Shifting the Spur Gear Transmission, and General care & Repair

[Jason Mills]

I am new to the unsynchronized world. Upshifts are sort of Ok, Downshifts are best when there is enough vehicle speed, that I do not rush... Yet I know some people proficient and all of the shifts are seamless, smooth, and appearing effortless... How does one get headed in that direction?

It would seem that low range would mostly be select the speed you want to travel at, and start in that gear for 1st, 2nd, and 3rd. With 4th being similar to the 2nd to 3rd shift in high range.. is that accurate thinking. I am not necessarily expecting to spend much time in 4th low, but I am wanting to have a much greater depth of understanding.

Please advise.

Also regarding the transmission health, it is Noisy... and seems to have lots of noticeable gear lash between drive and coast when in low range and first gear... Would a rebuild improve or solve ether of these characteristics?

[Matt Wilson]

In a nutshell, I would say it takes practice to get good at shifting the non-synchro transmission, especially downshifting. I am not 100% proficient, and haven't driven my '49 Power Wagon in a several years (it is down while I repair numerous things), but I was pretty good at it when I did drive it. My speedometer didn't work very well, but you get to where you can judge the road speed by eye, and you get decent at judging the sound of the engine speed needed for the gear you are downshifting into. You punch the gas pedal briefly to rev up the engine, immediately after letting out the clutch with the gear shift in neutral, then quickly push in the clutch, and as the engine (more importantly the transmission input shaft) is slowing down (because you've already let off the gas pedal by now), you pause briefly before shifting into the lower gear. You get good at knowing how long to pause, too - sometimes it's hardly any pause at all. You get to the point where you realize when you've over-revved the engine for the road speed. I suppose if the speedometer was working and if I had a tachometer, maybe I would start to memorize some of the approximate engine rpm's needed for the road speed indicated by the speedo and the gear I'm shifting into. Of course, looking at the speedo and tach and trying to do all that shifting and revving stuff might take too long, and it might be better to just go by your senses as I first mentioned.

As for gear ratios in low vs. high range, the transfer case high range is 1:1, while its low range is 1.95:1. The transmission gear ratios are: 1st - 6.4:1, 2nd - 3.09:1, 3rd - 1.67:1, 4th - 1:1. Reverse is 7.2:1, I believe. So 4th gear and transfer case in low would be the equivalent of 1.95:1, so it's not that close to being in second gear with the transfer case in high. Driving in third gear with transfer case in low would be 1.67 x 1.95 = 3.26, so it's pretty close to being in second gear with transfer in high. I'm not sure if that's answering your question, and I'm not sure I understood your question, but maybe it helps.

Regarding transmission noise, it depends on what kind of noise you are talking about. Are you hearing a noise that starts as a groan at low engine rpm's and becomes a higher pitched whine at higher rpm's? If so, then it may be just normal gear noise. The non-synchro transmissions have straight cut gears (spur gears) that naturally make a lot of noise, and there isn't anything you can do about it; it's just their nature. However, you shouldn't hear any such noise in fourth gear, because the power is going straight through the main shaft, not through any of the gears. If it's some other kind of noise you are hearing, then it could be a bearing going bad. I will say that the later non-synchro transmissions (which were installed starting around 1951 or 1952) were known for having a noisy third gear. Sometimes a rebuild would help, but sometimes not. I don't know as much about those gearboxes.

As for gear lash, you may be detecting slop in the drive train as a whole, not just the transmission. Some noticeable drive train slop is normal. My truck has noticeable slop, and it's especially noticeable in the lower gear ranges, and that's with a rebuilt transmission, differentials and new u-joints everywhere. The only unit that hasn't been rebuilt is the transfer case, and having crawled underneath the truck many times, I believe there is some slop there, but the it seems to run just fine, so I'm not sure that it is sloppy due to excessive wear. I suggest checking your u-joints and replacing as necessary. Also check the slip yokes of the drive shafts to see how much slop they have. It is common for those to be well worn. You could contact Vintage Power Wagons, Midwest Military, Dave Cirillo and possibly others to check availability of replacement slip yokes. They may be able to answer questions regarding the amount of slop in the individual gear units (transmission, transfer case, differentials). I am on a trip out of town currently, but when I return home, I could try to check these units on my truck and give you some idea of what I'm seeing. You'll need to remind me.

[Matthew Welcher PWA]

This is a fantastic write up.

Yes thank you Matt

[Jason Mills]

Thanks Matt,

I have spent a bit more time in high range and the shifting is slightly better. Yes you have answered my question in regard to shifting gears in the low range.

I have replaced all of the U-joints.. but I suspect the slip yokes on the front drive shaft are in need or a replacement, as there is play when the Yoke is moved up and down perpendicular to it's rotation. I will go looking for possible replacements or a repair option. As for gear lash.... I will see if I can explore how much slop is in first gear on the transmission. That seems to be where it is most noticeable when crawling through the woods. I rebuilt the Transfer case, but maybe investigating it's amount of gear lash is also useful, as low range is where it shows up most notably.
I appreciate your response and the sharing of your experience.