts of the correct hardness (a grade 8 is the minimum requirement) using new locking devices. Evenly torque the bolts to the specs in your service manual.

Using a pilot tool or old clutch shaft, slip the clutch disk in place against the flywheel. Make sure your clutch disk is facing the right way before installing the pressure plate. Attach the pressure plate to the flywheel and tighten its bolts evenly, half a turn at a time. If you don’t tighten down the clutch assembly evenly, you risk distorting it. Remove the pilot tool, then lubricate the pilot bearing with a little grease.

Pack the clutch release bearing with grease and slip it into its yoke. Put a bit of grease on the yoke pivot point too. Make sure you install the throw out bearing facing the right direction.

Transmission

Put the gearbox into high gear. Put your locator pins back into the upper bell housing holes. Slide the transmission into place. You may have to turn the U-joint yoke a little in order to get the splines on the clutch shaft to mesh with those on the clutch disk. Remove the locator pins, and tighten the transmission attaching bolts evenly.

Reattach the prop shaft at the universal joint. Hook up the clutch linkage, then adjust it to allow the correct amount of free pedal travel before it starts to engage.

Adjustment

Start your engine, depress the clutch, and bring the revs up slightly. Shift the transmission into reverse. If there is no gear clashing, adjust the clutch push-rod a little at a time until the reverse gears just start to clash when you shift. Now take the adjustment back out a little to where the gears don’t clash. This will give you a sufficient air gap. This method only works in gears that have no synchromesh, such as reverse.

Guide to rebuilding a clutch

This flywheel has serious cracks and scorching and must be replaced
The Flywheel is reground and balanced with clutch assembly, ready to install
A new pilot bushing is always a good idea when installing a clutch
Tighten the pressure plate in place evenly, a half-turn at a time
Install the throw-out bearing making sure it is facing the right way round
An old clutch shaft is used to align the clutch assembly
Long pins made from bolts with heads cut off help guide gear box back in
Diaphragm clutch
Bork and Beck type clutch
How a clutch engages and disengages
Adjusting the clutch pedal freeplay and travel

Words & Photos: Jim Richardson