Noisy Fifth gear, a worry or annoyance?

Trackrash · Trackrash's Garage

I just bought, freshened up, and installed a 915 trans in my '71 911.
There is somewhat of a whirring sound in fifth gear. It doesn't sound like a typical gear "whine". Best described as a slight whirring noise. It is not present in any other gear.
I replaced the first and second synchros. The fifth gear showed no noticeable wear on the teeth.
Under hard acceleration it is hardly noticeable. On overrun no noise. Most noticeable on slight acceleration

Is this a cause for concern or just an annoyance?

Thanks,

Peter Zimmermann · 04-26-2011, 01:34 PM

Did you look closely at the inside bearing surface, where the gear rides on a needle bearing? Did you see equally spaced lines on the gear's bearing surface? What else did you remove (housing bearings, etc.)? How did you set up the 5th/R slider?

Trackrash · Trackrash's Garage

The inner bearing surface looked smooth, no grooves, as seen in your tutorial pictures.

I adjusted the 5th reverse slider checking for clearance (1mm) between the slider and reverse gear while in neutral. I engaged reverse and fifth and it looked OK. Looked like the reverse gears were in the original position with reverse engaged. With fifth engaged I turned the input shaft by hand and it rotated smoothly.

I had the entire trans apart. I replaced the first and second synchros, second gear dog teeth, both the ball bearings, and installed a one piece bearing retainer.

I did notice some slight wear on the fifth/reverse guide sleeve where the slider would be when it is in the fifth gear position. Could that do it?

Thanks,

Peter Zimmermann · 04-26-2011, 03:21 PM

Hmmmmm... Did you happen to note if the pair numbers inscribed on the two gears that make up 5th speed matched?

Did you remove the center housing bearings, the ones that require removal of the very thin circlips? If yes, did you put the circlips back in?

Silly question, are you sure that you have enough oil in the trans to lubricate the gear properly? Oil should be to the bottom of the fill hole with the trans level.

Trackrash · Trackrash's Garage

Didn't check the gear numbers, but looked at the teeth and they showed no unusual wear.

I did not remove the center housing roller bearing races, and the outer races seemed tight.

Did I mention that the trans is out of a '72 911 which was wrecked?

I opened the fill hole and fluid started to come out.

dtxscott · 04-26-2011, 06:09 PM

Does 5th ride on the output shaft/pinion gear?

I had a similar noise in my G50 and found it loudest in 4th. It turns out the axle bearings were worn and noisy.

4th gear rides on the output shaft/pinion gear. When in 4th, the axle bearings would transmit the whirring noise from the bearings to the trans and amplify it throughout the car.

Trackrash · Trackrash's Garage

Fifth gear in the 915 is at the front of the trans, at the opposite end from the differential.

I first thought it might have something to do with the shifter adjustment. As if there was some tension on the shift rod which would transmit the gear noise. I have re-adjusted the shifter, but it did not make much difference.

john walker's workshop · 04-26-2011, 06:32 PM

old gears can make some noise, in spite of their appearance. each gear set probably has it's own little song if you listen carefully. too high a gear at too low an RPM will be noisier than it would be at a higher speed/rpm.

Trackrash · Trackrash's Garage

I was just looking and it appears that maybe I did not give the 915 enough clearance since it replaced the original 901. Perhaps there is some slight contact between the transmission and the body.

The bottom line is I can live with some noise, I just want to make sure there isn't some major problem.

michael pagano · 04-26-2011, 07:46 PM

If i go into 5th too early it makes a noise, must keep above 3k rpm for the noise to go away in 5th.

nkotselas · nkotselas's Garage

+1 on the last idea. I've got a low mileage 915 trans in my car and it whines at lower rpms then goes away. Depends how bad and what rpm it is i guess.
I wouldnt worry about it but i am no doctor
best of luck

Trackrash · Trackrash's Garage

I just pulled out my trans. I just could not live with the whirring sound in fifth gear. Apparently there is wear on fifth gear's teeth. Would this wear cause noise?

What about this guide sleave? Is this wear cause for concern?

Thanks for any advice.

Jon B · 08-16-2011, 07:45 PM

Gordon,

The gear is scuffed, from lack of lubrication. Common on 915 fifth gears, to varying degrees, which are highest off the oil level. The teeth should look the same on drive and coast.

It's getting difficult to find nice 915 fifth gears, and even visibly nice gears probably have 100K-plus miles of wear.

Trackrash · Trackrash's Garage

OK, I've got a new (to me) fifth gear, which looks very good.

Before I button up the trans I just want to make sure that the guide sleeve I have is serviceable. Is the wear on my guide sleeve going to be a problem? Does it look OK or should I look for a replacement?

Thanks for all the help.

Peter Zimmermann · 08-19-2011, 08:45 AM

I would not use that guide sleeve; straight into the scrap heap with it!

Trackrash · Trackrash's Garage

Just a follow up on this thread. I replaced the fifth gear set. Thanks, Steve Alarcon. The new gear set fixed the noise issue. The old gear set pictured above had a back lash of about .009". Right at the wear limit. Even so it was so loud that I could hear the whine echoing off K-rails when driving on the freeway.