1938 Plymouth Transmission Rebuild

[keithb7 675]

Another developing issue, which I suspect is present in 99% of these old trannies is gear clashing at 1st gear. It is a sliding gear with no synchros. Previous owners were likely attempting to downshift into first gear while still rolling. Not good. You can see 1st gear teeth that have rounded nubs on the leading edges, where it makes initial contact with its mating gear. 

See pics below. I see some small chunks are missing from some teeth. I found the little pieces in the bottom of the case. 

 

Two mating gear areas, sliding 1st gear on main and mating gear on countershaft, are seen below here. 

 

Having limited experience with the longevity of these gears... Are you guys reusing gears like this? Or replacing them?  Keep in mind I am reviving the car. Not restoring it. I'm Building a local summer cruiser to enjoy. However I do prefer reliability.  The engine makes what,  maybe 120 HP when fresh? With no more future student drivers , I might be able to nurse these 2 gears along?

 

 Thx. Keith. 

 

 

 

 

Edited December 3 by keithb7

[Dodgeb4ya 2,205]

IMO use the gears. If one or two teeth were missing 1/5 or more of a tooth

...Replace the gears....in your case easy driving more than likely A-OK.

You might want to check both the counter shaft and the end of the main shaft where the needle rollers run on to be sure there is no damage to the surface case hardening on the shafts.

Pitting/spalling.

 

 

Edited December 3 by Dodgeb4ya

[kencombs 223]

7 hours ago, Dodgeb4ya said:

IMO use the gears. If one or two teeth were missing 1/5 or more of a tooth

...Replace the gears....in your case easy driving more than likely A-OK.

You might want to check both the counter shaft and the end of the main shaft where the needle rollers run on to be sure there is no damage to the surface case hardening on the shafts.

 

Agree with this advice.  I 'll add this:  Use a fine grinding stone on a die grinder, or similar tool, to smooth the edges of the chips.  That removes stress risers that can further damage the teeth and eases the engagement.  A slight taper and no sharp edges is the goal.  Back when 'three on the tree' replacement and repair was a staple of the auto salvage and garage business, we did hundreds like that.

 

 

[RobertKB 525]

I agree they are re-usable but with the tranny completely apart and if you can get the replacement gears at a reasonable price, why not replace? Ron Whiting at Rovon Auto Parts is very reasonable and all his parts are NOS or NORS and made in the US or Canada. His number is 604-936-9312. Having parts numbers ready always helps. I do have a parts manual that gives the numbers if you need help. Ron is a great guy, his prices are in Canadian dollars, and he ships quickly. Pretty sure he would have the small parts package you want as well as anything else. Good luck on the tranny project!

[westaus29 16]

Hi Keith, I am a bit late joining this conversation but as I have just recently overhauled my 38 Plymouth gearbox I will pass on my learnings. I had previously done my 29 gearbox which is a bit simpler. 

 

I stripped the gearbox mainly because it was out and accessible and I am trying to do a thorough restoration. It had always shifted fine and sounded fine. When I stripped it, I found missing and broken needle rollers inside the input shaft where the mainshaft slots in, but no apparent damage. So am very glad I did strip it. My bearings were ok and gears were in decent shape with light damage to first and reverse so I kept them. Before going further I ordered a small parts kit. These are available from Andy Bernbaum for $65 and include all needles, thrust washers and circlips. I bought mine thru eBay from Mopar-Direct because of postage issues to Aus and am happy with the quality.

 

First and most important is a couple of lengths of 3/4 inch hardwood dowel, one exactly 6-1/2 inches long for cluster gear and one 1-1/16 inch long for reverse idler, see pic below. The lengths of these match the overall assembled length of each gear including thrust washers and make assembly quick and easy.

 

 

Secondly I found it much easier to work on the gearbox by bolting it upright using a simple L bracket mounted in the vice as per pics below. It helped to have a swivel vice.

