rust and pitting inside 915

[Quinlan]

Hi Everyone
I have a 915/63 Limited Slip transaxle that I use in my race car (79 911sc). I have been having problems with grinding while shifting into 3rd gear, and difficulty getting into 5th. I have new coupler bushings, new shift tunnel bushings and a new cup for the end of the shifter.
It has been at least 5 years since the gearbox was rebuilt (by a previous owner), so I assumed that it is time to get in there and refresh everything.

When I disassembled it today, I found a large amount of rust inside. There is also significant pitting on the shafts and the gears.

My question (for now) is: is this anything to worry about? Since it is a race car, I don't care about noise.
What would cause this? It is stored inside, in an unheated garage, over the winters.

Thanks!

[fanaudical]

The only thing I've seen cause rust/pitting in a gear box is the combination of moisture with lack of lubrication (as in no oil was in the gearbox or the gearbox had not turned for a significant period of time).

Did you clean anything before taking that picture? I don't see any evidence of oil film in that photo.

[Matt Monson]

How much oil came out when you drained it? That's pretty ugly.

[sc_rufctr]

Ugly but it wouldn't stop me using it.

Just replace what's worn and slap it back together.

[Quinlan]

Originally Posted by Matt Monson

How much oil came out when you drained it? That's pretty ugly.

About 2 1/2 liters came out when I drained it.

[Quinlan]

Originally Posted by sc_rufctr

Ugly but it wouldn't stop me using it.

Just replace what's worn and slap it back together.

I'm wondering if that is what happened the last time it was rebuilt. The bearings don't have any rust or pitting on them.

[yelcab1]

2.6 L is very low. You should get close to 4 liters. Although, I would just clean it off, replace what is worn, and use it.

[Quinlan]

Yes, it is low. I am religious about checking engine oil level, but despite the drips on the garage floor, I never "got around" to checking transmission oil.

The reason I measured it when it came out was that I was suspecting that low oil level might have contributed to my shifting problems. Is this the case?

Also, is there any benefit/detriment to cleaning off the rust (i.e. by bead blasting?).

Thanks everyone!

[Matt Monson]

Show pictures of the pitting up close. How does your crown wheel and pinion look?

I tell my race customers to change their oil every 6-8 events.

[Quinlan]

Originally Posted by Matt Monson

Show pictures of the pitting up close. How does your crown wheel and pinion look?

I tell my race customers to change their oil every 6-8 events.

Actually, it would have been 5 events between oil, so I am not that far off...

Here is the crown wheel and pinion.

[Matt Monson]

Other than the surface rust, I'm not seeing anything particularly scary. Look really closely at 2nd and 3rd gear. Those are the high use gears and the ones that wear through the heat treat and break off teeth. However, I don't understand why the oil would be so low after so few events.

[Reiver]

How is your air vent oriented? If in wrong (hole should point forward direction of travel if memory serves) it can suck oil out of the case giving you a low level and allowing for moisture to do whatever to exposed parts that set for a bit.

[Quinlan]

Originally Posted by Reiver

How is your air vent oriented? If in wrong (hole should point forward direction of travel if memory serves) it can suck oil out of the case giving you a low level and allowing for moisture to do whatever to exposed parts that set for a bit.

The air vent is properly oriented. I didn't do the latest fluid fill (my shop did), so I can't say for sure that the correct amount was put in in the first place, but it has been leaking for a while - drips here and there, but never forming a puddle. I suppose you add up all those drips, and it could come to something significant like this.