915 gearbox strange noise

[MaRu]

Hello everyone,

we just swapped gearboxes at a friends 911. The now new installed one however has a strange noise coming from the front of the transmission where reverse and 5th gear is located. But itīs only there in first to fourth gear, not in fifth. Itīs not engine rev related. Actually I can switch the engine of while driving and itīs there. The noise itself is a strange rattling , vibrating noise. Like if the gear of the starter wouldnīt pull out of the flywheel.
But itīs definitely coming out of the 5th gear area.
Could it be, that the needle bearing is bad but under the load of the gear in the noise disappears
Any ideas what might cause this?

Thank you very much!

[john walker's workshop]

Maybe the reverse idler to Rev/5 shift sleeve clearance is misadjusted and they're touching, or the thrust washer is missing from a previous repair.

[gled49]

Could be 5th gear needle bearing has imprinted groves on the gear, as commonly seen. The gear freewheels in all but 5th gear. But you should rule out things like sway bar touching the trans mount, or zero clearance shift coupler preload in the trans.

[MaRu]

Originally Posted by john walker's workshop

Maybe the reverse idler to Rev/5 shift sleeve clearance is misadjusted and they're touching, or the thrust washer is missing from a previous repair.

Yes, this is exactly what I think it sounds like. Iīll know more by Monday. Thank you!

[MaRu]

Originally Posted by gled49

Could be 5th gear needle bearing has imprinted groves on the gear, as commonly seen. The gear freewheels in all but 5th gear. But you should rule out things like sway bar touching the trans mount, or zero clearance shift coupler preload in the trans.

Thank you. Yes I ruled out all these things and will have the needle bearing by Monday when Iīll remove engine and trans and remove the front housing of it.

Iīll post pics when I find something.

[porschedude996]

I have a similar sound in 5th. Although my guess and others say overhaul. I’m interested in what comes out of this, please post pictures.

[Nick Triesch]

I find that if I leave out my hearing aids out on a drive, my Carrera is very quiet!

[gled49]

Maru, run your fingernail inside the 5th gear freewheel bearing surface, when apart to rule out needle bearing damage to the gear, if it’s damaged you will feel it.

['78 SC]

I had an irregular noise from the transmission since my '78 was new. Sort of a "thock" sound, that made me think something loose was getting whacked by a rotating part. No pattern I could discern, hot, cold, coasting, accelerating, etc.

Eventually I tore down the transmission to look for anything loose or out of place, but found nothing obvious. I was pleasantly surprised after reinstallation that the noise had disappeared. After reading John's comment, I'm wondering if the shift sleeve was misadjusted (from the factory) and I inadvertently corrected it during reassembly (thank you Mr. Zimmermann).

Originally Posted by john walker's workshop

Maybe the reverse idler to Rev/5 shift sleeve clearance is misadjusted and they're touching, or the thrust washer is missing from a previous repair.

Here's a photo of the slider and reverse idler. Does the wear pattern on the slider teeth suggest there could have be contact that caused my noise? Transmission has 55K miles.

[MaRu]

Alright, I found two issues. One was the washer just laying on the ground of the cover. Although I donīt think so because the rubber was tight I might have dropped it when removing the cover.
But the main thing was the 5th gear slider wasnīt adjusted probably. No itīs running, shifting,... without any strange noises!
Thanks for all the input guys!

[MaRu]

[porschedude996]

What tools are absolutely required to overhaul a 915? Given the bearings are not loose in the housing.

[MaRu]

Nothing special like with the G50.
A Puller for the bearings some bigger wrenches and best an old clutch disc to hold the shaft in the vice without damaging it. But you should know what you are doing. -at least a little bit to avoid things like we found...

[Matt Monson]

Originally Posted by porschedude996

What tools are absolutely required to overhaul a 915? Given the bearings are not loose in the housing.

I was going to link Pete Z’s 915 wiki but it’s gone! Anyone got a good archival link???

[Gonbad]

If you need to replace dog-teeth then you will need a press and a set of special plates that hold the slider and allow the dog-teeth to be pressed off and on without damage.

If you replace the main and pinion shaft bearings that ride in the case races you will have to setup the pinion depth and backlash again. Not necessarily the easiest thing to do without some special tooling.

You will also need a micrometer and dial gauge to measure tolerances and shaft runouts.

[eflight]

Originally Posted by Matt Monson

I was going to link Pete Z’s 915 wiki but it’s gone! Anyone got a good archival link???

Try this one

https://web.archive.org/web/20161104061059/http://porsche.wikidot.com/porsche-915-transmission-repair-tutorial

[Walt Fricke]

The key to diagnosing the problem here (short of pulling the nose cone to look) is the fact that the sound didn't happen in 5th (and, had it been tested, in reverse). In those gears the what - the transfer gear? reversing idler? - is either engaged (so it can't rub against anything, or when the tranny is in 5th, it has been moved far from the reverse (straight cut) gear, so the two can't rub. But if the operating sleeve setting is a bit off, when the sleeve is in the middle or neutral position (as it is for 1-4), then the two straight cut gears rub. In my case, I drove a long distance to an autocross - almost all in 5th, and the exhaust was loud. In 1st and 2d at the autox, everyone asked me what was going on. Other than the work of getting the setting right finally, and changing the oil again, no real harm done.

Not like wearing out the sharp synchro teeth.

[4litre6]

I feel like mine has a similar but different problem.
Originally it just had a worn synchro in first, so no major issue, but I quick changed it back to first gear one time and ever since it has a bad rattle/knock in all gears except first. Haven't pulled it out yet. Interested to see if anyone has enough knowledge of the inner workings that it makes some sense

Thanks,
Jim