915 rebuild

[magilla]

When I got my SC last Dec, I knew it wasn't shifting into first very smoothly. I brought it to my wrench who replaced the clutch, and that made a huge difference. I still feel that first is a bit tricky. I don't need to be at a complete stop anymore, but pretty close.

My latest scheme is this: buy a 915 in bad shape and rebuild it in my basement over the winter. I admit that I have little to no skills, but I am willing to experiment on this, since it won't disable the car for weeks if I screw up. It would potentially teach me some stuff about the car, let me feel like I am doing something for myself, and save me some $$, and give e a project for the winter.

My questions to you:

1) Is a 915 rebuildable with some patience, a few hand tools, and Bentley book?
2) Will it end up being cost prohibitive?
3) Is there a was to test it when I am "done" to see if it is working properly?
4) What am I not taking into consideration? So many seem to let someone else do it. Is there an art to it or simple parts swap?
5) I am smoking something I shouldn't?

Any thoughts would be greatly appreciated.

Magilla

[Doug Zielke]

Mag,
Others here will give you more definitive answers to your questions, but IMO you need a better book than the Bentley to rebuild a 915. Taking a look here through my Factory manual....shows a whole lot more info compared to the Bentley.
Good luck, and let us know how it goes should you d-i-y.

[bhaubold]

Magilla,

The labor on a rebuild (out of the car) is about $600. If you have to buy some special tools (you will) that will make the difference even less.

I would consider having a pro do it. If you have to do it twice, that really takes the fun out of it.

But, if you must.....

[makaio]

Do it!

Even though my tranny is in great shape, I have thought about finding a 915 in need of help and teaching myself how to re-build it.

With this board, the right tools, and lots of time, you should have no problems!

[tbitz]

I am planing on rebuilding my 915 over the winter.

[ChrisBennet]

Magilla writes:
1) Is a 915 rebuildable with some patience, a few hand tools, and Bentley book?
I've done a few of these and I have all the special tools. It isn't rocket science but I do learn something new every time I do one. You aren't that far away (I'm in southern NH) , I could help you do yours or show you how it is done. I'll be doing at least one 915 this winter. I wouldn't buy one to practice on.
2) Will it end up being cost prohibitive?
Syncros, dog teeth, sliders and new fluid will run around $600.
Replacing just the syncros ($100) is a false economy IMO.
3) Is there a was to test it when I am "done" to see if it is working properly?
There is a little jig you use to set them up/check and you can check the shifting by pushing/pulling the shift rod coming out of the transmission.
4) What am I not taking into consideration? So many seem to let someone else do it. Is there an art to it or simple parts swap?
If you don't have a hydraulic press you'll have to use someone's. The 2 tools for removing the dog teeth are kinda expensive (~$100 ea).

If you decide to have someone rebuild it, Gary Fairbanks is a very respected 915 rebuilder in nearby Conn. I get my 915 parts from him at very reasonable prices.

-Chris

[CraigD]

Chris,

Your pricing seems to low. I just checked and for my 901 tranny, 5 syncro rings, 5 dog teeth, and a gasket set are already $650 and I don't even have fluid or sliders. Do you replace all the dog teeth or just certain ones? Which sliders do you replace? Do you replace all the syncros?

I have been contemplating rebuilding my 901 due to baulky shifts into 2nd unless short shifted. If I focus on problem areas then I would replace 1st and 2nd dog teeth, 2nd 3rd slider, and 1st thru 3rd syncros. These repairs would be about $500 with a gasket set. Maybe this is more in line with what you intended.

[old_skul]

This is something I've thought of doing just for the heck of it myself. But to tell the truth, unless you can't get into first *at all* without being at a full stop, or unless second gear sounds like a box of Captain Crunch, it's not worth it. My car goes into first gear while still rolling so I'm happy with it.

But it sure looks like a fun project for the winter

[ChrisBennet]

Originally posted by CraigD
Chris,

Your pricing seems to low. I just checked and for my 901 tranny, 5 syncro rings, 5 dog teeth, and a gasket set are already $650 and I don't even have fluid or sliders. Do you replace all the dog teeth or just certain ones? Which sliders do you replace? Do you replace all the syncros?

I have been contemplating rebuilding my 901 due to baulky shifts into 2nd unless short shifted. If I focus on problem areas then I would replace 1st and 2nd dog teeth, 2nd 3rd slider, and 1st thru 3rd syncros. These repairs would be about $500 with a gasket set. Maybe this is more in line with what you intended.

