unexplained shift improvement - 915

[LWJ]

Background: I bought an 80 SC with 100k in June. Trans was rebuilt 10-15k miles ago by a shop with a good reputation for P-cars. In particular, the receipt lists a new synchro and slider for third.

Third gear has been less than perfect since my purchase. Third gear is simply "sensitive" both engaging and disengaging. Even if rpms are perfectly matched or double clutched, it may not always cooperate.

So I read up on PP and have replaced with Swepo 201, new trans and motor mounts, new bushings, cable adjust, etc. Here comes the funny part: often times after I do a little work, the car SHIFTS LIKE BUTTER. Then, given a few miles, it goes back to it's old reluctant ways.

In the last week, it has shifted VERY WELL. I wouldn't call it butter, but then, it is a 915. I am extremely pleased with the shifting improvement and would like to keep this new-found level of ease. I am writing to try to diagnose what happened and make sure it doesn't change. Here are my thoughts.

1) The hardware put in for the rebuild was poor quality and it took 15k miles to "wear in." Seems improbable but possible.

2) As I lifted the car by the sump plate, this caused my engine/trans alignment to change slightly (hence the temporary butter-shift) but it progresses to a less desirable alignment. Now it is in a nice alignment?

3) Clutch didn't fully travel back and disengage?

4) Other scenario?

Thanks in advance! I don't think I could own a 911 without the help of the Pelican Parts Brain Trust.
Larry

[Tyson Schmidt]

The synchro and slider wore in. Some feel great right out of the box, but most need to find themselves.

[Jammo]

LWJ,
don't want to be a 'harbinger of doom' here but that is a 'textbook' symptom of the old side fork plate problem.

On the R/H side of the box (looking at the box from engine) is a plate held on by a few nuts. Inside there is a fork (support) that is attached to the plate. It is not uncommon for the internal fixings to come loose allowing the fork to move about. It will carry on working but if this fork becomes detached and/or the nuts jump off inside it can result in TERMINAL damage,,,,, and I mean gear lock up expensive.
I cannot guarantee that this is the problem (remote diagnosis) but I have done quite a few of these and cured (saved) them,,,, however, I have had boxes here in ribbons due to this internal detachment.
I would check it out.
Hope this helps mate.
Regards

Jammo.

[LWJ]

Jammo,
Thanks! I believe one drains the trans fluid and 4 quick bolts to check?

That was on my list to check before it improved. Thanks for the heads up.
Larry

[Peter Zimmermann]

Call the shop who did the trans, ask if they measure every installed synchro. The measurement spec is 76.3 +/- 0.18mm. If they do measure each synchro (if they get defensive & blustery at the question they probably don't) it's possible that you ended up with one that was slightly irregular in shape, and it took 15K miles to break in (the slider has been cutting away at the friction material a little at a time). If they don't measure, you might have gotten a bad one out of the box, or the dog tooth ring should have been replaced due to wear and allowed a good synchro to expand too far. In either case, if it has "healed" you should be good to go for many miles.

[Jammo]

Originally Posted by LWJ

Jammo,
Thanks! I believe one drains the trans fluid and 4 quick bolts to check?

That was on my list to check before it improved. Thanks for the heads up.
Larry

Hi Larry,
That's right. Get the box (tranny) warm and drop the Swepco (much easier if warm) then do the check - is the work/repair still under some sort of warrantee? If it is get it back to them. If not, there is no better place to start than the plate/fork.
This all assumes, of course, your selection linkage is correctly set up, that can be a tad difficult if the garage is not too 'specialised' with 915's.
I dont disagree with Peter's explanation/advice either, but for the little work to, if at least ease your mind, then do check the plate/fork for sure.
Hope it helps.
Regards

Jammo.

[Formerly Steve Wilkinson]

Interesting that the one thing he doesn't mention touching is the shift coupler.

[sc_rufctr]

All good advice so far but I would check everything external as well.

A complete list of what to check.

