915 Rebuild - Overview (my tips!)

[Shawzy]

I recently completed the rebuild of my '79 5-speed 915 trans.

I am a firm believer in learning from other's failure and success, and therefore I would like to share my experience here.

First off, I must thank all the guys on the board here with their support and knowledge. Very appreciated!! I could always sleep soundly knowing one of you would help me out...

My hardcopy resources were the Bentley and Haynes manuals.

Tools:

Big vice
Homemade shop press (bottle jack, 3/4" threaded rod, steel plates)
Old clutch disc (input shaft lock)
Hammer and chisel
40-250ftlb 1/2" Torque wrench
5-50 ftlb 3/8" Torque wrench
Scrap aluminum plate to use when clamping parts in vice
Rubber mallet
Metric wrench/socket set
Shift fork jig (special P-tool - get it from Pelican)

The lowdown on parts replaced:

SYNCHRO RINGS 1-5
THRUST BLOCKS 1-5
ANCHOR STOPS 1-5
BRAKE BANDS 1-5
SYNCHRO HUBS 1,2,3
OPERATING SLEEVES 1/2,3/4
MAINSHAFT BALL BEARING
GASKETS (input shaft fore and aft, diff, etc. etc.)

My cost was about $2000 CAD not including Swepco and a few tools I treated myself to.

The rebuild on the whole went well, although I can't comment on it's performance because I have yet to put it back in the car! I will add to this thread to post the conclusion.

Here are some tips that I picked up along the way while doing my rebuild.

Trans Dis-assembly:

Take pictures/vids! Pics in the manual are sometimes ok, but I was able to refer to my pics when re-assembling to ensure all ok. Also great to prove to the buds you actually did what you claim...

Shaft Dis-assembly:

I used the inertia method to dismantle my input/output shafts. All went fine but after talking with experienced rebuilders I would have put the whole shaft in the oven to 250C to help it along (make sure the wife isn't home...takes a while for oil smell to rid the kitchen...)

Mainshaft ball bearing:

My ball bearing on the input shaft came apart in my hands upon removing it. I replaced it, before learning from a pro I met later that some bearings come apart and some don't. FAG bearings do not come apart. My SKF bearing did. One has to use judgement as to whether the bearing should be replaced. My old one was not worn or scored, so in hindsight I would have re-used it.

Synchro Hub removal:

This was a tough one for me. I didn't have the tools so I resorted to the hammer&chisel technique. Worked ok, but I could have done things a bit differently. Use an all-metal chisel; i.e. no polymer handle. I used a wood chisel with a poly-handle. It took forever, because the handle absorbed all the blows and didn't transmit energy to the hub. Next time I will use a masonry chisel sharpened accordingly.

DON'T USE A DREMEL. I tried this and managed to put a nice ding in my gear. Trouble is, it's tough to know when you have gone through the hub.

I have heard a bearing splitter with a shop press can be used to remove hubs.

Gears 1-5:

After working on all the gears (with hubs, rings, etc.), I had a problem keeping 3,4,5 straight from one another. I mixed up 4th with 3rd, and then 5th with 3rd. Stupid yes, as there are differences to tell them apart from one another (size, spline direction on 5th for eg). For the first-timer I would recommend taking a permanent marker and marking each gear 1-5 just to keep it all in order. I had to disassemble my input shaft 3 times because of the mistakes i made!! Ouch - that was a pride basher.

Shaft Re-assembly:

I heated the necessary parts to about 300C and it all slipped on nicely. Once assembled, DON'T TORQUE AND STAKE THE MAINSHAFT 41MM NUT UNTIL YOU HAVE CHECKED THE ASSEMBLY. This goes back to my earlier comment about mixing up gears. Ensure spline direction is the same on each gear for the shafts. Try and mesh the input and output shafts together as they should be when in the trans. Better yet, put them back in the trans to ensure they can mesh. I made the mistake of torqueing and staking my mainshaft nut and discovered it wouldn't mesh with the output shaft because I had put 4th and 3rd on in the wrong order. The next attempt, I put 5th on in place of 3rd!!! It was late, and I was cranky and tired. I won't work on my baby when in that state again! Needlesstosay, I had to get another mainshaft nut.

