VCCA Home Forums GENERAL DISCUSSION New User Forum 1931 transmission

My 31 tranny jumps out of 3rd gear under 'no load' condition. The gears and shift fingers are good and the gear is fully engaged when the tower is removed.
I have tried another tower with same result.
I suspect the input shaft is wandering but how to check?
All comments appreciated

Most often the internal surface on the gear on the input shaft and the high speed sliding gear are worn (tapered). It is often due to a bad pilot bearing allowing the input shaft to move.

Thanks Chipper,
I'll investigate and report back.

I can notice a very slight taper on the input shaft gear teeth in the contact zone.
I have another box which has a step in the gear height in the mesh area and a more pronounced taper in the leading edge of the gears of the input shaft.
I will rebuild this second box and see what happens.
Also, which casting numbers on the inside or outside of the casing are relevant.

The taper on the meshing surface of the gears is what forces them apart. I don't know of a fix for that condition short of replacing the gears. The greater the taper the faster the gears will separate. It might be worth trying to flatten the taper on the gear face for both gears and see what happens.

FYI (and others) the change from power applied to decelerating seems to help work the gears apart. The taper on the gear edge helps the gears to mesh smoothly. It is taper on the gear faces that force them apart.

Chipper, your explanation is great but there is one more partial fix. We have 6-11% grades around here and my '46 pickup(slightly different tranny I know) would often pop out in third on them. Since I had new pilot/ inputshaft/ tranny gears I figured to live with it. Talking to Jim Carter one while visiting his business I told him the problem,he reached under the counter and pulled out a brass bushing(after market back then) that the shifter shaft ball rides in the tower. Way heavier than the thin original,it let me install a much heavier spring under it. I was skeptical but for about $15 why not.It did not totally solve the popout but made a heck of a difference.
If you did'nt mind the appearence,A bungy from the shifter to the seat would REALLY solve it, grin. Seriously, I also was told by some 'Ancient' at a swap to look closely for wear in the shift gate plate and/or shift rod ball which prevents satisfactory engagement depth. Never got around to pursuing that. DFC

DFC,
You are correct that wear in the shift gate plate, shift ball can prevent total gear engagement. It is not the primary cause but important to check. The primary cause is wear in the pilot bearing or bushing that allows the shaft to wobble and produce the taper in the meshing surface of the gears.

My '28 Canopy Express has the problem of occasionally (mostly when applying power just after deceleration) jumping out of third gear. I have learned to drive it with my right leg resting on the floor shift lever. I know that it is double duty as that the right foot operates the accelerator pedal. Works for me. Don't know if it helps remove the taper from the gear faces or not. Got to believe it does.

Thanks to all for comments, which I appreciate
I am rebuilding the second box, which appears to be a later version, with the input shaft gears being cut down to about half height over the meshing zone( which is about half the length of the gear). Obviously third gear has similar pcd.
I have also discovered the input shaft pilot bush has been replaced with a bearing. The input shaft from the second box had a 10thou slop, so we are machining the input shaft sufficient to accommodate a sleeve to fit the bearing.
The gear selector fingers have been welded up and filed back to give a nice fit and the these take the top gear up to full mesh.
The detent mechanism in the tower also seems to be positive and firm, although this is a subjective notion.
Also new bearings are being fitted front and back.
Will keep you informed and post some photo's in due course.

On a related issue what is the intended lubricant for the front trunnion bearing.
Grease, oil or a combination. I note gear oil can pass thru the rear bearing and a grease nipple is fitted above the trunnion.

Once again thanks.

A pilot bearing was originally used in 1931 engines not a bushing. It also might be a good idea to check to see if the crank is centered in the clutch housing. If it is not, then the transmission input shaft may wobble causing problems later.

A sealed or shielded bearing can be used on the front of the transmission but removing the rear (toward transmission) seal or shield is recommended (so the bearing can be properly lubricated by transmission lube). It is particularly a good idea if the front bearing retainer is worn. Never use a sealed or shielded bearing in the rear as the lube from the transmission also lubes the U-joint (or trunnion). The grease "nipple" originally was a factory installed plug to be replaced with an alemite fitting. The alemite fittings were intended to be used with the "grease gun" filled with "heavy oil" (600W or SAE 160 oil). Grease does not properly lube the U-joint.

While you have the transmission removed check the fit of the front propeller shaft bushing. If the shaft can be noticeably moved up and down then the bushing (and shaft?) is worn. It is best to replace the bushing and original cork seal. That prevents lube from overfilling the rear end and eventually flowing out of the axles onto the brake linings. A modern lip seal is available to replace the original cork.

Thanks to all for comments, in particular, Chipper
I will forward a more detailed response once all is done and dusted.
At the present time we have fitted the second box with the cut down input shaft, it does not jump out of gear, however oil leaks from the front of the box are a problem..
This leads to another question? Is the front gearbox bearing meant to have an oil seal on the clutch side.
The box we repaired did not have one, just an oil groove to redirect the oil back into the box.

MickCno1
There was no "seal" as such in the gearbox. The groove you mentioned and reasonably tight clearance was all that kept oil in the gearbox, if there is excess clearance around the groove the oil will get past.
Note Chippers comment about the later type "sealed" bearing with the inside seal removed as an effective remedy for this situation.
Tony

thanks Tony et al,
I just completed 1000klm maiden journey, used about half a litre of gear oil. I will replace the front bearing with a sealed unit as per Chippers comments.
My bigger problem is engine oil loss, but I will create another post on that matter.