VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 1938 Input shaft

My gearbox (transmission) drops out of top on overrun. I have pulled it out and it is pristine but we noted that the input shaft which goes into the spigot bush has had 1/2" cut off it. That means that instead of the input shaft protruding 6 3/4" from the casing it protrudes 6 1/4". That would not help it to stay firmly in place. So I grabbed another three boxes out of the collection and find that another shaft has also been shortened 1/2". Why? The distance from the back of the bellhousing up into the spigot is 7" so 6 3/4" would be right. Can anyone suggest why this would happen.

Terry, I don't have an answer about the shorter input shaft, but have you clocked the transmission tunnel with a dial gauge, because it is a common problem . My 1939 slips out of top if I coast down hills using engine back pressure. My car has the original engine and bell housing and gearbox.

Hi Jack. Yes it is a common problem to the point where some cars have become undriveable.I have looked at the dial gauge thing and the shimming to correct and I think that if the design is so sensitive then I won't bother. I know of one car that has had the gearbox rebuilt three times and that has only made matters worse. I will install a NOS clutch gear and see what happens. I would be interested to know how often your car drops out and at what sort of speeds. Mine does it on steep hills and when I back off at over 80 kph.

Terry, I haven't driven my 39 for a few years on an high speed road, but i remember it was at the high speeds going down grades in top gear , and it would kick out after a while of doing this. I just reselected top and it would stay in gear for a while , and then pop out again. For my gearbox , I replaced the two main bearings and the needle rollers in the aft end of the input shaft, and that was all. I was thinking that the general driveline vibrations which occur was the cause, and slowly shakes the gears out of line. My other thoughts are that when you drive the gears wear in a certain way , and then when the load is reversed it causes the gears to want to separate , instead of biting in to each other. .

I was pretty sure that you have discussed this issue here previously..

The problem you are discussing is more often caused by gearbox to engine misalignment (.003" out can be enough in some cases). Check the bell housing faces are parrellel and the gearbox face is square to the centreline of the main shaft, not really possible without dismantling the whole thing.
Tony

Jack I have discussed some of this before and mention has been made of misalignment. I have since had the gearbox dismantled and found that two out of four gearboxes have the clutch gear shortened by half an inch. I as asking why this would happen and at this stage no-one knows.

Well I have had the input shaft lengthened so it is now perfect. The bearings were replaced with sealed bearings, a new flanged spigot bush was installed and the detent springs were replaced with new.The result is that the car now no longer drops out of top gear. Another car has had the transmission gone over four times and it still drops out of top. Another transmission was acquired and following from my experience the replacement input shaft was measured and found to 1/2" longer than the one in the car. Where are these short input shafts coming from? Why were some cut short? Who knows?

Any update on the reason for long and short input shafts?

I found the same thing when going through spare/used input shafts to use for my '37. I have a total of 8 input shafts for 1937 or 1938 transmissions. Three of them have the short nose and 5 have the long style. Two of the ones with long noses had two grooves through the splines and the other three with long noses had solid splines (see picture of the two long nose styles). Perhaps some are service parts but I have seen all three styles in 1937-38 complete transmissions.

https://www.dropbox.com/sh/vp4qm0fsreagwhz/AACaWz2JLxpSvbZhoWHR1TX_a?dl=0

By the way - I'm using a long nose input shaft with the two grooves in the splines. I don't know which is correct for 1937, but this one seems to be in the best condition of the ones I have.

Tom

I have had my transmission in four times, two at one shop and two at another. However mine did not drop out of gear and everything is good now. The last input shaft that was installed was about half an inch shorter and I don't believe it was cut.

In conversation I asked about what causes them to come out of gear. He pointed to the engagement splines on one of the front shafts I have. The splines were worn on a curve at the tips where they first engage. He told me that causes them to jump out of gear.

Do you know that the teeth are fully meshed when shifted to drive?

It may be that the slector or something in the shifting mechanism is not sliding the gears all the way home.

That was the ailment with my 41 several years ago.

Charlie

Well here we are back again some four years and 3000 miles later. The transmission works well enough but still falls out of top gear on the over run going down a steep hill. Fortunately the brakes are good however it is simply dangerous. It won't pull back unless I push it into neutral and then clutch in and pull back.Of course it will then slip out again. Before I pull it all out again there has been a suggestion that instead of running 90 oil I should try 140 which I ran in the 28. Any thoughts?

In all cases I have found that off-line wear between the input shaft gear and the third sliding gear is the problem. The wear occurs when the input shaft is out of line or wobbles. Bad alignment of the transmission or bad pilot bearing (or bushing) is the primary cause.

Replacing the worn parts (pilot bearing, input shaft and/or third gear) have solved the problem. I have been able to keep from getting worse by replacing pilot bearing and holding in gear when changing from engine pulling to retarding.

Thank you Chipper. I will show the gearbox man. The problem is that he has used the best parts out of three gearboxes and can't do any better. I will try and locate NOS parts but they are scarce here. I doubt that using 140 grade oil will solve the problem but it is easy and reversible.

wHEN THE NEEDLE ROLLERS BEARS AND SHAFT SURFACE THEY ROLL ON ARE WORN IT WILL JUMP OUT OF HIGH GEAR, ALSO WHEN THE BALL BEARINGS GET LOOSE.

If the input and output shafts arent perfectly lined up all non overdrive gearboxes will jump out of top gear, mostly on the down hill closed throttle situation. I have found a couple that disengaged on the up hill full throttle but not a common occurrence.
Tony

I might need some help here completely describing this fix.

Recently a fellow VCCA member (John Wolf) helped me rebuild the 3 speed transmission for a '49 3100 pickup. One extra step he had me do was to insert a washer/spacer between the internal side of the inner race of front bearing for the input shaft and the face of the gear on the input shaft.

He showed me that this moves the shaft to the rear slightly which gives more positive engagement especially in high gear (direct drive). It was not a real thick spacer because you need to leave some clearance between the rear end of the input shaft and the main shaft.

John has rebuilt many 3 speeds over the years and has used this fix to reduce the problem of jumping out of high gear.

This might not applicable to your situation but might be worth considering.

Thanks for that. I will keep it in mind.