 

 

I was unable to remove the input shaft as a first step. It fouled the cluster gear and I dont like the idea of hammering the bearing as a way of getting that clearance, especially as my bearings were fine. So for me first step was to start at the back and remove the mainshaft complete with clutch assembly, taking care to hold the assembly together. Next I removed the countershaft, replacing it with the dowel, and let the cluster gear drop into the bottom of the box. This allowed me to remove the input shaft complete with bearing and circlip, then lift out the cluster gear assembly, then tap out the reverse idler shaft and likewise remove it ( cant quite remember if had to remove the reverse idler gear first???).

 

Assembly is the reverse. The dowels make it easy to insert the needles in the reverse idler and cluster gear, and position the thrust washers at each side. for the cluster gear, the steel thrust washers go next to the gear and needles, the bronze washers go on the outside. Note there are 31 needles to each bearing and I found they fit like a glove.

Pic Z1 shows needles being installed using dowel to hold them in place

 

 

Z2 shows thrust washers about to be installed

 

 

and Z3 shows the cluster assembly with thrust washers ready to go back in the box.

 

 

To insert the larger needles in the input shaft recess is not quite so easy, The last one feels like it wont go in but a bit of pressure from a wooden dowel and it should pop in. They fit so tight they wont fall out while you assemble them into the box. See pic Z4

 

 

Next install the cluster gear assembly and the reverse idler assembly, leaving the cluster gear in the bottom of the box. Install the input shaft, then lift the cluster gear into place and install the countershaft. Make sure countershaft and reverse idler shaft rear slots line up for the retainer plate. Then replace the mainshaft.

 

If you decide to pull the clutch gear apart you are in for some fun. I have some pics somewhere of a tool I used to reassemble it. As I recall it was a tin can of appropriate diameter. I could find it if you want.

 

Edited Wednesday at 02:38 PM by westaus29
wrong data

[keithb7 675]

Thank-you @westaus29 for taking the time to write down your experience. The info is excellent and very helpful.

 

Are your wood dowels 3/4” diameter? 

[westaus29 16]

Yes 3/4 inch as bought from hardware store. Have just corrected reverse idler shaft length, had some extra bits in toolbox and picked the wrong one. Just took the top off my spare box and confirmed length is 1-1/16 inch. By the way, a bit of slop in bearings is to be expected as they are simple ball bearings. The input and mainshafts are steadied at both ends by bearings and arent going to move far.

[keithb7 675]

Hardwood, Oak ¾” dowel acquired.  Another good ol’ made in the USA part, I’m pretty sure. We don’t seem to grow or harvest any oak up here in Canada. 

I’m not planning to disassemble the clutch gear!  Hopefully I don’t have to. 

 

I ordered up USA SKF roller ball bearings. Numbers are:

 

Front main pinion (input) shaft 207ZNR. 

 

Output shaft bearing which is pressed into park brake housing 207S

Edited Friday at 03:49 AM by keithb7

[westaus29 16]

Just remembered we have gone metric and dowel would be 19 mm not 3/4 inch. Difference of .05 mm not a problem. True Oak is hard to get here, we have "Tasmanian Oak" which is actually a native eucalypt, and in WA we have Jarrah which is a prized dark red very hard eucalypt.

 

Thanks for documenting the bearing numbers. I'll add it to my files.

Jim

[keithb7 675]

@westaus29 Reading though your instructions, there are a few things I can't quite wrap my head around.  When pushing the countershaft out, it was pretty tight. When putting it back in, if the slot for the lock plate is not lined up exactly as needed, how does one twist the countershaft to align?  

 

Also, what is the proper technique to push the reverse idler gear out the back of the case? I can't access it from both sides of the outside of the transmission case, as I did with the countershaft. Thanks, Keith

[westaus29 16]

I had a bit of trouble aligning the countershaft but have done it three times now, twice on my good box and once on the spare. The best way I found is to insert countershaft and reverse idler shaft  only part way so their far ends are just starting into the case. That holds everything together while you align the shafts and they are easier to twist. Tap the retainer plate in place at this stage and slowly tap both shafts in all the way. I put a bit of sealant here and there to hopefully minimise oil leakage

 

To get the reverse idler gear out I used a long brass drift inserted at an angle thru the input shaft hole. It doesnt seem to be as tight a fit as the countershaft.