The Stovebolt Site Forums The Shop Area Driveline Slipping clutch 1940 Chev 4 speed.

Just recently on the road I have detected some slippage while shifting to 3 and then to 4th gear. 1st and second seems to be alright.

So I installed temporarily a tach and now can see the engine over revving by about 4 to 500 rpm then it seems to grab and works fine. Easing off the throttle after releasing the clutch also let it grab better.

I have a few nasty oil leaks from the ( suspected ) valve cover and possibly the side cover for the lifters. I say suspected because it is a very tight COE and can't see sh*t
once the fan has blown the oil all over......

Now I suspect I may have a rear main seal leak.

Engine is a rebuilt stock 261... refaced flywheel, new clutch and pressure plate...all less than 250 miles on the new speedo.

The truck is a Canadian WW II that has been totally restored.
Axle gearing is 6.5 with 37 in. tall 10.50x16 tires.....top speed 45mph.

Comfortable at 30 to 35 MPH doing 2000 rpm..... when shifting from 2nd to 3rd engine turns about 1800RPM....double clutch... release the clutch pedal and floor the gas the engine speeds up to about 2100rpm than grabs ....same happens from 3rd to 4th gear...engine speeding is a bit more pronounced......

Not loosing enough oil from all the dripping to add any additional oil yet...... seems to drip more when crankcase is full then self limiting at the halfway mark.

I plan on removing the cab completely late Fall early Winter to get to the bottom of this....... will need to drive the truck with the engine covers totally removed to detect source of the external leaks..

Meanwhile what can I do to ease the problem....?

How do oil leaking from the rear main seal ( if that is the case) get to the surface of the clutch plate??? won't the spinning flywheel just shed it inside the bell housing>> is there a possibility that oil fog is being generated inside the bellhousing?? would removing the bottom bell housing sheet metal and spraying brake cleaner
inside help in any way??

If I do need to replace the rear main seal ( and I do not know what I currently have in there) what is the best seal to use???

I am prepared to remove the emgine completely out of the frame and put it on the engine stand to fix it for good!!!

From what I have read it seems that once the clutch plate is oily it is ruined for good....are there other solutions??

Comments, suggestions, questions welcomed!!!!!

Bob C.

Are you using the 6 volt flywheel?
I have seen plenty of early pressure plates that have been rebuilt/refaced where enough metal has been removed so the clamping force is greatly reduced.

Check your cluch pedal for free play. It should be about 3/4" to 1" of travel. To test, drive truck in second or third with a bit of throtle on and push on the cluch pedal untill it starts to slip, If it slips right away you need to adjust the free play.

A clutch will start to slip in the highest gear, first.

Ed

I am using the 1959 261 flywheel with stock 11 in. clutch pack.

Also running the 59 rebuilt 12v starter.

There is very little to adjust that can be done on this system.... I have about 3/4 in. free play....only adjustment is the screwed tail end near the fork.

I usually avoid using the bull low first gear and start in second gear on flat surface.... and it seems to grab nicely in second gear.

I am wondering....because of the very low gear ratio an weight inertia of the 6000+ pound truck can the engine overrev and slip just because the engine can respond faster than the clutch can grab??? at say 2000 rpm.....

When shifting from 3rd to 4th once the clutch is released and I floor the throttle the carburtator sucks with a violent growl and the vacuum drops to about 5 in. then the engine races ahead of the clutch.....

Would rebuilding the clutch pack with stronger springs give more bite......

Remember this truck was designed around the 216 at 85 bhp and now running a 261 at a listed 140bhp.

Appreciate you insight and comments.

Bob C.

I have the same issue with my 1/2 ton '50 with a full pressure 235 and a dinky 216 clutch pack while pulling way too heavy of a load on a trailer. I found that gradual acceleration after letting out the clutch pedal will eliminate the slippage in all gears. If I goose it under load, it slips.
I did have the puny pressure plate resurfaced, which may have robed me of clamping force, as Pre '68 Dave suggested.
Carl

Temporarily sidelined with a sprained ankle which prevents proper usage of the clutch.....

