915 Transmission Rebuild

[NSW911]

After finishing a considerable amount of work on my '86 911 last year and with the car looking great following a full respray,one defect continued to nag away at me. Overall the gearbox had performed reasonably well in the three years since I purchased the car but there was no denying the fact that 1st gear synchromesh had virtually disappeared and was not functioning too well on 3rd gear either.

Never having tackled a job like this before I resolved that should I do so I would have to be very well prepared, both knowledge wise and tool wise. I buried myself in all the literature I could find on 915 transmission repair making good use of Peter Zimmermann's fine set of tutorials as well as all the information I could glean from the Bentley service manual.

There are a number 915 transmission special tools listed in the Bentley manual, some of which I found to be essential with the others being in the "nice to have" category.The tools I found to be indispensable were,-

(a) P37a---------Input shaft lock tool.
(b) P260a--------Shift rod holder.
(c) P355a---------Input shaft mounting tool.
(d) P252a---------Input shaft collar nut socket tool (41mm)
(e) Synchronizer gear press tool.

To these tools must be added a suitable bearing press plus a torque wrench with a scale up to at least 250NM.



Needless to say, one must have a proper stand on which to mount the box prior to commencing work. I built a 911 specific engine stand some time back integral with my work bench which proved very suitable for the task at hand.



Box installed, oil drained and all ready to commence work.

I encountered little difficulty in dismantling the box and shafts making sure I placed all parts down in the order they came off the shafts to simplify the reassembly process. First gear dog teeth and synchro ring were badly worn as expected as was the third gear synchro ring. Giving into the "while you're in there" impulse I wound up replacing the synchro rings, brake bands and snap retaining rings on all forward gears along with the sliders between 1st/2nd gears and 3rd/4th gears. The original main and pinion shaft bearing clamping plates were replaced with a single stainless steel retainer plate,-----the Wevo XT-032.



First step in disassembling the pinion shaft.

Some difficulty was experienced in installing the new dog teeth ring into the 1st gear hub.
Try as I may I could not stop the ring tilting to one side as pressure was applied from the press; the problem eventually being solved by applying just a touch of pressure, rotating the assembly 90 degrees and applying a touch more pressure, rotating the assembly a further 90 degrees and so on till the ring was fully seated.

More problems arose when attempting to install the new snap retainer rings into the synchro hubs. These rings are not unlike large "C" shaped circlips and have to be expanded to accommodate the "land" above the retainer grooves in the hubs. When attempting to expand the snap ring using suitable circlip pliers I would invariably nudge the freshly installed synchro ring out of its dog teeth embrace resulting in the whole procedure having to be repeated-------to no avail. How to secure the synchro ring before attempting to seat the snap ring? The answer was as simple as it first appeared complicated!



Simply by surrounding the synchro ring with a piece of baling wire and then reducing the diameter of the synchro ring by coiling the wire at the ring gap I was able to stop the ring leaving the embrace of the dog teeth when accidentally touched by pliers or wandering fingers. There still remained the problem of installing the big and strong "C" retaining ring!



As illustrated, this problem was solved by expanding the ring just enough to grab part of the "land" above the groove and then using a pair of ordinary clamps to stop the ring rising up and out of place when circlip pliers were again used to expand the ring. By alternately expanding the ring slightly and then "walking" the clamps around the circumference of the hub towards each other, all five snap rings were eventually seated.



Success!

Next was the installation of the upgraded single piece bearing retainer plate. The old plates,------one for the main and one for the pinion shaft were a surprisingly loose fit over their respective bearings in strong contrast to that obtained with the new one-piece plate. Very tight! So tight in fact that heat had to be applied to the retainer plate in order to properly seat the bearings.

All that remained now was to reassemble the main and pinion shafts before installing the whole lot back in the box followed by the all important setting up of the shift forks.



Pressing on the final pinion shaft bearing.



Completed main and pinion shaft. The new one-piece clamping plate is clearly visible.

Before attempting this transmission rebuild I took particular notice of the fact that there had been no appreciable drive train noise experienced when driving the car, leading me to believe the box and differential carrier bearings need not be changed. This conclusion was borne out------as much as it could be------by close inspection of all bearings following disassembly of the main and pinion shafts. As a result the original bearings were reused, the differential carrier bearings remained untouched and the original number of pinion head depth shims were used in reassembly.

Back to those shift forks. Being new to the task I spent a considerable amount of time going over the setting up of the shift forks followed by much checking and rechecking of all I had done. I have to admit I was haunted by a job I had finished years ago when, following some gearbox work on another car I discovered to my horror a large spacing washer sitting on the work bench AFTER all work had been completed with engine and box back in the vehicle. Nothing like that was going to happen this time!

With all checks complete I proceeded to fit new seals to the input and gear shift shafts prior to closing up the box, mating it to the engine and reinstalling the completed unit back into the car.

I was very pleased with the test run------ the box performed flawlessly with next to no discernible drive train noise. No doubt there was a large dose of beginners luck in achieving this result with perhaps just a small percentage due to the preparation I felt was necessary before taking on a job like this for the first time.