Graig,
Yes, my numbers were a little off. For a 915 I pay approximately:
$110 for all 5 syncros,
$200 for both sliders (1/2 >$100, the 3/4 is <$100)
$285 dog teeth $75 1st & 2nd, $45 ea for perfect used 3-5
$40 for a gasket kit (or $3 if I'm just replacing the paper gaskets)
$40 for a gallon of Swepco
__________
$675 total

I can't find the latest receipt but I found a couple of old ones.
The receipts don't have dates on them so I'm guessing "when".
I get my parts from Gary Fairbanks. See the back of Excellence
or Pano for his number.

Last year
1-2 slider $90
1-2 syncros $45
1-2 dog teeth $150

Year or two before:
5 synchros $100
1-2, 3-4 sliders $180
1-4 dog teeth $300
gasket kit $35


No matter how you cut it, it isn't cheap but it is still a lot better than the shiney catalog prices.
-Chris

[Bill Douglas]

Guys (Guys in the know, Warren & Chris), what do brake bands and excelerrator blocks do?

My guess is, using the example of moving from first gear to second, the brake band slows the gear down so that the syncro ring has half a chance of matching the speed of one gear to the next. And... When shifting down a gear the excelerator band speeds it up to match the much faster first gear.

A lot of 915 gearboxes have trouble between 1st and 2nd because this is the biggest ratio difference and there is more slowing down and speeding up going on.

So, do we need to put more emphasis on relpacing the brake band and excelerator block, instead of just the synco's, sliders and dog teeth. Or have I got it completely wrong?

Bill, 79SC.

[ChrisBennet]

Originally posted by Bill Douglas
Guys (Guys in the know, Warren & Chris), what do brake bands and excelerrator blocks do?

My guess is, using the example of moving from first gear to second, the brake band slows the gear down so that the syncro ring has half a chance of matching the speed of one gear to the next. And... When shifting down a gear the excelerator band speeds it up to match the much faster first gear.

A lot of 915 gearboxes have trouble between 1st and 2nd because this is the biggest ratio difference and there is more slowing down and speeding up going on.

So, do we need to put more emphasis on relpacing the brake band and excelerator block, instead of just the synco's, sliders and dog teeth. Or have I got it completely wrong?

Bill, 79SC.

Bill,
If I'm one of the guys "in the know" than we're in trouble.
The syncro is responsible for matching the speed of the gear to the slider. Sort of like a clutch.
My understanding of these things is incomplete but by examination it looks like when the syncro is "twisted" on the gear it pushes against a block ("energizer") which in turn pushes against one end of a brake band. The other end of the brake band pushes against another fixed block ("stop") which can't move relative to the gear so the band is comressed and bends/bulges out. I think the bulging out either expands the synchro ring to lock it into the slider or serves to cushion the shock of engagement. Someone "in the know" should answer this for both of us. Without a picture I doubt this will make sense, sorry.
-Chris

[silver912e]

Just an FYI:

There are some really nice pictures of a 915 rebuild at:

http://instant-g.com/Projects/915Trans/

[john walker's workshop]

the bands, when energized by the initial contact of the shift sleeve as it begins to drag on the syncro ring, expand and keep the shift sleeve from passing over the syncro until the gear speed is matched to the speed of the shaft that the shift sleeve turns on. then they relax and let the shift happen. picture a spinning wheel moving against a stationary wheel. they rub for a moment, then match speeds. the force created as the stationary wheel is brought up to speed, is the same force that pushes the brake bands out against the syncro. when the friction surface on the syncro ring wears off, the shift sleeve can't speed up or slow down the selected gear very well, due to lack of friction between them. the brake bands lack the force to keep the syncro expanded, and the shift sleeve passes over the syncro prematurely and gashes against the engagement teeth, causing wear on both the teeth and the sleeve. sleeves wear out normally from rubbing on the syncros, so they get replaced on a rebuild along with syncros and as many pressed on syncro teeth as needed. it's a good idea to replace at least 1st and 2nd on 915s due to the fact that the teeth break off a lot.

[ChrisBennet]

Originally posted by john walker's workshop
the bands, when energized by the initial contact of the shift sleeve as it begins to drag on the syncro ring, expand and keep the shift sleeve from passing over the syncro until the gear speed is matched to the speed of the shaft that the shift sleeve turns on. then they relax and let the shift happen.

Thanks John!