A tip from a friend on torqueing the mainshaft nut - I welded a sacrificial 1/2" drive socket to the center of my old clutch disc (needed as the input shaft locking tool). This allowed me to hold the 41mm nut in a vice and torque the shaft onto it. Worked like a charm and I got an accurate torque. Apparently there is a chance of bending the input shaft this way, so I reccomend supporting it accordingly so it can't bend. I think that is why Porsche has a separate input shaft holding tool that goes on the opposite end of the splined end, and super deep 41mm socket. Most people don't have this arrangement however and all is well.

Trans Re-assembly:

Once I got my shafts in order, I was ready to put it all back together. After putting on the shift fork jig, I had trouble torqueing the 36mm input shaft nut to the required 181 ftlbs. I ended up having to rig the trans up so it could be supported horizontally. If one has an engine stand, no problem (I don't). When horizontal and well supported, I could put all my weight on torque wrench without the trans spinning in my buddies hands.

I had difficulty getting the mid-section of the trans back on with the shift-rod - I ended up ripping my paper gasket. I therefore suggest practicing putting the mid-section on a few times before putting the gasket on.

After the top section was on, I was replacing the 5th/reverse shift fork bolt and accidentally dropped it. It found it's way deep into the trans of course so I had to take it all apart to get it out. So, take extra care when working with bolts/washers.



Well, that's about it. I will edit this if I have forgotten anything, and will add to the thread when the trans is back in the car and ready to fly.

I hope this helps anyone intending to do a 915 rebuild. It was a great experience, and not as scary as I imagined. The only little parts to watch are the shaft detent plugs and springs, reverse switch pin. Everything else is big and German. If you have the time and patience, I say GO FOR IT to anyone thinking about it. After all, the board here is always ready to help!!!!

[0396]

Good read..thanks for the education and advice.

[tvanbroekhoven]

Great writeup for aspiring 915 rebuilders (eg me :-) )

How long did the rebuild take ?

[Ricks911s]

Thanks for the write-up. I will likely do this one day. Please be sure to give us an update after you install the trans and let us know about the differences you noticed after you drive it.

[john walker's workshop]

you didn't mention the captive detents in the diff housing and center housing. got them in the proper position?

[sc_rufctr]

Great write up but this was written 7 years ago...

[john walker's workshop]

oh.

[Shawzy]

Well, 7 years later and all is well. Haven't been on the board for a long time.

Trans felt fantastic when all together, but one thing that helped even more was replacing the plastic bushings on the gear lever itself. About $30 and 20 mins. Tightens things up nicely.

Trans took about 1000km to break in, ie. to get smooth shifts. This is normal i am told. No regrets I would do this again if needed and it would take half the time Took me about 1 week for rebuild, but i would bank on 3 as once the engine is out you will want to do a bunch of other stuff (valves, etc.)

This board is the only reason I attempted such a task, besides the fact i love getting my hands covered in Porsche grease. heehee.

Cheers

[0396]

Question to those before me.

After reading several 915 postings.. I've noticed different postings relating to the 'size' of sockets need for the work.

Here's what I been able to gather ..27 mm . 36 mm - for the tail cover section and a 46 mm for M/S...Is this correct? For once I open her up, I hate to stop any progress I've made because of insuffcent 'tools'.

Please advise- thanks all

[0396]

Question to those before me.

After reading several 915 postings.. I've noticed different postings relating to the 'size' of sockets need for the work.

Here's what I been able to gather ..27 mm . 36 mm - for the tail cover section and a 46 mm for M/S...Is this correct? For once I open her up, I hate to stop any progress I've made because of insuffcient 'tools'.

Please advise- thanks all