1. Clutch adjustment. Make sure the cable retracts when someone steps on it as per the secs in the manual.
2. Engine trans mounts. Make sure they're in good order.
3. The linkage from the shifter back to the gearbox. Check all bushings and connections. Especially the clamp at the shift coupler behind the seats under the covering panel.

4... Shift fork. After draining the oil remove the plate from the bottom of the gearbox and inspect. There are three studs and nuts holding the shift fork to the cover. Make sure they're tight and check that the shift fork won't move when pushed from the side.

Here is how mine looked when I checked earlier this year.



I also found some surprises which lead to sourcing a second hand gearbox from an 83. Diff bolts head!

[LWJ]

Excellent! Thanks everyone. Great advice. I will check the fork pronto. (Let's hope no diff bolts come out!) Yes. I am familiar with the coupler adjustment, I guess that is what I meant by "etc."
Thank you all,
Larry

[Walt Fricke]

Larry

If your clutch doesn't fully disengage, you couldn't get the car into 1st or (especially) reverse to begin with, at least not without grinding you'd have noticed. No way the gear you want to select has anything to do with how the clutch works.

Where you jack the car up has nothing to do with this issue. You haven't bent anything.

You say you also had trouble getting OUT of 3d gear? Wow. The normal issue is having to force things getting in - and having grinding sounds while doing so. Peter's theory of an oversized (to start with) synchro split ring wearing to size might fit this.

In addition to matching engagement tooth speeds (synchronizing) when engaging a gear, this ring also is what holds the car in gear. When the engagement teeth are fully engaged, the synchro ring is nestled inside of the slider in a groove whose ID is a bit larger than on either side of it, so it compresses when going into gear (which is why it is a split ring), and expands when in this middle groove. This keeps the engagement teeth from just spitting eachother apart again. When the ring is way way worn out, it won't expand enough and you have to hold it in gear with the shift lever.

I think that 3d may be the most common synchro to go bad, as there is a tendency to speed that 2d to 3d shift. You say the synchro and slider were replaced. Does the parts list also indicate replacement of the "dog" teeth? The engagement teeth part which mates with the slider? That usually is more worn than the slider teeth on the side of the slider. It is a press fit into the side of the actual 3d gear. Assuming these were also replaced I'd vote for the delayed run-in theory on that score also. At least as long as now it is shifting well for you.

Sourcing these engagement teeth parts is tricky. You cannot purchase them from Porsche. You can only purchase a whole gear set, with these teeth already installed. I doubt anyone purchases these, as the cost is so high from Porsche. So everyone uses aftermarket ones. One hears of quality control issues there from time to time. One shop says none of the aftermarket engagement teeth have one of the dimensions quite right - a slight concave surface where the synchro ring first hits. Another reason to cross fingers and root for the wear-in theory for the improvement noted.

Hard to imagine a decent shop overlooking a loose shift lever fork during a rebuild. The plate has to come off to get the box apart, and there it is. On my own trannies I have always slipped a wrench on the nuts that hold it before putting it back, just to make sure. I'd suppose a real mechanic would do the same, takes about 5 seconds if all is well to begin with. Hard to see just how a loose fulcrum fork would not cause issues shifting into other gears also.

However, if you don't mind draining and refilling the box for a look-see, not a tough job. At least it is doable without removal and disassembly of the whole tranny. Kind of fun to look around inside if you want to figure out how shifting works, though what you might want to see you probably can't. You need to have a replacement gasket on hand. However, you can do without the gasket if you have the orange Loctite anaerobic case sealant, which will do a fine job on clean, oil free mating surfaces.

Formerly Steve is suggesting maybe the fore and aft adjustment of the shift coupler in the car got off a bit, then maybe slipped back to a good place when tugged in the other direction. If so, though, then shifting into 1st and 5th would most likely be affected, one would suppose.

A similar, though more evil, potential has to do with the clamping of the 3d/4th gear shift fork (the one that is U shaped sort of like a tuning fork, and slips around the center of the slider and causes it to slide one way or the other. It is held onto a rod with a pinch bolt setup. This is often disassembled with the transmission, and reset on reassembly. If not pinched enough, the fork can move on the rod.

Pray for wear.

Walt