1974 GMC 4905A Clutch Issue

[Mechanical Music Man]

Hello All,

I have an issue with my clutch. The person that I bought the bus from put in a new one. The issue is I can't get it into first gear without shutting off the engine. That, or it will go into first if I am driving it and come almost to a stop. If I am going under 5mph it will go in. But if I goof that up I am left with shutting down the engine. It has a four speed Spicer transmission. We tried adjusting it, but to no avail.  I searched here and found a thread where it is mentioned but that's it. I also went to a large shop that deals in Detroit Diesels and other Transmissions. Of course only the mechanics that worked there for like 30 years might have an idea, but mostly scratched their heads.  Hope someone can point me in the right direction.

Thank you in advance!

[pabusnut]

Music Man,

Welcome to the world of Spicer Transmissions in a 4905!
Unfortunately, this is how they all operate, unless you want to rebuild the tranny.  You can try dropping it in 4th, and then to 1st, but mine won't do that either. 

I have to shut down, put it in 1st, and then start in gear w/ clutch to the floor.

I think there was one guy who rebuilt his back to factory spec to restore this functionality.

Steve

[j.m.jackson]

Same issue in my 5303 with the 4 speed spicer. It's alot better once its warmed up (15-20 minutes on the road), but dead cold, gotta shut it down to get it in first.

Also, be aware to not start the engine cold with the clutch pedal down. The engine doesn't have enough oil around the crank bearings to deal with the thrust load of the depressed clutch.

[bevans6]

Sometimes the root cause is worn splines on the input shaft of the transmission.  My clutch got quite bad, despite being essentially brand new, and when I closely inspected the input shaft the splines had .015" deep wear marks where the clutch rides when the clutch is engaged.  The wear grooves lock the clutch discs up against the cover plate/idler plate and flywheel when the clutch is disengaged and that keeps the gears spinning so you can't engage a gear.  many truck shops inspect and replace the input shaft on a transmission as part of a clutch job.  I have heard of something called a spline saver, but I don't know what it is.

[dtcerrato]

We have the 4104 with four speed Spicer. It is the nature of the beast, but not bad enough to have to stall the engine to get it in first gear. For almost two decades now - at the recommendation of the late "Deans Coach" we run a full synthetic 50 wt oil called Shell Dentax (no longer available) and the day we changed it the issue of hard to get in first gear got much better. After 18 years we're still running the same batch of oil - it has an extended service interval of 500,000 miles. There must be an oil of equivalent spec in case anyone wanted to give it a try. We swear by it for the different personality it has given our shifting patterns...

[pabusnut]

DC, can you actually get your trans into first from neutral without shuttin off the engine?  If that is so, I would try your magic transmission oil!!

I see O'reilly Auto sells Castrol Syngear CD-50 for $185. for a 5 gal pail. 
How many gallons does the spicer hold?

Steve

[busfan]

Seems to be a common problem, I am currently following the book adjustment specs for my 1968 4503 suburban.  For the clutch adj, you will start at the front and verify the pedal lever is 4.5" from the plywood. Next find the yolk and adjust it 1.75" to the front of the bulkhead(take out the pin and rotate to adjust)..I am currently on this step, I figure there is a raceway if I take apart the galley way near the rear of the drive/isle? maybe accessible via the front compartment on my S6m? hmm..

After those steps you go to the 90 ish degree clevis pin in back engine compartment, block the clutch pedal down up front and measure that distance from center of clevis pin, should be 1 7/8"

Now block the clutch pedal up lift up so bottom collar is touching plywood floor, measure distance from clevis pin at bell crank to bulkhead should be 3/8"

From there manual goes into adjustment for normal wear manual is 1963 November Sec 5 pg 156-9

Good luck to us all!