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CCKW and Chevy Clutch Adjustment

Stephen Keith 2005 

With a GMC, if you want to find out about the "GMC Clutch", then adjust it
for free-play like all but one of the GMC (CCKW) manuals states. One manual
(the correct one) states that you adjust the clutch so it just barely
disengages about 1/2 to 3/4" from the floor. The free play is what ever it
is. As the clutch disk wears, the pedal will come up higher and higher until
there is no more free play.

> From TM10-1105 Maint Manual Model CCKWX-353 July 8, 1941

"To obtain maximum mileage between adjustments, lengthen rod so that clutch is fully disengaged when pedal is 1/2" from the toe-board. This should give 1" pedal travel before clutch begins to release. If travel is less than 1" diaphragm spring should be carefully inspected."

The free-play is determined by the clearance between the throw-out bearing
and the fingers. The throw-out bearing is pulled away from the fingers by
the pedal return spring. As the disk wears, the fingers protrude more from
the pressure plate and the free play available becomes less. When there is
no more free-play, the throw out bearing will be riding on the fingers and
the clutch disk, at a minimum needs to be replaced. No adjustment will make
any difference. If you keep driving, the clutch will start to slip and/or
the throw out bearing will wear out.

The overriding concern, is NOT free-play but over stressing the clutch
diaphragm spring. If you continually over stress it, it may eventually twist
and become useless and you will not be able to disengage it.

If you adjust for the free-play amount, when the clutch disk is new, you
will be way over bending and over stressing the pressure plate diaphram
spring disk.

If you adjust the clutch to just barely release as the pedal touches the
floor, you will not be over stressing the clutch diaphragm spring and the
clutch will have the same release point for most of the life of the clutch.

Rebuilding the GMC Diaphram Clutch Pressure Plate

If you machine the pressure plate face, you have to do it in known thickness
increments AND you have to ADD known thichness shims behind the pivot points
for the diaphragm spring. Again, most of the manuals for the CCWK/DUKW do not
mention this. Failure to do so results in the diaphram spring fingers stick
much farther out of the housing, decreasing pedal efficiency and rendering
it pretty much junk!

Stephen Keith 2005 

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Site opened 1/7/2005 Last modified: 03/12/2013 17:52