Clutch Gear/Synchromesh Studies

[keithb7]

Today my new NOS Chrysler clutch gear (aka synchromesh) arrived. I took a little time to study the new one, comparing to the old one. In my quest for understanding, I thought I would post my findings here and see what you folks think.

 

Looking at the inner contact teeth of the gear, this seems to be a wearing surface to me. I'll go out on a limb and say, maybe the highest wearing area of the clutch gear? This area contacts a ramp/cone shaped ring on an adjacent gear. When we shift between 2nd & 3rd we push this clutch gear, the brass teeth bite the adjacent gear ramp, and that action speeds up the clutch gear. The two adjacent gears speed match and mate together nicely. The new gear assy here you can see nice fine lines cut into the brass teeth surface. I'm not sure but I think this material is some type of brass/bronze alloy.

 

Pic 1 here is an example of nice clean, new teeth with lines cut in them, to cause friction I believe.  This is all coated in oil when in use inside the tranny. Special oil is specified for use that is not too slippery as I understand it. Modern GL-5 and GL-4 gear oil is not ideal for use in our old manual transmissions. I read that GL-4 is easier on brass parts, however GL-5 can cause brass parts to wear prematurely. These oils are recommended for extreme pressures conditions as found in a hypoid crown & pinion gear set. For our old Mopar trannies GL-1 oil is often recommended and available at Napa (USA, not available in Canada). Oil that is too slippery, (too high lubricity?) may not allow the clutch gear here to speed up or slow down quick enough as we force it into place with the stick shift. Causing a grind.

 

The next photo shows my old pitted and worn clutch gear internal teeth. They are pretty smooth. The brass looks a little beat up. Perhaps small particles of dirt, or bearing steel, embedded in it. It probably had years of use, running around with bearings that are wearing out. The pinion bearing in this tranny was found to be particularly worn and rough upon disassembly.  So were the metal thrust washers on the counter shaft. This worn material, steel, goes somewhere. Likely suspended in the oil,  and travels throughout the tranny during service. I'd wager it gets crushed into, and embeds itself into softer brass alloy metals, like these inner clutch gear teeth, when they contact the adjacent ramp ring and are forced to mate. The friction lines are long gone. Worn away. Imagine tiny metal particles of steel in the oil, working like sandpaper on these softer brass parts. Wearing them away. Keep your tranny oil clean folks!

 

I did try to measure the inside diameter of the old gear versus the new one. These teeth appear to wear on a taper so an accurate measurement is challenging with a snap-T gauge and a micrometer.

 

 

 

Edited June 11, 2020 by keithb7

[keithb7]

[keithb7]

A few more pics of the old and new clutch gears for comparison. This is the new clutch gear. That's vintage, dark anti-rust grease still on there in places.

 

 

Edited June 11, 2020 by keithb7

[keithb7]

Old gear assy here. The teeth appear to be a little thinner here. Maybe. I hope to have a nice smooth, easy shifting tranny when I reassemble it soon.

 

Edited June 11, 2020 by keithb7

[Sniper]

Good work and thank's for the update

[Adam H P15 D30]

Use the new fluids that are specified for manual transmissions to avoid the blocking ring wear.  Gear oil has changed so much I wouldn't even use the NAPA GL1 anymore.  As discussed on another thread, the Redline MT fluid seems to work well in our old gear boxes, I know it works great in mine.  Vast improvement in shifting verses the NAPA GL1 that it replaced.  When it shifts easier and grinds less, the blocking rings are doing their job better.

[keithb7]

I fired up my car tonight to test drive after my recent new clutch gear replacement.  The tranny feels great. No grinding at all. Smooth clean tight shifts up and down between 2nd and 3rd gear. I can hardly believe how good  this '38 car feels as it keeps improving. Tranny countershaft was sealed up with anaerobic sealant at the rearward end. Been full of oil for 4 days no leaks yet. Seems good. Finger crossed.

 

All the talk recently about different oils to use in the tranny. We know that Redline MTL works really well in these old 3 speeds. I did a little more reading and researching. I decided to try Caterpillar TDTO (Transmission Drive Train Oil). SAE 50. I work at the local Caterpillar dealer. The old timers here tell me it's a great oil for my needs. Cat tells us to use it many applications for axles, final drives, transmissions.  So I figured I'd give it a try and report my findings.  Early indications so far are I have nice smooth shifts. I also placed a very strong magnet inside the gearbox. Max distance from any moving parts. It is submersed in oil. It will collect any future metal wear particles and help keep my tranny oil clean.

 

So, the question remains did my shifts improve from the new clutch gear? Or from removing the GL-5 and putting in a better suited oil? I'm not sure I'll ever know as I did both. I had some odd other noises going on that I seem to have quelled as well. I installed a new pilot bushing, I trimmed up the rear motor mounts so they fit properly, and I found & replaced a broken spring in the trunnion at the front of my drive shaft.  I am happy with the results.