10 replies to this topic

[foreverfixin]

Hello there experienced engine builders! Now that I’ve sorted out the primary gear end float (and I have John Guess to thank for some guidance in this regard) I’m looking at refitting the flywheel housing and clutch assembly. Can I just ask if I’ve got things straight in my mind? . . . the driving straps are bolted to the flywheel with a spacer underneath, in my case these are 2mm thick. In operation the diaphragm pushes on the other end of the straps which in turn push the pressure plate away from the clutch plate. The maximum amount of ‘bend’ in the straps can surely only be the same as the thickness of the spacers before they come into contact with the flywheel before being able to go no further. Am I right in thinking that 2mm of movement of the pressure plate is sufficient for clutch disengagement, or am I misunderstanding how the clutch works? Any comments would be welcome. Thanks.

 

[nicklouse]

No the spacers have nothing to do with the distance pushed. But they do need to be set correctly so that the spring is level. Or you are not getting the correct clamping force.

 

have a watch. https://youtu.be/myx0mnLYv4g

[Ethel]

The backplate barely has to move to release the friction plate sandwiched between it and the flywheel. As Nick says the movement is in flattening the diaphragm. It provides the clamping force by trying to return to its stress free state. Pushing the pedal actually increases that stress, but the flatter it gets the more those stresses are inside the diaphragm and the less they're trying to pull the backplate towards the flywheel.

 

If you got the spacers so the diaphragm went perfectly flat there'd be zero sideload on the crank thrusts when the clutch was disengaged  , but it wouldn't be able to push itself back when you let the pedal go 

[Spider]

,,,,,,The maximum amount of ‘bend’ in the straps can surely only be the same as the thickness of the spacers before they come into contact with the flywheel before being able to go no further. Am I right in thinking that 2mm of movement of the pressure plate is sufficient for clutch disengagement, or am I misunderstanding how the clutch works? Any comments would be welcome. Thanks.

Yes.

The limit of separation is what ever these spacers are, before the straps on the horns of the Backing Plate will 'bottom out' against the flywheel.

The clutch only needs around 0.9 - 1.0 mm of movement to release and when all parts are new, you'll get about 1.5 mm movement.
 

[foreverfixin]

Thanks for your replies guys. I wasn’t far off then. However the diaphragm works the towers on the back plate are pushed away from the clutch plate (by not very much) thereby separating the sandwich. The friction surfaces of my flywheel and back plate are actually not in bad condition so I’m intending to reuse them as they are but with a new clutch plate and diaphragm (which I don’t have the equipment to pressure or torque test as shown in AC Dodd’s video). I guess that’s an important stage though because if it slips there’ll have to be some machining done and that’ll be better done now than later down the line. I’m also not sure about centralising the back plate when bolting on the diaphragm which obviously can’t be done until the flywheel is torqued up. Any thoughts?

 

[Spider]

When assembling on the bench, you can use the Primary Gear to centralise the Plate, the splined nose of the gear will fit fairly neatly in the back of the Flywheel.

With the assembly torqued up, look carefully at the Bevel Spring in the Diaphragm, it needs to be flat to generate the best clamping pressure. Often, because of normal wear of the friction faces of the Flywheel and Backing Plate (and I feel the material on the new plates are thinner too these days), the Bevel Spring isn't quite flat and needs to 'go a bit more' in order to get it flat. You can set it up in a press and use a DTI to measure how much further it needs to go, and then have the Horns on the Backing Plate machined back by that amount. If you are unable to measure it, take the assembly to a Clutch Specialist or most engine machine shops can measure and machine it for you.

[foreverfixin]

Thanks Spider, I now know what I need to achieve and have had a play with assembly. Following a procedure I read elsewhere I laid the back plate on two wood blocks, put the new friction plate (thinner than the old one oddly enough) on and then the flywheel. I could see that the horns were bending the straps upwards so fitted thicker spacers in order make them parallel with the flywheel. Feeling good at this point! I went on to fit the diaphragm and progressively tightened it down. Checked the bevel spring and decided it probably would need to go in a bit more. Then I noticed that the straps were not parallel any more.

 

My query now is should the adjustment of the strap spacers be before or after fitting the diaphragm?

 

As I don’t have a press to push the bevel spring flat I’m going to have to use a clutch specialist as suggested. I imagine that the horns will need reducing in which case I might as well have the back plate and flywheel resurfaced at the same time.

[KTS]

i've just been through this process and used the method from the HickeyRaceEngineering youtube videos on flywheel & clutch setup, which doesn't require a press to work out how much to take off the backplate horns - i would recommend watching the videos to get a good understanding of what is involved, and why.

 

..a very brief summary of what i did is on my build thread below

 

https://www.theminif...15#entry3707972

[foreverfixin]

KTS, your reply and build thread are helpful. I have watched the HRE videos and have to say that the guy explains the diaphragm set up very well indeed.

 

Having removed the straps the bevel spring is now flat so I need to get the horns reduced by the thickness of the straps plus whatever comes off the flywheel and back plate in the resurfacing process. Then in the final assembly (if I understand correctly?) I will need to fit spacers thick enough to ensure that the straps are parallel to the flywheel.

 

All that remains is for me to find a machine shop with experience in this type of clutch to carry out the work. I have made some enquiries and am awaiting replies. If anyone has any suggestions or recommendations for such a specialist in the Oxford area I would be pleased to know. Thank you.

[Ethel]

I had a thunk.

 

How about using your flywheel puller to compress the diaphragm, shouldn't take much adapting.

[foreverfixin]

I don’t have a flywheel puller. Reading about other TMF members issues with removing flywheels I didn’t bother buying one but hired a heavy duty one from Guessworks which worked a treat.

 

I have now found a local specialist who has experience with this type of clutch so will be happy to have them do the set up including any machining. They are Fiennes Engineering at Filkins in West Oxfordshire previously known to me as restorers of RR and Bentleys amongst others.