cause of 3rd gear slipping out?

[sr325]

Hi Guys,

My 3rd gear slips out, from time to time. Does anybody have any sense of whether this means a full rebuild is required or is it more likely to be something in the forks that can be repaired just by pulling the top of the transmission?

[mcgarrett]

When our old transmissions jump out of gear, it is usually due to worn spacer shims at either end of the main gearset shaft, or a combination of other worn parts that are misbehaving. The best approach is to pull the gearbox and check everything to see what's going on. Fits and clearances are critical.

[rotorwrench]

The high gear is engaged with dogs on the end of the input shaft that engage with the intermediate gear. If the dog teeth have a lot of wear or the spline grooves are worn in the intermediate gear then that could allow for easier demesh of the 1:1 ratio drive. Straight cut gears generally don't have much of a thrust load either way but excessive gear/mainshaft end play or movement could be troublesome in this respect. Condition of the shift tower components could also have an affect. If a person takes the shift tower off the lower case then a visual inspection can be performed on this stuff to see what the condition of the gear train and shifter components are in. I'd look for wear on the bottom of the shift lever where it engages the shift forks. A lot of wear won't allow the intermediate gear to move as far as it should.

[Tom Endy]

Attached is a very old newsletter article about jumping out of gear. The causes are still true today.

http://www.santaanitaas.org/wp-conte...ut-of-Gear.pdf

Tom Endy

[Synchro909]

Originally Posted by rotorwrench

The high gear is engaged with dogs on the end of the input shaft that engage with the intermediate gear. If the dog teeth have a lot of wear or the spline grooves are worn in the intermediate gear then that could allow for easier demesh of the 1:1 ratio drive. Straight cut gears generally don't have much of a thrust load either way but excessive gear/mainshaft end play or movement could be troublesome in this respect. Condition of the shift tower components could also have an affect. If a person takes the shift tower off the lower case then a visual inspection can be performed on this stuff to see what the condition of the gear train and shifter components are in. I'd look for wear on the bottom of the shift lever where it engages the shift forks. A lot of wear won't allow the intermediate gear to move as far as it should.

Those dog teeth usually are not cut square but have a slight angle (about 3°) holding them in mesh. Once they wear, the angle is reduced or even reversed and the gears don't stay meshed.
I stopped that happening on one occasion when we were hundreds of miles from a workshop by using a battery powered angle grinder to recreate the angle. Was it a "farmer fix"? You betcha but it got us home.

[CWPASADENA]

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Originally Posted by rotorwrench

Condition of the shift tower components could also have an affect. If a person takes the shift tower off the lower case then a visual inspection can be performed on this stuff to see what the condition of the gear train and shifter components are in. I'd look for wear on the bottom of the shift lever where it engages the shift forks. A lot of wear won't allow the intermediate gear to move as far as it should.

It may require a full transmission rebuild but first look at the shift tower.

Also, it is possible a badly misaligned flywheel housing can cause enough mis alignment of the input shaft which can also contribute to this problem.

My opinion,

Chris W.

[dave in australia]

A few causes, worn bearing between the input and output shaft. Worn input or output shaft bearing. Worn teeth on input shaft where top engages when selected. Worn input shaft support bearing at rear of crank. No shims between the flywheel housing and engine block causing a misalignment between the crank and input shaft. Bent selector fork not allowing the detent ball to engage the shaft dent, this was the cause of my A jumping out of top.

[Bick in New Zealand]

I'm having that problem with my Mitchell. I have done less than 1500 miles. It just slips quietly out, usually when going down hill or over a bump.

[Synchro909]

Originally Posted by Bick in New Zealand

I'm having that problem with my Mitchell. I have done less than 1500 miles. It just slips quietly out, usually when going down hill or over a bump.

I've had that happen but I put it down to the soft rear engine mounts allowing the whole of the mechanicals to move back and forth in the car while the shift lever stays in the same place, pulling it out of gear. With the Burtz engine, I went back to solid mounts because it runs so smoothly and I've not had a problem since. The car feels better on the road too.

[WHN]

Is the car new to you? Has it been doing this since you got it? How much maintenance has t had over the years?

In my opinion, you are going to have to rebuild the transmission. Even if your shift tower is contributing to the problem. There are most likely other issues going on.

If your shooting for a trouble free car. Pull the transmission and check it out. At this point look at your clutch and pressure plate, replace the throw out and pilot bears. Check your u-joint too.

Welcome to owning a 100 year old car.

Enjoy.

[sethkestenbaum]

Swap out the shift tower with a known working one and see if that "fixes" the problem. Do not bother with a full rebuild unless you need it.

[Big hammer]

Remove your tower and remove the plungers and spring, clean the bore where the plungers live. The plungers need to move easily and the spring holds them in the notches on the shifting rails. The shift should snap into each gear

[sr325]

Hi Guys, Thanks again for so much helpful input. I just discovered the Burtz engine last night. So, that completely changes everything. I'm pretty sure I'm going to go that route. Which means that once I get into it I'll have everything out and it will be a good time to go through the transmission too. I recently purchased this car online and don't have any information on it's history. So, I'm just discovering things as I dig into it. Sadly, it's in worse condition than I had hoped for. But, the body is basically nice and clean (no rust). So, all I have to do is mechanical restoration. First thing is to get rebuilt (cast iron) brakes/hubs all around and rebuilt steering and then get it back on the road and see how it handles. Then I'll plan out my engine/transmisison etc., rebuild for next winter.

[Terry, NJ]

I had a similar problem with one of my A s. Unfortunately, I had to pull the trans and disassemble part of the trans (upper part, not the reverse or the cluster). The problem wound up being the back of the input shaft. The grooves that engage with the shaft when it's third (Third gear is direct drive, no meshing of gears) were worn. I wondered if it was a cheap replacement input shaft. I had other shafts that seemed just as old (probably original) that didn't seem so worn at this point. In fact I used one of them and it's working fine.
Terry

[CWPASADENA]

Originally Posted by sr325

Hi Guys, Thanks again for so much helpful input. I just discovered the Burtz engine last night. So, that completely changes everything. I'm pretty sure I'm going to go that route. Then I'll plan out my engine/transmisison etc., rebuild for next winter.

If you are going with a Burtz Engine, I would strongly recommend you also go with a Mitchell Transmission. Better gear ratios available and synchronized in second and high. Far superior to the original transmission.

My opinion,

Chris W.

[Bick in New Zealand]

Originally Posted by Synchro909

I've had that happen but I put it down to the soft rear engine mounts allowing the whole of the mechanicals to move back and forth in the car while the shift lever stays in the same place, pulling it out of gear. With the Burtz engine, I went back to solid mounts because it runs so smoothly and I've not had a problem since. The car feels better on the road too.

That would be right with mine, as I have flexible mounts front and rear. Thanks for the tip.

[1929]

Originally Posted by Bick in New Zealand

I'm having that problem with my Mitchell. I have done less than 1500 miles. It just slips quietly out, usually when going down hill or over a bump.

My Mitchell will do the same when going down a steep hill.