915 tranny 5th/rev detent question

[OhioRick]

Greetings Pelicans,

Just wrapping up a full rebuild on my 1983 SC 915 box (with kudos to Pete Z. and his tutorials). I think I made a last minute mistake with a detent.

This is after a long process of pinion and main shaft bearing repair, one piece thrust plate, mostly new bearings and synchro/slider replacements. I also assembled and disassembled many times adjusting final drive preload, pinion depth and backlash to get it just right.

Problem: I just mounted my center case section and installed the final 5th/reverse gears and forks. About to check the final clearance between reverse idler and slider gear and find that the fork and rod are not held in place.

My 1/2 and 3/4 shift rods are interlocked so only one can be moved at a time, so I think those detents are correct, but the 5/rev rod moves freely.

Question: did I mess up the last detent(s)? Isn't this rod supposed to be held in place by a detent so I can set that gap? Under what shift positions should it be locked? How about free too move?

Further: I checked for the detents to be visible and in place before installing the rods, but don't remember installing something called a "sleeve". If I didn't, I wonder if the pin is out of place and not pushing the detents far enough?

Would appreciate help from somebody who has been through this. Thanks.

Rick

[KTL]

The detents don't really "lock" the selection of the gears. They just provide a spring pressure on the metal pills to hold the pill against the notch/relief in the shift rods. It keeps the rods from moving by themselves when a gear isn't selected on that rod.

You can overcome the pressure of the detent spring by simply pushing on a rod by hand and you will select whatever gear you want. So, you should be able to select two gears at a time no problem by using the individual fork rods, not the main/shifter shift rod, for 1-2 and 3-4

Here's some basic info. from the '72-'83 manual. The last image shows you a cross section of how the vertical detent should be engaging the rods

[swade]

Awesome post KTL.

[OhioRick]

Thanks a bunch for that explanation and factory illustrations. I'm kicking myself that I missed such a small detail after everything else I went through.

I have a sneaking suspicion the small piece I found in a box later is the sleeve I should have installed with the pin and detents. I will pull things apart tonight and hopefully get it right.

Count me as another satisfied Pelican forum user. Your help is invaluable to us amateurs.

Rick

[Jon B]

Originally Posted by KTL

The detents don't really "lock" the selection of the gears. They just provide a spring pressure on the metal pills to hold the pill against the notch/relief in the shift rods.
It keeps the rods from moving by themselves when a gear isn't selected on that rod.

Kevin, just to be clear about their purpose, these detents do lock an adjacent shift rod whenever 3rd, 4th, 5th or Reverse gears are selected.
The detents do not merely keep the rods from moving by themselves.
In the illustration below, the 5th/Reverse shift rod (11) has been locked by the long detent (9), pin (6) and short detent (4), due to the 3rd/4th shift rod (3) being engaged.

This is done so that two adjacent shift rods, in this case the 3rd/4th and 5th/Reverse rods, cannot be moved into gear simultaneously.

Originally Posted by KTL

[john walker's workshop]

Did you pop out #2 and see if #4 is visible above the shaft it's supposed to be under?

[OhioRick]

Guys,

Found that I forgot to install the sleeve. Just had to remove the center section to remove the roll pin and install the pin, spring, and sleeve properly, and now it works as designed.One gasket trashed, but crisis averted.

I do believe the detents in the dif case are actual interlocks that prevent the 1/2 shift rod from actuating when the 3/4 is in gear (and vice versa). But the 5th/rev detent is spring loaded, allowing me to push past the detent as I installed that shift rod.

All is good again (at least until I get in reinstalled and see the final results of over $2000 in tranny parts and machining). Again, thanks for all the help.

Rick

[KTL]

Great to hear you discovered the problem Rick. No worries about the gasket. They're so thin and fragile it's a wonder we get them in place the first time w/out harming them. I just use a very thin application of Curil T or Hylomar blue (guys who work on Hewland gearboxes seem to really like this) sealant in place of the gaskets anymore.