Would I do it again? I most certainly would! The tools and parts were expensive but the knowledge gained and pleasure experienced in following the task through was immeasurable and far outweighed the total dollar cost of the whole exercise.

My two cents worth.

[Jcslocum]

Oh man this is sooooo timely!!!!

I have the engine going back together right now and the trans was to be next on the stand.

Do you want to lend/lease/sell the tools???

[yelcab1]

You make it look easy... and it definitely is not easy. Doable, not easy.

[RD911T]

Well done. Cunning solutions to the problems you faced. Nice bench vise. Is that a Wilton? I didn't know there was a bench vise that could be rotated on it's side like that. Very handy. I would love to know some details about it.

[Peter Zimmermann]

NSW911 - Good job!

[Dpmulvan]

Nice job!! How much for all the specialty tools?

[Quinlan]

Originally Posted by Dpmulvan

Nice job!! How much for all the specialty tools?

And where did you find the tools?

[KTL]

Excellent job!

I agree 100% that a must-do part on any 915 rebuild is to install a one-piece bearing retainer. The separate retainers do a very poor job of supporting the bearings. You can use the factory 930 retainer, or the nice aftermarket retainers from Guard, CMS, WEVO, Carquip, etc.

There are some substitute tools you can use instead of the specific factory tools

>355a shaft holding tool can be duplicated by using an old clutch disc. You don't even have to disassemble the clutch disc. Just insert some M8 bolts into the clutch hub holes and it will lock the rotation

>252a long socket can be made with a 1-5/8 socket holed out and an extension welded to its side to create a crowfoot style socket.



You of course must adjust your torque wrench setting to account for the crowfoot socket offset. The offset pivot effectively lengthens the torque arm and therefore you set your torque wrench to a lower amount

Torque Wrench Adapter Extended Equations Calculator - Engineers Edge

Those P226, P225 synchro teeth press tools are nice to have. Be careful to anybody thinking of buying a new one, as Pelican has experienced problems with SIR Tools getting the dimensions right on their P225 (3/4/5 gears). The tool is cut wrong and grips the upper ring (actually the groove for the circlip) of the synchro teeth too tightly.

I have one of the bad ones and it's very obvious the fit is wrong. A proper tool will lightly capture the ring. Whereas these bad tools clench the ring and effectively press it into the fixture when the bolts are fully tightened. Then when you apply the hydraulic press, you risk fracturing the synchro teeth. Ask me how I know...... I figured i'd give the bad tool a try on an old junk gear and it broke the ring w/out applying much pressing force.

By the way you don't need to formally press the new synchro teeth onto the bare gear. Freeze the gear and heat the synchro teeth in a toaster oven or on a hot plate. Just be sure to give the gear plenty of time in the freezer. And then put WD40 or your favorite rust prevention spray on the married gear & dog teeth right after assembly. It will rust very quickly (like, right before your very eyes) if you don't treat them with something.

[OsoMoore]

For those doing this job without all the tools of the OP - I did this at home in my lesser-equipped garage and my misadventures might be helpful.

Here's a link to the wrap-up post in my thread. Read the thread for more details.
Can't Shift Into First

[Trackrash]

I love the C-clamps on the synchro ring. Nice job.

[yelcab1]

I welded my own long socket, and made a locking tool out of a clutch disc. I bought the synchro tools, then sold them. Should have kept the tools.

[Swoboda]

Great write-up! Thanks for sharing.

[Geneman]

Fantastic. Please do the same rebuild on a g50 and document it so well! Thanks. Frank

[geoff_k]

What a timely article. I have an 84 car that is in need of a rebuild.
I hope you don't mind me making this article a favourite and in the future contacting you. I just up the road at Toronto, Newcastle.

[NSW911]

Hi guys and thanks for all the input.

RD911T,------ The vise illustrated in my post was obtained many years ago when I began putting my garage together. Unfortunately it has no brand name on it and I’m no longer in possession of any receipts.




Hopefully these photo’s will help you identify the brand.

Dpmulvan and Quinlan,------My special tools were sourced as follows. Prices excluding transport costs are in US dollars.

P37a----------$165-----Design911 UK
P260a--------$106-----Pelican Parts
P355a--------$168-----Freisinger Motorsport (FREISINGER MOTORSPORT - Porsche: Historische Rennwagen, Restauration, Tuning, Fahrzeuge & Teile)
P252a--------$385-----Design911 UK
Synchronizer gear press tool (1st/2nd gear)--$103---Pelican Parts.

Hope this helps.

geoff_k,------check your messages; I have sent you a pm.

Cheers all.

[oldie44]

Here are my versions of P355a and P252a.
The P355a uses the 1st/reverse shift sleeve guide from a 914 transmission with a tab welded on to clamp in a vice.

[Mick_D]

I think I'm headed down this road with my '76 915. It's been out of the car for a year and last week I drained the fluid- and there was a great deal of water in it.
THis may be over my head.

[SiberianDVM]

Lowes has a vise that looks just like that one except it's blue.

[tito911s]

A well documented piece of work. Many thanks -- much appreciated.


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[Mick_D]

I know this thread's over a year old, but I think I'm gonna want to open up my transmission and see what its condition is, and I am wondering if I can do that without taking it entirely apart.