My first line of defence for at least temporrary fix willbe to remove the bottom sheet metal pan on the bellhousing and spray large Quantities of break cleaner on the clutch pack while some one else depresses the clutch pedal..... messy but curious to see if it will dry up the clutch facing.

I am still having a hard time understanding how a leaky rear main seal on one side of the flywheel allows engine oil to reach the other face of the spinning flywheel??

Will update with results of my fix.

Cheers

Leaking oil that may hit the front side of the flywheel is going to be flung around the whole bell housing, then will run along the top of the bell housing, dripping onto the clutch pack, and easily soaking both surfaces. Even if it only got to one side of the disk, you'd loose nearly 1/2 of the friction area.

Usually oil on a friction material like clutch or brake shoes will tend to make them grab. But I know that diesel fuel will cause a clutch to slip. I once lost 14 gallons of it thru the clutch on my PowerStroke. It did finally stop slipping in top gear after I got the leak fixed. I initially washed it out at the local squirt-a-car, using the soap cycle and then rinse from the same source point as the leak in the engine valley.

Sorry about the bum ankle.

Thanks for reminding me that the clutch has two sides that gets clamped......

Will try degreasing/cleaning first opportunity I have.....

Been walking on a sore ankle for a week now and just found out it is broken and needs a cast.

Nuts!!!!

Bob, I went thru a similar thing in my '40.

After taking the clutch assy out I found that several of the fingers of the diaphram were collapsed and so there was lost pressure .

New clutch assy and all good again.

Hi Dusty

When I rebuilt the 40 Chev I used the clutch pack/flywheel from the 1959 261 c.i. engine and was totally rebuilt..... that is the 11 inch plate with HD springs......

Crawling about on my bad leg... not up to crawling underneath yet!!

Cheers

One can use a suitable stick or rod to work the clutch while leg heals. Worked for me.

Ed

Well I still have a slipping clutch........ and need some insight from you guys.

The sore ankle proved to be a broken shin bone....walking on a air inflatable cast cause my lower back to go out of wack....... painkillers where fun but did not cure anything.....general weakness in my legs landed me for an angiogram.....which concluded with a Quad by pass.....BUT I am till vertical.......

.....and my clutch still slips..... I can see the rpm needle zip up at least 500 rpm if I floor the gas going up a hill.....

Now I have removed the flywheel sheet metal cover and sprayed two full can of brake cleaner..... while a helper kept the clutch pedal floored.

Still slips. WHY......??

Only two source of oil...... the input shaft of the transmission ( 4 speed non synchro circa 1940) or the rear main seal on the crankshaft.

Before any spraying I inspected and found no oily residue on the front side of the flywheel....however the front of the tranny, shift fork, throw out bearing where very oily.......

Oil level in the tranny is even with the upper drain hole.

Is it possible that the leaking is from the tranny........ how common is that???

The tranny was totally rebuilt with new seals and new bearings/races.

350 miles on the engine and runs good with no oil burning ....... could my rear main be leaking??? how would it be visible???? engine seems to run best with 3 1/2 quarts and 1 1/2 quart contained in the two PH 8 spin on filters.............running shell 15W40 T4 Rotella diesel with zink supplement.....oil just been changed with filters.

One car show in late August and then considering pulling the tranny out to reach the clutch.

Since I have the top removed, and the center floor section can be removed..... it will be easier to use the overhead chain block for the removal/re-installation but would sure appreciate finding the cause so it will not need to be done again...... but it will be a [censored] to disconnect the four wheel drive linkages and E-brake and driveshaft to the Transfer case and the gas lines and floor gas selection switch (dual fuel tanks) just to remove the four tranny bolts..... the truck will be on four secure axle stands for this process.

I will have a capable helper and now quite capable of crawling underneath with the best.

Any insight, suggestions, comments are welcomed.

Bob C.