Sorry for my misstatement (thanks as always to Jon B for his corrections on me) about the function of the detents. My belief was based on how you can indeed select two gears at once. With the tail case off the transmission, you can put the transmission in 5th and then push one of the other two shift rods forward. I believe it's 1-2. Then you have the trans in two gears at once and the gear shafts are locked. Incidentally this is sort of a half-a$$ed way of locking together the mainshaft and pinion shaft which allows you to loosen or tighten the pinion nut inside the 5th-reverse guide hub.

[Pazuzu]

Originally Posted by OhioRick

I do believe the detents in the dif case are actual interlocks that prevent the 1/2 shift rod from actuating when the 3/4 is in gear (and vice versa). But the 5th/rev detent is spring loaded, allowing me to push past the detent as I installed that shift rod.

Rick

The simple pill between 1/2 and 3/4 is to prevent both from being engaged at once (each shift rod pushes the pill into the detent of the other one, preventing it from moving at all).

The complex detent which you just fixed prevents 3/4 and 5/R from being engaged at once. The pill-rod-peanut actually acts as one long pill, you cannot push the 5/R shift rod around if 3/4 is engaged. If you can, then you have something still wrong with that detent.


You can engage 1/2 and 5/R at once, they never made a lockout for that.

[Jon B]

Originally Posted by Jon B

In the illustration below, the 5th/Reverse shift rod (11) has been locked by the long detent (9), pin (6) and short detent (4), due to the 3rd/4th shift rod (3) being engaged.

Let me somewhat correct my description of the last illustration.
It could represent the situation I described, where the 3rd/4th shift rod is engaged and the 5th/Reverse rod is locked, but it could also represent a situation where the 3rd/4th shift rod is in neutral, and neither rod is locked.
The position of the 3rd/4th shift rod in the illustration is unclear, and the detents would be in the same positions shown in either situation.

The only function of the short detent (4) in the illustration is to lock the 3rd/4th shift rod, or to assist in locking the 5th/Reverse shift rod. There is no spring pressure on that detent.

[john walker's workshop]

There is really no reason to remove that 5th/reverse detent stuff on a rebuild. It ain't going anywhere.

[KTL]

Agreed 100% with JW that the vertical detent doesn't need to be removed. But if you've done any major cleaning of the middle case section, especially media blasting (which should be soda so it dissolves), then you'd be advised to remove that detent assembly.

My limited experience with this issue is from inheriting a scrap 915 complete case from a friend who had planned to clean it up. It's scrap because it was run without oil and all sorts of damage occurred to the gear shafts & differential. So the case sections can no longer be trusted. He was planning to do something like display it in the garage and therefore he blasted it in a cabinet at work.

Cases look pretty on the outside but the vertical detent was all crapped up with media on the inside. Those detent parts were a serious pain in the butt to get out of the bore. I was glad the case is junk because i'm sure the bore is boogered up pretty good from removing the parts. I wasn't being careless just because the case is junk. I worked for a long time to free the pieces from the grit.

I wanted to salvage the detent pieces for spare parts and use the case as a "cutaway" section for checking/setting the forks and clearance of the tabs on the rods

Thanks to Jerry Austin (Porsche84 here on the forum) for the idea

[Jon B]

Originally Posted by KTL

But if you've done any major cleaning of the middle case section, especially media blasting (which should be soda so it dissolves), then you'd be advised to remove that detent assembly.

I agree, Kevin. Also if you rinse the case in an aqueous tank or power wash it, then it's a good idea to remove those pieces.
The lower, tapered detent (9) can trap water, and cannot be completely cleaned & dried without removing it.

[OhioRick]

I really didn't want to ignore any potential chips or grit that might get caught in the detent area, so I removed them for cleaning. Unfortunately, I just missed the sleeve, which is an important detail.

As soon as I re-seal one cam tower, I will put things back together and see my results. I'm confident now that things will work well, but I need the weather to improve. I'm getting tired of working in a cold garage.