Please could someone explain how the clutch works?

[Keith]

Ok it's got me baffled, I've removed my old engine from my car, and removed the gearbox.

I can see the input shaft on the gearbox and the clutch's release bearing (which I expected to be completely different). Last night I removed the pressure plate, clutch assembly and flywheel from the crank.

But, for the life of me I can't figure out how the 5 parts work. So how does the clutch pedal disengage the gear? I guess that the clutch pedal operates the release bearing which stops drive to the gearbox and then when you take your foot off the clutch the release bearing goes back into the clutch assembly and the gear turns?

[cajef]

See http://www.dorwey.co.uk/clutch.htm

[Keith]

Cool site, but the clutch doesn't really move as it says, it's held in place by a metal cradle (pressure plate) and the release bearing moves and does something.

The clutch operates a cable which moves and arm, which pushs the release bearing away from the engine.

**still confused**

[Wig]

Coming out of the gearbox is the splined shaft. on the shaft inside the gearbox is the thrust bearing, when you press the pedal this bearing moves forward on the shaft - you should be able to replicate this movement for yourself.

The thrust bearing when moved forwards presses down on the fingers on the clutch cover. As the fingers are depressed this releases tension on the springs inside the clutch assembly, allowing the friction plate to come away from flywheel.

The gearbox splined shaft is driven by the centre of the clutch assembly i.e. friction plate, when the friction plate is pressed against the flywheel this turns the friction plate and as a result turns the splined shaft.


When the clutch is in release mode the flywheel and clutch cover are spinning at engine speed with the friction plate/splined shaft inside the clutch assembly remaining stationary.

Because the clutch cover is spinning at engine speed this means the fingers are also spinning, but in released mode the thrust bearing is pressing against those fingers whilst they are spinning. Hence you will see on the ends of the fingers a shiny spot. And as you have a moving object (fingers) pressing against a non moving object (thrust bearing control lever) you need an intermediary bearing (thrust plate bearing) to stop the movement from causing wear to the lever, to dampen the vibration, and to help the movement overcome the friction against the non moving part.

As to the exact process by which depressing the fingers causes the springs inside the clutch assembly to lose tension - I don't know.

[BenL]

http://auto.howstuffworks.com/clutch.htm

Not a massive amount of info on the 1st page but maybe after some drilling through other pages it will have more relevant info for you.

[Keith]

Thanks Wig :)

Great explanation, now to fit the flywheel, clutch and gearbox to my new engine and hope it works!

[cajef]

OK try this one http://www.clutchwizard.com/basics.htm

[Wig]

I just edited in a bit extra about the thrust bearing above.

When looking at the friction plate, be aware it is just like a car tyre. There should be raised pads of friction material standing proud of the friction plate surface, so there should in effect be a "tread" kind of like a car tyre.

When a car tyre is bald it has no tread, if your friction plate is similarly bald it will have no raised tread. And if it is REALLY WORN the friction material will have worn away so much that the metal rivets (within the friction plate) will be worn aswell and will look shiny and worn flat (like if you filed the top of a screw head).

friction plates wear out very slowly, but if you have no tread left you should change it, if you have worn rivets you must change it. The thrust bearing should last the life of the friction plate so you should be ok to leave that alone.

[Conor]

Make sure that when you fit the clutch, that you use a clutch alignment tool to ensure the friction plate is central otherwise it'll shake itself to bits if you manage to get the gearbox on.

[epninety]

Normally, if it's not well enough aligned you won't get the gearbox on, because the gearbox input shaft mounts into the spigot bush (or bearing) in the centre of the crankshaft. Final alignment occurs the first time you press the clutch pedal down and release the driven plate (movement will only be tiny fractions of a mm when you do this).

(I have seen some dodgy engine conversions with no support for the end of the input shaft, but they generally don't last long enough to count.)

Conor is right to suggest an alignment tool though, or you can spend many happy hours trying to mate the engine and box together with no success :mad: If you haven't had much practice it can be a pretty frustrating experience at the best of times.

[Keith]

Make sure that when you fit the clutch, that you use a clutch alignment tool to ensure the friction plate is central otherwise it'll shake itself to bits if you manage to get the gearbox on.

The rebuild guide I'm using suggests

With the plate held on loosely by the cover either use a clutch centering tool if you have one (sometimes they are supplied with the clutch kit) or try my method of getting a ½" socket extension bar and some electrical insulation tape.Use the tape to wrap around the end of the extension bar until it fits snugly in the center of the clutch plate and the hole in the end of the crank. You should then be able to gently move the plate until it is centered. NOTE: this may be a trial and error time here so be patient and you’ll get it right eventually

This rebuild is frustrating, I don't mind buying the tool, but I will try without first and then head to halfords after a few hours :rotfl:

[epninety]

I confess I've never owned a clutch alignment tool either, but it does do a much better job than most of the alternatives. I usually use various old sockets on a piece of threaded bar, and check the final alignment by eye / finger. Every time I do it though (probably 1-2 times a year) , I make a mental note to buy an alignment tool!

[Keith]

I bought the tool in the end, local motorfactors had it for £8, so bought one.

Flywheel change went well, clutch went on, and then the final bolt sheered, toss. Not sure if I should drill it out or not :S I removed the other 5 and used thread lock on them aswell as torquing them to the correct torque.

[cyclonebri1]

I bought the tool in the end, local motorfactors had it for £8, so bought one.

Flywheel change went well, clutch went on, and then the final bolt sheered, toss. Not sure if I should drill it out or not :S I removed the other 5 and used thread lock on them aswell as torquing them to the correct torque.


Your clutch will be out of balance if you dont fit a new screw and could result in life shortening vibration. (the clutche, not yours:rotfl: )

Also it's not true that the clutch will vibrate if not centred. As said you will not relocate the gearbox input shaft if it's too far out, and even if it is slightly out the first time the clutch is depressed on completion the friction disc will "self align the last little bit as it is floating when the peddle is depressed.;)

[Wig]

Oh drat! You snapped a bolt.

Clutch cover bolt? Did you tighten them down each a little bit at a time in a five pointed star like pattern (http://en.wikipedia.org/wiki/Pentagram)- or 6 depending on number of bolts.

The torque should only have been between 15 - 30 Nm depending on bolt size (M7-11/12mmsocket or M8-13mm socket), this is where it is useful to have small torque bar more accurate at low levels. I'm thinking if you snapped the head off you must have been torqing them too much.

As it is done now I would be tempted to just go for it, the flywheel is balanced, but I can't see a tiny missing bolt head having much effect. I'd suck it and see.

[epninety]

There's a fair amount of force on that sheet metal clutch cover plate... I'd want it to have a full set of bolts if possible. A distorted clutch could make it unpleasant to drive in the future. It's not going to be corroded in, so a left handed drill should have it out in a jiffy.

Easy enough to be not worth the bodge!

[Keith]

I'm going to leave it as it is for now, and fit the gearbox when I'm next off work on friday, the flywheel is a spare lighter version I had left over so if it does effect the drive of the car, I'll bin it and refit the original.

Trouble is, I've no idea if the engine I am fitting is in good condition, or if the poor gear change I suffered prior to engine failure was due to bad linkages, so it's a case of get it all together and hope atm.

If the car, drives badly, but the engine is fine then I'll whip the gearbox off and change the clutch, if the gearchange is poor with new linkages and oil then, I'll dump the gearbox and probably fit a new clutch and change the flywheel then.

At the moment, I just want to hear the bloody thing runnning!