The GMC CCKW

I was talking to a gent in our truck club who has over 50 years experience
as a heavy vehicle mechanic. He has owned many CCKW, DUKW,
Studebakers and Diamond T.

He told me in his opinion the cause of gearbox failure was that the circlip groove in the mainshaft that holds 5th gear in place was too shallow.

His solution in the past was to machine the groove wider and deeper, then fit 2 circlips. I did not think to ask if they were the same circlip as original.

The clip you refer to is burried down inside the 5th OD gear on the main shaft. If you look at the manual, the procedure for removal leaves a LOT to be desired! You need three "picks" to try and open the clip up without springing it. Next time I have to do it, I will try to make a tool for it.

I believe that there was also an early and a later gear. The latter gear had more holes in it I believe to let oil in.

Can I say that this failure is the same reason I in my truck?When I loose the clutch pcan't run in overdrive(5th) at 20 m.p.h.edal I hear a noise.What I will have to do?

You must decide if this noise is not normal.

Some people I have spoken to told me that 5th gear is always very noisy

If the noise is too bad, it would be best to NOT USE 5th gear

If the circlip fails, either because of a too shallow groove design, or
a worn thrust washer, your gearbox would be badly damaged.

If you decide the noise is abnormal, I believe at least a partial gearbox dismantle would be needed.

it really helps if you can find a mechanic who knows these trucks to diagnose the severity of the noise

My 5th gear only not engaged successfully.The noise is why this not occurs.When I loose the clutch pedal(running) I notice this noise at 20 m.p.h.Initially I thought it is because I went in low speed(10 m.p.h.).When my truck is stoped with the engine working,I get this engaged.I didn't know the overdrive has a failure.Thanks.