915 Shift fork check with an unexpected outcome

[sc_rufctr]

Hi All

I was changing the transmission fluid today and because of some recent information in another thread I thought I would check the shift fork... (The one bolted into the bottom of the transmission)

I had been experiencing some issues with shifting but I put that down to wear and tear. I Drained the tranny and removed the shift fork. At first it looked fine but on closer examination I could feel the the fork was free to move around on the alloy carrier. Maybe 2mm at the most but I was lucky as the all of the nuts and washers were still there.

One nut was so loose I was able to undo it by hand. The stud was also loose and came out too easily. Part of it had broken inside the carrier. The other two I had to use a spanner but it took very little effort to undo them. The other two studs came out with no dramas... They were all loose.
I feel confident I can fix this but...



Now the surprise...

This fell out when I removed the alloy carrier off the shift fork from the transmission.
I'm pretty sure it's a differential bolt head. I found the threaded part in the oil pan with the old transmission oil.
Looks like my car will be off the road for a while

[kwikt 911]

Same thing happened to my 915. Sccccaaaarrrrrrry!!!!!!!!

Kevin

[sc_rufctr]

Kevin

What was the outcome?

I intend to pull the motor and tranny out tomorrow. Strip the tranny and check everything.

The tranny was always a bit noisy. Like a constant wine but it shifted OK and seemed to be fine. I have since looked into the drain hole with a torch and spun the wheel at the same time. I think more than one bolt has come loose. I'm guessing at least 3. I also saw one bolt part way out and the locking tab seemed to be the only thing holding it into position. I could have been a lot worse. I'm lucky I checked this when I did.

Do these have a habit of coming loose?

[Peter Zimmermann]

That part is the head of a broken ring gear to diff carrier bolt! That is not supposed to happen!

[sc_rufctr]

Originally Posted by Peter Zimmermann

That part is the head of a broken ring gear to diff carrier bolt! That is not supposed to happen!

Thanks for that. Have you seen this before?

I plan to strip the gear box, order new bolts and locking plates and re assemble but I'd like to know why this happened?

Incorrect torque of the bolts? Or something else like failing ring and pinion gear (doubtful)...

Do you "Locktite" these bolts or rely solely on the locking plates?

[sc_rufctr]

Does anybody know the torque settings for the differential bolts?

I've looked at my "Haynes" manual. Not given... It's a 1978 SC...

Thanks in advance.

[Peter Zimmermann]

Originally Posted by sc_rufctr

Thanks for that. Have you seen this before?

I plan to strip the gear box, order new bolts and locking plates and re assemble but I'd like to know why this happened?

Incorrect torque of the bolts? Or something else like failing ring and pinion gear (doubtful)...

Do you "Locktite" these bolts or rely solely on the locking plates?

I have never seen that problem, but if the diff has been apart and the bolts were put in using an impact wrench anything's possible!

For installation the manual requires that the bolts be clean and dry and the surfaces between the ring gear and carrier must also be clean and dry.

Torque is:
1. Non-locking diff - "11.9" bolts = 83.1 - 87 lb/ft
2. Locking diff - "11.9" bolts = 101 - 108 lb/ft
3. Locking diff - "12.9" bolts = 108 - 116 lb/ft

You might find a failed R&P...

[sc_rufctr]

Thanks Peter

I'm guessing you're a trans guy. Maybe a dumb question but would you use Loktite on these?

I know it goes against the advice of clean and dry for assembly but can you see any harm in using Loktite?

[sc_rufctr]

Up for the weekend...

I'll be pulling the motor and will post pics of what I find. (differential)

[sc_rufctr]

Motor is out. Gearbox is on my work bench with the covers off. Will post some interesting pics in the morning.

[sc_rufctr]

Here are some pictures of the diff and ring gear... This is how it looked when I removed it from the box.

The three bolts were found at various places in the trans case. One "bolt thread" and "head" were sitting on the shift fork plate. One "thread" was in the front cover just under the 5th/Reverse shift rod and one "complete bolt" and "head" were just under the alloy wheel for the speedo in the diff area.

All of the remaining bolts were loose! Can either be wiggled by hand or removed without tools.

Thinking about what could have caused this I'm guessing at some point someone did up the bolts by hand.. Went away for a coffee and a sandwich and came back and installed the locking plates and re assembled the box. They forgot to torque the bolts!

I'm ordering 12 new bolts and 6 locking plates tomorrow morning... After seeing this I don't want to risk using any of the old hardware.
The internals of the case and side plate are scared but OK. I'll stone off any damage and clean the case thoroughly.

I feel incredibly lucky. Had any of the loose hardware jammed in the gears or ring and pinion it would have become a trail of oily junk in about .1 of a second.

Comments welcome





The scrunched up piece of steel is the remains of a locking plate!...



You can see some damage on the ring gear at the tip. This is the worse of the damage I can see.

[sc_rufctr]

Here are the internals. There is some wear on the synchro rings etc but to me they look pretty good.

Is it OK to "flip" the synchro rings rather than buy new ones? I only ask because they don't look half bad to me. The're still rough to the touch but with some shiny patches as seen in the pics.

This is fifth... The synchro ring looks pretty good. Still may replace or flip with the others.



