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Spitfire/Herald Clutch Part Numbers


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Looking for some knowledge that's maybe been lost to the sands of time.

What do AP Borg & Beck clutch numbers actually mean, I rediscovered a few NOS 6.5" clutch covers I had in storage, and they seem to fall into two categories:

  • Box and clutch marked 76149/35 with a splash of blue paint on the diaphragm springs.
  • Box and clutch marked 76149/24 with a splash of white paint on the diaphragm springs.

Are these actually different in anyway? I thought maybe it was something to do with clamping force, but struggling to find much online...



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gosh  takes me back to dealing with the AP guys on truck warranty   most of these manufactures use numbers as a production run year. product and week 

76 may be the year  149 spec  /24 week but thats a bit of guesswork 

the paint is applied on the load test there will be other numbers stamped on the drive strap rivet heads  

but not dealt with these for 25 years so memory hard drive is a bit wiped 


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Thanks Pete, did a bit more digging... There are other numbers on them that look like the production run, e.g. 74D45 sounds like 45th week 1974 to me, the 76149/XX numbers are definitely the part numbers. AP website mentions that colours tell you the clamping load, but seems to be different for every product line:

https://apracing.com/drawings/2018 Product Catalogue/High Performance Clutch.pdf

Doesn't look like they make 6.5"/165mm clutches anymore either...

Mini pages mention the same thing, but again, the colours for those clutches don't really help me:



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2 hours ago, Pete Lewis said:

might be worth a call to firstline who make the B &B units now 


Well, I popped off some emails to B&B/FirstLine and also to AP Racing this morning, and hats off to them, B&B/FirstLine already replied, with scans of original technical drawings from 1965 for the 76149/xx series! White has a higher clamp load.


Dark Blue: Load at Peak:  556 - 485 daN, Load at Valley 227 - 156 daN.
White: Load at Peak:  579 - 507 daN, Load at Valley 325 - 254 daN.

Although I don't exactly know what load at peak and load at valley mean...

But also seems the blue spring units are designed for a thicker friction plate:

Gauge plate thickness (from their drawings this is thickness of the clutch plate):
Dark Blue: 0.286 - 0.284"
White: 0.261 - 0.251"

Diaphragm material thickness:
Dark Blue: 2.30-2.22 (mm?).
White: 2.45-2.37 (mm?).

Probably more than anyone wanted to know, and yet I still don't quite know if they're interchangeable, suspect not.

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coil clutches have a thicker disc than diaphragms so the hysteresis of the clamp loads are differeing   if you measure the input loads against the clamp loads and the distance of travel you get quite a variation in loads as the unit  loads and unloads  so the pressure pressing the clutch will start off higher and the pressure on the pedal returning is lesser

use the wrong disc and you can really upset clamp and pedal loads , a thick disc will reduce the pedal loads to silly light but you get slip very quickly 

if you track the data its a bit like the infinity symbol  its not a straight line load for input and output 

you need a load cell . DTI and pressure cell to map all three against each other 

bathhroom scales and ruler wont work well  ha ! 


had a browse on youtube but nothing shows the way the clamp and release loads are configured 



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