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Gordo's 915 Transmission Rebuild

I've decided it's time to rebuild my 915 transmission.

The trans I'm using is difficult to shift into 1st and 2nd gears in spite of efforts to improve the performance (Adjustments, Swepco, Wevo stuff etc.).

I'm initially planning on replacing the 1st and 2nd gear synchro assemblies (synchro rings, slider, and 1st & 2nd gear dog tooth rings) - but will see what I find as I open and inspect...

I also have a couple of "upgrades" planned - adding the Wevo 915 Gate Shift Kit, and swapping in a 15:30 / 2.00 2nd gear set: 2nd Gear Swap for a Street 915 Transmission - Better Acceleration?

Looking forward to the job, the help from this great community, and hopefully better shifting and performance in the future.

--------------------------------------

915/63 vs the 915/73 Transmission

I had 2 transmissions to choose from for my rebuild:
- A high mileage (200k + miles, no history) 915/63 trans that was originally in my 1983 911SC or,
- A high mileage (200k + miles, no history) 915/73 trans that came with the 3.2L engine that I upgraded and installed in my SC.

I've opted to rebuild the 915/73 although it's currently installed / in use. Mr. Zimmerman's excellent tutorial How-To: Porsche 915 Transmission Repair Tutorial Part 3 - Porsche Wiki notes that the 915/73 is much less likely to suffer rear pinion shaft bearing race wear issues, requiring machine shop repairs.

--------------------------------------

Disassembly Tip

Followed Mr. Zimmerman's tutorial, but adding a few notes to clarify a few minor points:

- Removing the 5th/R idler required bit of a head scratch. I think the transmission needs to be placed into 3rd gear to allow the reverse gear to align for removal.



I initially attempted to pull the 5th/R slider sleeve assembly outward and past the reverse idler gear (as per the tutorial) but found that the assembly wouldn't budge.

I shifted gears (by grabbing the shifter shaft and pushing / pulling) and found this allowed the 5th/R slider to move outward (beyond the detent) and the gears to align, allowing removal of the reverse idler gear set.

Outside of that small note - disassembly was textbook.

--------------------------------------

More to come as I inspect and replace...

Gordo
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Last edited by Gordo2; 01-17-2017 at 05:34 PM..
Old 01-01-2017, 07:45 PM
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Break a Leg Brother Gordo!

Other upgrade you should consider is the one piece bearing bracket from CMS.

Jim
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Old 01-01-2017, 07:49 PM
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Initial Inspection

Quote:
Originally Posted by DRACO A5OG View Post
...Other upgrade you should consider is the one piece bearing bracket from CMS.

Jim
Thanks Jim - I've been eyeing that one or the Wevo bracket.


---------------------------------------
Disassembly Continued

I fashioned a main shaft lock using an old clutch disk (alternative to buying the P37A factory tool). I found 2 bolts that fit through existing holes in the clutch disk, and attached a couple pieces of flat stock. I attached the flat stock to 2 mounting points on the transmission housing.



This locked the main shaft, and pinion gears (in gear / not in neutral), allowing me to remove the 27mm and 36mm nuts with an air impact wrench.


---------------------------------------

Initial / Assembled Inspection

As I suspected, my 1st gear dog teeth are shot:




Likewise, my second gear dog teeth are rather round...



Will get some better photo's once disassembled.

I was surprised to find that the teeth on my 3rd gear slider sleeve appear rather worn, although the 3rd gear dog teeth look like new (3rd shifted nicely). Also the 3rd gear synchro ring looked like it had some wear (shiny surfaces).

I may be adding a few more parts before all is said and done (do it once, right the first time...).


