what exactly allows you to smoothly shift into 1st from a standstill?

[Beau G]

My friend is taking his car in for a 1st gear rebuild. I'm asking on his behalf although I'm also interested in knowing too. What is usually the culprit when a 915 gearbox crunches when shifting into 1st from a standstill. Any help is appreciated.

[Peter Zimmermann]

All things considered, the culprits can be;
1. Driver technique
2. Clutch out of adjustment or bad (including cable difficulties)
3. Linkage/bushing/coupler issues (doubtful based on your "crunch" comment)
4. Synchro problem relating to worn synchro ring, worn/broken dog teeth, bad shift slider, incorrect internal shift fork adjustment

Does your friend pull the shifter back into second, or up into third (whichever gear is smoother) prior to putting the car into first? Depending on that answer, when driving a 915 with a "weak" first gear synchro, always ease the shifter back into first gear - before stopping - and leave it in first while stopped. This eliminates the hurry up/stress of engaging first gear after the light turns green. Basically, the car might be fine, and because of limited info, the problem might be the driver!

[Sirius Driver]

I'm the guy with the car, Beau got the thread going for me while I drove back home tonight. Anyway, I'm a good 915 driver, but ever since I received this car it's being a pain in the trans department. Okay, so not long ago I did the clutch and, I had my friend replace 1st and 2nd dog teeth gears, and a bearing on my 915, since they were both missing teeth. In doing so, 2nd gear is now awesome, but 1st gear is somewhat worse. Before he worked on it, I could only go into 1st gear if I went into 2nd or 3rd before going into 1st. While slowing down, If I came out of 2nd or 3rd I could easily enter 1st gear as long as I was coasting below 5mph. After the new dog teeth gears, I still have go into 2nd or 3rd or 1st will grind, and now I cant go into 1st if I'm coasting. I can ease into 1st from a standstill but if I shift into 1st like in any other healthy 915, The gear will grind. I'm ready to take it in for a 1st gear rebuild, again. I'm definitely nice to the tranny, and I know how to shift from a standstill. thanks! any insight before I decide to spend $

[timmy2]

How are your shift coupler bushings and the shifter and tunnel bushings? Too much play and poor alignment = poor shifting, grinding etc.

I replaced all of mine and shifting is very smooth, I have no real trouble shifting into first, stationary or moving.
I used pelican for the tunnel and shifter bushings.
I used ivangene's (the coupler whisperer) delrin bushings for the shift coupler in the rear of the tunnel.

[will hung]

If you paid to have 1st and 2nd rebuilt before, I would take it back to the original mechanic and have him sort it out.

[Grady Clay]

To add to Peter Z’s good list: “It is designed this way.”

When you are sitting at a stop, in neutral with the clutch engaged, you suddenly disengage the clutch and attempt to engage first gear: this is an ‘up-shift’.
Even with a new syncro ring, this can have a ‘clash’ of engagement dogs.

Porsche chose to increase the first-gear ‘down-shift’ syncro function at the expense of the ‘up-shift’ syncro function.
They did this by increasing the ‘servo’ function on the ‘down-shift’ direction of synchronization.
This left the ‘up-shift’ direction without a separate ‘brake band’ and far less servo function from only the ‘stops’.

You can either pause before attempting to engage 1at gear, allowing the clutch disc, input shaft and gears to slow or ‘touch’ another gear syncro first.


For decades, I have modified 1st gear by replacing the engagement dog ring with a 2nd gear part.
This uses the equal bi-directional servo syncro of 2nd gear.
While this reduces the servo-syncro function down-shifting into first, it does away with the ‘up-shift’ almost non-syncro of the original design.
Yes, the later 1st-gear sliding sleeve with asymmetrical dogs works well with the 2nd-gear dogs.

I ‘compensate’ for having less ‘down-shift’ into 1st gear servo syncro function by waiting until the 911 is traveling slower or I double-clutch downshift at higher speeds.



Nothing seems to totally prevent occasional ‘balking’ of the engagement of 1st gear when the gears are all stopped (car stopped and disengaged clutch having the input shaft gears stopped.)
This is due to the blunt ends of the engagement dogs being occasionally situated where they don’t allow engagement.
Porsche tried using asymmetrical teeth on the 1st gear engagement (sliding sleeve) and 1st gear engagement dogs.
There was marginal (undetectable?) improvement.

The solution here is to have the input shaft gears turning by going from clutch engagemed to disengaged. This uses the ‘up-shift’ function of the 1st gear syncro to align the engagement dogs.
Another solution is to engage another gear (2-5) so it rotates the engagement dog alignment slightly.

Best,
Grady

[Peter Zimmermann]

After reading Dalton's post, I wonder if the last "repair" included symmetrical dog teeth on first gear, being used with an asymmetrical slider? I've found this type of mistake in transmissions I've repaired, but I've never really driven a car with the miss-matched parts. Or used parts were used, or...

[Grady Clay]

Pete,

I have seen Factory virgin 915s with the asymmetrical dog teeth originally on 2nd gear with no ill effects. (Afternoon ‘lager effect’.)
I regularly have used the asymmetrical teeth against 2nd gear dogs installed on 1st gear with excellent effect. (See post #6 above.)

Best,
Grady

[wayner]

Sometimes at a stand still, shifting from neutral into second then into 1st before you pull away will let you engage 1st so that you can pull away.

[JoeyD911]

Let me get this straight, your tranny shifts like crap going down into first and grinds a little bit upon performing the shift, sounds to me, as a 915 owner as well, that everything is adjusted perfectly!

[Sirius Driver]

I'm gonna take some time to read Grady's response a little better, and really get a grip on this. I'm used to going into 3rd gear before upshifting into 1st, so I'm not really ticked about that, although it's what prompted me to open the gearbox. What's messing me up is the fact that I can no longer shift into 1st if the car is moving slowly. I suppose my previous gears were so worn that they were actually allowing me to. I want to either be able to take off in 1st without engaging 3-5 first, or be able shift into 1st while coasting. Arrrrrrrrgh! Gonna get to work, will be back in a couple hours. Thanks Guys!

[tobluforu]

Great info and it sheds light on the thread I created yesterday.