Lotus Elan

Clutch cover (pressure plate) torque rating

PostPost by: 1owner69Elan » Fri Jun 26, 2020 1:43 am

I started this topic under another heading for the Voigt 5-speed installation but figured it is a more generic issue of potential interest.

In pursuit of knowing what pressure plate rating I had for my uprated engine, I found that there is a serious lack of markings, nomenclatures, and identifications, for the various pressure plates extant. Even the manufacturers are not always indicated.

The standard pressure plate is generally marked with a green dot (s) on the diaphragm fingers. This is rated for 120 lb-ft and is notionally adequate for the standard Lotus Twincam which ranges from 104 to 113 (depending on series and publication). For a more powerful engine, alternative pressure plates with higher torque ratings are available.

The pressure plate that was installed when my engine was recently redone to a high spec (181 bhp, 143 lb-ft) is a Borg & Beck with an orange dot on the diaphragm. No one, not my engine builder, the supplier (Dave Bean), the Internet, or this Forum, has been able to provide any information as to the torque rating for this "Orange" unit. Dave Vegher, who concentrates on race cars, is not familiar with road-going versions of the pressure plate and simply relied on Dave Bean (Ken Gray) to supply a "heavy duty" plate. But, Bean cannot say what "Heavy Duty" actually equates to in terms of torque rating.

So, I finally hit upon the idea of measuring the diaphragm spring force of my "orange" plate compared against a.) a standard (120 lb-ft) and b.) the plate provided by Voigt with my 5 speed purported to also be "standard". The torque rating is proportional to the clamping force provided by the diaphragm. I thereby I figured I could create a force-displacement curve for all three pressure plates and then scale from the known "green" standard plate @ 120 lb-ft
to derive the ratings for the other two pressure plates.

Three pressure plates (left to right): "Green standard(120 lb-ft)", "Orange uprated", "Voigt standard"
IMG_5577.jpeg and

I created a test rig from my bench press, a dial indicator, and a bathroom scale. The scale was needed because the press reading (in tons) is too coarse for the force measurement.
IMG_5576.jpeg and

The force displacement curves derived look like this.
Clutch Pressure Plate Curves.jpg and

The "green" standard and the Voigt plate are identical. Scaling the orange plate, and measured at a displacement 0.175 inch, which is the static load compression of the friction disk - when bolted to the flywheel, I arrived at a ~165 lb-ft torque rating for the "Orange". This happens to be consistent with the uprated twincam AP Racing pressure plates.

In conclusion, I am now comfortable in using the "orange" plate going forward.
'69 Elan S4 SE
Street 181 BHP
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PostPost by: mark030358 » Fri Jun 26, 2020 6:23 am


Great forum this....
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PostPost by: 2cams70 » Fri Jun 26, 2020 9:25 am

Thanks for sharing. That is good reference information
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