Thanks, Gordo
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Last edited by Gordo2; 01-01-2017 at 08:23 PM..
Old 01-01-2017, 08:17 PM
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Yeap, pretty much what I went thru upon tear down, ended up with new 1-4 syncrhos, bands and sliders. I really did not want to tear her donwn for a long long time,
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Old 01-01-2017, 09:49 PM
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Quote:
Originally Posted by DRACO A5OG View Post
Yeap, pretty much what I went thru upon tear down, ended up with new 1-4 syncrhos, bands and sliders. I really did not want to tear her donwn for a long long time,
Same here.
I think the third gear wear is common lot's of hard downshifts there. Replace all syncros, sliders and dog teeth. I've read that it's important what brand part's
you use. Don't know the the name but one brand fit's better and is of higher quality. I found this out after my rebuild.
Old 01-02-2017, 05:24 AM
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Curiosity question: what are the three grooves on the gear teeth for, oil movement?
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Old 01-02-2017, 07:59 AM
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Old 01-02-2017, 10:11 PM
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Thumbs up

Quote:
Originally Posted by gomezoneill View Post
Same here.
I think the third gear wear is common lot's of hard downshifts there. Replace all syncros, sliders and dog teeth. I've read that it's important what brand part's
you use. Don't know the the name but one brand fit's better and is of higher quality. I found this out after my rebuild.
I recommend the German OEM ones, though R&S have supposedly corrected their issues but why take a change for a few more dollars. In fact, got the Synchros & Sliders from CMS because I had them press on the 2nd gear synchro for me for $25.00, I really did not want to screw things up for $25.00, Sorry Wayne, I would order here if you offered the service.

Break a Leg Brother, PM me if you have any questions.
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Old 01-03-2017, 12:36 AM
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Looking forward to continuing to follow this as I hope to embark on a similar project in the near term.


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Old 01-04-2017, 04:37 AM
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Have you checked your pinion bearing race? They tend to become loose and spin in the case.
Old 01-04-2017, 06:24 AM
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I wonder if there's someone who rebuilds these and sells them with a core exchange?
Old 01-04-2017, 06:29 AM
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I did a 915 rebuild a couple of summers ago.

Don't get the cheaper synchro rings. I did and had to waste a week sending them back for new ones. Their wear coating was missing in spots.

You can put the transmission in two gears at once (3rd and reverse iirc) and then you don't need a shaft locking tool.

I had a local Porsche transmission shop press off my old dog teeth and put my new ones on. They charged me $20 and did it in 5 minutes.

When putting the shafts back together, one gear at the end is a very close friction fit. Put it in the oven at 250 degrees for 10 minutes and then you can drop it right on.

Use lots of plastic baggies and label everything. Use zip ties to keep bushings and other parts in the correct order. Take lots of pictures.

You can read about the issues I encountered and overcame here: Can't Shift Into First

Good luck!
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Old 01-04-2017, 06:42 AM
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subscribed! thanks for sharing!
Old 01-04-2017, 07:03 AM
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yes...thanks for posting...not enough pics of transmission rebuilds.
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Old 01-04-2017, 09:44 AM
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what are the parts costs for a rebuild like this?
Old 01-04-2017, 10:16 AM
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Quote:
Originally Posted by Mick_D View Post
I wonder if there's someone who rebuilds these and sells them with a core exchange?
Then you'll lose the matching numbers for the vehicle not worth it.
Old 01-04-2017, 12:11 PM
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It's a must-do to replace the engagement sleeve when you've got dog teeth that are so damaged like that. The receiving teeth on the sleeve are going to be similarly blunted by those mangled dog teeth trying to engage the sleeve. Although it does seem like the sleeve is a much harder steel than the dog teeth. I say that because the sleeve rarely shows as much blunting on its teeth. The other thing to consider is the synchro friction ring wears on the inside diameter of the sleeve. That said, it would seem that the synchro ring is the sacrificial part, similar to a brake pad, since it sheds its friction coating and appears to show more wear than on the sleeve itself. Nonetheless the fact that the sleeve is taking wear on both its teeth and its inside diameter is a very strong argument to replace it when doing a thorough refresh of the trans.

