Found possible root cause of many 915 shift problems

[wayner]

Good suggestions

I'll report back.

[Jdub]

I'd only add that the static fork bolted by four nuts to the plate you can remove from the bottom of the 915 does get loose. Remove the plate, clean it up, loctite, and new gasket. I am sure tho that whoever rebuilt the trans must have seen to this as well.

This is an often-overlooked issue with the trans. Worth inspecting at your fluid change intervals.

[wayner]

Here is my update on this shifting issue so far:

Anyone following this thread knows now that everything is new front to back except for the throwout bearing. That bearing may or may not be significant to my problem though.

I drove the car to work one day adn got caught in stop and go traffic and detours. I probably spent 1.5hrs of clutch in clutch out creeping along. I also spent about an hour total to and from being able to use most of the gears, although double clutching up and down.

I parked it after that. All sorts of fears raced through my mind such as "did I forget to fill the new transmission (shifting was THAT bad).

After pouting for a few days and letting the car sit, I drove it to Chris Kirby's (My mechanic, the guy I trust anytime I need to call in an expert in my area).

The funny thing is that on the day that I drove it to his place, it shifted like a formula car!
(I've driven a formula car and my 911 may have even been better).

The question is WHY is it better?
I didn't do anything to it?

Chris drove it and he said it shifts fine.

I'm wondering: Break-in period perhaps?

[X JBM X]

That's the craziest thing I ever heard (well maybe not the craziest...)

Break-in period is the only thing I can think of, but it doesn't explain why it would shift better after sitting for a few days, when it shifted terribly right beforehand.

[seth agnew]

You did not mention whether or not you replaced the pilot bearing..I have seen a few where the PILOT bearing in the flywheel was not replaced and they would get sticky/not turn very well- consequently, the input shaft is still spinning at a slightly higher rate of speed than it should be. Given that you didn;t replace the t/o bearing, i would be questioning the pilot as well. May not be your issue but from what you are describing, it is certainly a possibility...

Was not 100% clear from your description though- do you have physical difficulty/stiffness engaging every gear, or is the actuation smooth but it grinds in all gears? Also, do the gear sets engage smoothly with the engine OFF?

[wayner]

Regarding CRAZY - On the way home on that day downtown, it may have been shifting better than at the beginning. THe next time after it sitting was when I noticed it though. My guess is that it got incrementally better throughout that day and I didn't clue in until the next drive.

Regarding the throw out bearing, no I have still not replaced it.

Regarding shifting, it would grind if I wasn't careful (mainly 1st) but terribly stiff everywhere, even shifting out of gears. It seemed that double clutching when going up a gear let it enter the next gear without wrestling with the lever as much. Double clutching downshifting ( and rev matching) was a necessity. Stiffness was not as noticeable without the engine running.

The transmission was rebuilt by a shop when the previous owner had the car apart. I am the first one to use the transmission since.

I'm not sure which bearings specifically were replaced. Here is the parts list he gave me in the book of receipts that came with the car.

SETH you may be on to something.
I suspect that the pilot bearing in the flywheel may not have been changed. I know that I didn't think to do it.

[wayner]

I should also add ( for those who haven't followed my build thread) that this is the first time in 12 years that this engine and fresh ( 12 year old) transmission have been run since the restoration began 2 owners ago. I don't know if that adds anything to the diagnosis

[ganun]

I think its time to change the oil, Swepco for a 915
Right?

[GaryR]

Kendall




http://www.pelicanparts.com/More_Info/KEN-LSD-7745.htm?Kendall%2080W-90%20GL-5%20Gear%20Oil%20With%20LSD%20Additive%20(1%20Quar t)

[wayner]

I agree with changing the fluid after putting a few miles on a fresh transmission, and who knows what fluid was used when it was rebuilt, but would fresh fluid in a fresh transmission go bad from sitting on a shelf for 12 years?

I don't know

It's at my mechanics now to take care of carbs etc. he says it now shifts fine without having done anything.

Strange. Maybe a pilot bearing not working well after years of sitting? Maybe it freed up after some miles?

Next winter when I drop the engine I will replace the throw out and plot bearings as a precaution

[wayner]

Originally Posted by X JBM X

That's the craziest thing I ever heard (well maybe not the craziest...)

Break-in period is the only thing I can think of, but it doesn't explain why it would shift better after sitting for a few days, when it shifted terribly right beforehand.

Another clue:

When I brought the car home the weather up in Canada was unseasonably cold.

It has been much warmer since. I wonder if this had anything to do with the dramatic difference in shifting? (my drives were short)

The fluid has been changed to whatever my many years of experience 911 technician uses, and it shifts well all the time. No adjustments have been made, just driving time and then later a fluid change.

(BTW, he said it shifted well when I brought it to him before he changed the fluid, but he kept it in a heated garage and the weather had warmed up. No idea what fluid was originally in it when I got it).

[Reiver]

Just changed motors and found my throw out bearing to be toast even tho it only had about 35k on it.
Shifting was bit dodgy and it did cause a metallic whirring/clicking noise in the tranny that seemed to be at the shifter...I could diminish the noise by moving the shifter forward/back a tad while in gear.
When we pulled the tranny it was obviously the culprit. The clutch/basket/flywheel were in great shape. Bad bearing.
Replaced and shifts as new.

[wayner]

Thanks!

I'm suspecting that the drag caused by the other oil when cold was part of the issue. It certainly improved:
1) when warm and
now is consistently better with
2) the replacement oil.

It has never been as bad as it was when I first brought it home.


I still haven't ruled out a bearing issue. Next spring I will be able to make the observation once the new ones go in. (throw-out and pilot bearing for good measure).


P.S.
As a sidenote, I replaced the manual my transmission oil in my truck with the same grade that it came with, but royal purple brand synthetic, and it went from shifting like butter to barely shifting when the winter months came. After I went back to the brand the manufacturer recommended, all was good again but my synchros were worn from all that heavy handed shifting. There might be something to staying with dino oil when cold (which goes against everything I've been told about synthetic). I hadn't considered that this might be the issue with my 911 too.

[Reiver]

Wayner,
Some trannies require a certain amount of friction to operate properly and some synthetics are way too slick.

[wayner]

Why do they get better when they are warm? Very puzzling