Fifth Shift sleeve... This looks almost as good as new to me.



Input shaft set (3rd and 4th)... Obvious synchro wear but otherwise pretty good... Spiders seem good to me but what would I know? Comments please. (I have more pics)



Pinion shaft set (1st and 2nd)... Again worn Synchros but there is noticeable wear on one of the "spider" teeth... 1st I think

[Peter Zimmermann]

Well, where do I start... Looking at your pictures I see a junk R&P, a throw-away 2nd gear, possibly a throw-away 5th gear (the picture could be better), junk synchros (I never flip, or recommend flipping them), 1st gear dog teeth are wiped out, and, of course, with all the metal floating around in there you'll have to replace all the bearings. I can safely recommend that you start looking for a replacement trans with a known history, yours will cost $4K + to repair properly, and just about every tool in the book will be needed.

[sc_rufctr]

No good news?...lol

I guess I'll start looking for a second hand trans. Thanks for the input...

To my untrained eye I didn't think it was that bad. All the bearings I've removed so far looked good. No scoring or scratches. Even the needle bearings look good to me.

The diff bearings are grey and again look good. No scratches or scoring.

I understand what you're saying and it's hard to argue otherwise but this trans was shifting ok and running quiet before I took it apart.

Had I not removed the shift fork as a check I would never have found the loose bolts. So it seems at odds with what you're telling me. But thanks again for your comments.

[efhughes3]

The Permatex residue would seem to show that some hack was working on that at some point.

[sc_rufctr]

Originally Posted by efhughes3

The Permatex residue would seem to show that some hack was working on that at some point.

I think you're 100% right. The un torqued diff bolts confirm your observations.

Ahh what to do?... I guess I need a good second hand gearbox.

[sc_rufctr]

"Peter Zimmermann" please comment...Some better pics of the five gear... Plus one of the Gear Bag

Fifth again.



Top two, 3rd and 4th



Bottom two, 1st and 2nd



Gears complete.

[sc_rufctr]

"Peter Zimmermann" ~ Thanks for your input so fare.

Better pics of the Ring and pinion gears...

Why do you say this has failed? Is it the wear pattern on the contact surfaces or something else?

I'm a complete novice with gearboxes. I thought it looked OK but as I said I'm a novice...

[Peter Zimmermann]

Originally Posted by sc_rufctr

"Peter Zimmermann" ~ Thanks for your input so fare.

Better pics of the Ring and pinion gears...
Why do you say this has failed? Is it the wear pattern on the contact surfaces or something else?

Your post #12:
Pic #1: The furthest tooth to the right appears to have wear through on the case hardening
Pic #3: 2nd gear teeth are worn through the case hardening
Pic #4: Severe dog teeth wear, especially 1st

Your post #17:
The pics are not quite clear enough (everything should be washed, dried, and re-shot) but it looks like an asymmetrical slider was paired with a symmetrical dog tooth ring on your 1st speed - heavy wear on the dog teeth make it impossible to tell.

Your R&P post:
The lower picture that shows the chunk missing from the bottom tooth - that tells it all. If you put that part back in I suspect that the case hardening will peel back from the damaged area, and after an unpredictable period of miles you will have a really big failure when that tooth is weakened to the point that it breaks.

Bearing "appearance" is not important at this point, you have had tiny bits of hardened steel come off of gear teeth (I suspect that 3rd speed also has root wear showing), and those bits have been circulating through the transmission, and bearings. I would not feel secure taking a chance on reusing those bearings.

Root wear on primary gear teeth has become a regular problem, and I don't think that anyone can tell you for certain how many miles a gear with the below pictured wear will last compared to a gear re-installed with no visible root wear. I also don't think that anyone can tell you how long it takes (type of use, type of oil, etc. all play a part) for a gear with tiny root wear to turn into the major wear in the second picture. It's all becoming a bit of a crap shoot...

[sc_rufctr]

Peter, Thanks again for the input...

After reading your comments I'm inclined to clean everything up as best I can, seal up the gear box and reinstall. I'll obviously replace the diff bolts and locking tabs but that it. The rest goes back as is.

Hopefully I'll have long enough to source a good second hand gearbox. From what you're telling me I could easily spend the same amount of money fixing this one so it makes sense to buy a used gear box with better internals.

I have been considering doing a G50 conversion. The nicer shifting, hydro clutch and newer design makes more and more sense as time goes by. After reading your replies I would guess a lot of older 915s would be in a very similar condition to mine.

A local independent "Porsche" mechanic can do a complete G50 conversion for about $12,000... That includes everything from supplying the G50 box to the pedal cluster and shifter mods. He also recommended doing the "shortened bell housing" mod so I can keep my torsion bars in the rear.

Please comment about the G50? What should I look for when presented with a potential candidate? I know there are some updates recommended for the G50. Should I look for these or is it something that can easily be done afterwards?

After what's happened with my 915 I'm inclined to have the G50 opened up and inspected by a trans specialist before I part with any money. I know that's probably not necessary with a G50 but it can't hurt.

Meanwhile I'll strip my gearbox right down, clean everything up and take some better pictures and post them here. If only to provide information to other people here.

Peter