915 trans - mainshaft noise at start-up

[fanaudical]

My 915 started making an odd noise at start-up if the car has sat a day.

Upon cold start-up with car parked in neutral, the trans makes a low, audible whirring noise. Push the clutch in and it stops. Back the car down the 100ft driveway in reverse, stop, put it in neutral - noise is still there with the clutch out. Drive the car ~100 ft forward in first gear, stop, put it in neutral - and the noise is gone.

My theory is that there is a bearing on the input shaft that drains all its oil over time when warm and shut down. Driving forward dumps some oil on it from the differential turning.

Does this sound plausible? Which bearing would this be? And is this the precursor to worse problems?

Thanks in advance for the help.

[Matt Monson]

There are a few things I would suspect. One is the mainshaft outer case bearing, 999.110.025.00.

But imo, the more likely culprit is a needle bearing. There is one running under the 5th gear or could be one of the couple needle bearings associated with the reverse gear.

How long do you have until it fails catastrophically? Nobody knows. Could be 100 miles, could be another 10,000.

[fanaudical]

Thanks, Matt. I'll try to isolate the location of the sound. From the driver's seat, it sounds like it's up front (which would support the theory of 5th or reverse). At least those are relatively easy to get to.

Assuming it's one of the needle bearings - Is there any driving behavior that causes these to fail? I try to stay out of 5th gear unless I'm consistently above 60 MPH.

[Matt Monson]

It’s just the least lubricated area. When you’re driving steady state the oil stays more towards the main case and the differential. Keeping your oil topped up and having a good service interval is really the only thing you can do.

[PeteKz]

Do you park at a significant up or down angle?

Otherwise, keep an eye on your magnetic drain and fill plugs to watch for excessive fuzz buildup. Bearings will shed metal usually long before they fail.

[fanaudical]

Thanks, Matt. I like to think I'm regular on service. I've got maybe ~25k miles on this transmission since it was professionally rebuilt.

Pete - The car parks flat but at the top of a driveway with significant uphill climb. I'll take a look at the drain plug. I just did a fluid change last year - I don't remember much in the way of fuzz - will go look at notes.

[Matt Monson]

It just occurred to me that only the 5th gear idler will spin in neutral. It’s not a reverse issue.