I think we may end up finding the pinion bearing race is loose on this one too. Even though Porsche revised the main/final drive case section for the 915 in the Carrera 3.2 cars, to use a smaller bearing on the input shaft (intention was that the smaller bearing up top leaves more "meat" in the housing below to support the pinion race), the later transmissions also seem to show loose bearing races pretty often. My racecar has a 1984 transmission and I found that race super loose. Granted, a racecar is pretty much always being leaned on so it's going to put a hurtin' on that bearing a lot more often.

https://www.youtube.com/watch?v=xmNmxm_AT0c

So I had to have both of my bores sleeved for steel sleeves to reinforce it and I sent it to CMS for the work. Roger & his staff do a really nice job and turn things around in a timely fashion.

https://californiamotorsports.net/collections/porsche-915-transmission-parts/products/cms-915-bearing-bore-repair

I also strongly recommend you get the singular bearing tensioning plate for the trans. CMS, Guard, WEVO and Carquip make a nice one. It does a MUCH better job of supporting those ball bearings than the original thin individual tensioning plates. Also take a hard look at those bearings which reside in the tensioning plate and strongly consider replacing them. They seem to wear the most. Especially the pinion ball bearing. When they do wear, they let the shafts walk around inside the case a bit. When the shaft walks, that seems to help loosen the bearing race.

When replacing the dog teeth on the gears, I would carefully cut/split the dog teeth ring off the gear to remove it. Pressing them on and off loosens the tight fit between the ring and the gear hub area. You can easily install the new ring by heating it up in a toaster oven or cook it in hot oil and it will drop right on the gear hub. Also freeze the gear to help the ease of fit. Just be sure to shoot the gear with WD-40 or whatever right after installing the ring because it was start to rust almost instantly.

Also be sure to take a good look inside the reverse idler gear pair for needle bearing shadows. Same goes for free/loose 5th gear down below on the pinion shaft. With reverse idler being at the forward end of the transmission and up high, acceleration/forward movement migrates the pool of oil in the trans to the rear. So the front end of the trans isn't always getting as much oil as it wants everywhere.

BTW I also have an electronic copy of the factory 915 + Sportomatic repair manual (only goes up to 1981 but still a very helpful technical reference) if you would like me to send it along to you.
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Old 01-04-2017, 12:36 PM
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Mine hasn't had matching numbers since whoever pulled the 75 motor out and stuck in a 1976 motor, whenever that was. They did retain the 11-blade fan and put a pop-out valve in the CIS box, though.

I'm not worried about collector's or investor's concerns. I want to drive the thing to death.


Quote:
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Then you'll lose the matching numbers for the vehicle not worth it.
Old 01-04-2017, 12:40 PM
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Gouge

Side note - here's a cool animated video that does a great job of explaining how a manual transmission and synchro's work

https://www.youtube.com/watch?v=wCu9W9xNwtI

Check it out - I watched this a few times and it gave me a much better understanding of how this stuff all works together.

-----------------------------

Quote:
Originally Posted by KTL View Post
It's a must-do to...
Man Kevin - that post was full of gouge. I hadn't considered cutting off the dog teeth - was planning to either find a local shop that could remove it or buy the tool - after all, I Love Tools ...

Meanwhile your observations regarding the pinion bearing race is a bummer - especially considering I pulled the engine/trans to rebuild vice rebuilding the 915 I already had on the stand. Hope to find time to remove the bearing retainer and gear sets this weekend to see what shape they are in.

-----------------------------
Mick_D,

I'm in for cost of whatever it takes to do it right - I'm currently guessing I'm going to come in around $1,000 (minus Wevo and new 2nd gear set). This is primarily for the 1st & 2nd synchro parts and tools (assuming I buy vs send to a shop). The rest of the disassembly and inspection will tell the remainder of the story.

I will make sure to list parts and costs as I proceed - I don't normally do that, it helps me remain in denial...

-----------------------------
OsoMoore,

Did you end up going with the OEM brand as per Drako's recommendation, or did you buy another brand?

-----------------------------

Thanks for the input folks,

Gordo
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Old 01-04-2017, 06:32 PM
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Quote:
Originally Posted by Mick_D View Post
I wonder if there's someone who rebuilds these and sells them with a core exchange?
There is. Tom Amon does this. Check out his website Mobile Works West. He has a couple of rebuilt 915s and I think a G50 for sale right now. You can buy them straight up or get a lower price with an exchange. The prices are very attractive and Tom is an excellent rebuilder/mechanic.
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