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How Much To Skim From Pressure Plate Horns


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#1 unburntfuelinthemorning

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Posted 31 August 2019 - 09:46 PM

To get the diaphragm to sit flat on my Borg & Beck clutch when it's all bolted up I need to skim 40 thou off of the pressure plate horns.

 

I was going to get a little more skimmed off so as the clutch linings wear the diaphragm returns to the flat position thus increasing the clamping force.

 

The question is how much extra to skim from the horns in addition to the 40 thou?

 

The distance between the surface of the lining and the rivets is 40 thou.  If the clutch was half worn the friction disc would reduce in thickness by 40 thou. 

 

Should I get an additional 40 thou skimmed from the pressure plate horns (total 80 thou) so when the clutch is half worn the pressure plate is then flat?

 



#2 DeadSquare

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Posted 31 August 2019 - 10:51 PM

To get the diaphragm to sit flat on my Borg & Beck clutch when it's all bolted up I need to skim 40 thou off of the pressure plate horns.

 

I was going to get a little more skimmed off so as the clutch linings wear the diaphragm returns to the flat position thus increasing the clamping force.

 

The question is how much extra to skim from the horns in addition to the 40 thou?

 

The distance between the surface of the lining and the rivets is 40 thou.  If the clutch was half worn the friction disc would reduce in thickness by 40 thou. 

 

Should I get an additional 40 thou skimmed from the pressure plate horns (total 80 thou) so when the clutch is half worn the pressure plate is then flat?

 

No.



#3 ACDodd

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Posted 31 August 2019 - 10:51 PM

No skim only to get the diaphragm flat when new.

Ac

#4 unburntfuelinthemorning

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Posted 01 September 2019 - 07:03 AM

OK, thanks.  My reason for asking is due to this paragraph from a Hillier's text book which states 

 

"When fitted to a clutch mounted on a flywheel, the spring is compressed just beyond the point where it is flat (point A on the graph).  Set in this position, the reduction in thickness of the friction facing due to wear causes the spring to 'release' and the force to increase."

 

It seemed to make sense but I wasn't sure how much they meant by "just beyond".

 

199Yx6W.jpg



#5 unburntfuelinthemorning

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Posted 01 September 2019 - 05:04 PM

How much thickness does a clutch lining lose during the initial bedding in? 



#6 Moke Spider

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Posted 01 September 2019 - 06:29 PM

I do skim a bit more than just making them flat, to allow for plate wear.

 

I work out how much has to come off them to get it flat and then measure on the plate, from the friction material to the rivet head only on one side though and add that to what has to come off. In this way it's 'over' by half the amount it can wear, when new and 'under' by close to the same amount when fully worn. I'm not sure if this is how much 'a bit' is that Hillier's might be referring ?



#7 KTS

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Posted 01 September 2019 - 10:50 PM

if you haven't seen it before, this is a really good article on clutch setup and operation 

 

https://www.minimani...FAQ__non_Verto_



#8 unburntfuelinthemorning

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Posted 02 September 2019 - 08:21 AM

if you haven't seen it before, this is a really good article on clutch setup and operation 

 

https://www.minimani...FAQ__non_Verto_

Thanks.  That article and Des Hammill's book is where I started with this.

 

There seem to be quite a few opinions on how to do this correctly.  Some threads on this forum even say that the diaphragm spring should never go beyond flat at any point in it's operation or it will go the other way and won't come back.  I don't agree with this. 

 

I think I'll tread my own path with this taking into account all the differing views and go slightly beyond flat but not as much as Moke has.  So similar to the graph.

 

Thanks to AC for putting up a very helpful video in another thread showing how to check for spring flatness with a spirit level.



#9 Moke Spider

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Posted 02 September 2019 - 09:19 AM

 Some threads on this forum even say that the diaphragm spring should never go beyond flat at any point in it's operation or it will go the other way and won't come back.  I don't agree with this.

 

When the pedal's depressed, this is exactly what happens, only they do come back,,,, or I've never had one that hasn't !



#10 KTS

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Posted 02 September 2019 - 09:41 AM

what i thought was quite interesting on the minimania item was this graph showing force/deflection suggesting a used blue pressure plate would stand up to 90thou deflection from flat without any significant additional forces involved/required, yet only 45thou deflection is required to fully disengage the clutch, so in theory you could get away with 30-35thou pre-load (i.e ..past 'flat' ) at initial assembly

 

with an orange, it appears anything more than 60thou deflection of the spring starts adding significant extra loads onto the crankshaft thrusts, so you'd probably want to avoid starting at anything more than about 10thou pre-load on the spring

 

it's a nice article, but there is a caveat in that as the data is based on only 3 clutch springs there's no telling whether they were a representative sample or not  O_O



#11 unburntfuelinthemorning

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Posted 02 September 2019 - 12:28 PM

 

 Some threads on this forum even say that the diaphragm spring should never go beyond flat at any point in it's operation or it will go the other way and won't come back.  I don't agree with this.

 

When the pedal's depressed, this is exactly what happens, only they do come back,,,, or I've never had one that hasn't !

Here's what I mean from another thread http://www.theminifo...myth/?p=2874088



#12 Moke Spider

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Posted 02 September 2019 - 07:15 PM

 

 

 Some threads on this forum even say that the diaphragm spring should never go beyond flat at any point in it's operation or it will go the other way and won't come back.  I don't agree with this.

 

When the pedal's depressed, this is exactly what happens, only they do come back,,,, or I've never had one that hasn't !

Here's what I mean from another thread http://www.theminifo...myth/?p=2874088

 

 

I recall long ago, hearing that some of the clutch reconditioners used to turn the bevel spring 'inside out' and refit them the other way around. At that time, I actually tried this on a Mini Diaphragm. I couldn't do it. I could get it to go well over easy enough but it sprung back.

 

Next time you have one on the bench, try it.

 

Also, if you already done it, have a look at how the clutch works;-

 

IQpttzZ.jpg

 

Item 22

 

It effectively turns it inside out every time your foot goes on the pedal.

 

Just reading a some of the posts in that thread, re: the Overthrow Nuts. When first released, the Mini wasn't fitted with these, but they did soon appear and this was with the Coil Spring Clutch, long before the Diaphragm types came about. They were fitted to prevent breaking the centre main bearing cap and that's all.



#13 DeadSquare

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Posted 02 September 2019 - 07:55 PM

In my ignorance, I used to think that I was being clever to fit double springs in the clutch.

 

By chance at an International meeting, I stopped and looked in the back of the AP van, where a technician was examining a burnt Formula 5000 clutch plate.

 

I told him about my wheeze of fitting extra springs from a Massey Ferguson baler, inferring that something like that might have stopped the clutch burning.

 

He patiently pointed out where the clutch setting should have been, and then said  "AP aren't stupid you know.  Have you ever noticed the piffling spring on friction shock absorbers ?.  With the clutch engaged, it takes about ten times the torque of a standard Mini to make the clutch plate slip".

 

I walked away, humbled.






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