915 rebuild input shaft gone?

[mmasse]

I am rebuilding my 915. I decided on the rebuild because I was having issues down shifting from 5th to 4th on the track.
I started inspecting the gears and the first thing I noticed is that the input shaft (1st gear) seems to be in bad condition. Teeth are sharp as razors. See pics, please

What are my options? Can this be repaired or I need to find a new shaft with matching gear?

[john walker's workshop]

The mainshaft 1st gear is normally like that. The gear teeth look a bit worn, but need a close up, clearer pic of the tooth surfaces.

[Gordo2]

Was waiting on someone with more experienced eyes to chime in - you couldn't get more experienced than JW...

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Your main shaft 1st gear looked similar to what I found when I rebuilt my trans.

When I rebuilt my trans, I recall most advice regarding gear wear & replacement was oriented toward uneven wear of the gears (pattern of wear on the gears) - where replacement could be needed if a bearing wore out and caused abnormal / uneven gear wear.

I think it is unusual to find gears that need replaced - most wear & poor shifting is associated with worn synchro components.

Transmissions really aren't that complex to rebuild - but similar to engine re-builds, assessment of what needs or should be replaced can be immensely difficult (assuming you don't have $$$ to burn...).

Good luck.

[john walker's workshop]

Here's one from my stash.

[mmasse]

Hi John
I just took some more pictures. Beside the wear, it looks as if the two first gears are not well aligned. Thoughts?

[mmasse]

You can see the other gears are perfectly aligned, tough

[john walker's workshop]

My example shows the same haze mark toward the bearing as yours, so that's normal.

[mmasse]

Thank you!! Now I am relieved

[Matt Monson]

I'm going to disagree. See what I've circled? That relief line is the hardness starting to wear through. I bet a thorough cleaning and inspection will show that line isn't the only one and that's it's on multiple gearteeth. And those bearings are junk.

[mmasse]

Thank you Matt.
Bearings are all going to be replaced. I was trying to see if input shaft could be saved.
Beside $$ replacing 1st would give me the opportunity to go with a taller ratio, which might be benefitial since I have a 993 with supercharger on it and current 1st gear is very short

[KTL]

My first reaction was the same thing Matt mentioned- the rollers in the bearings for the diff housing look NFG due to the scoring around them. Also in that picture Matt highlighted, take notice of the dog teeth for 1st gear. The asymmetric teeth are really blunted. Good teeth have a pronounced single-angled tip as shown in the pic below. Yours are unforunately quite rounded.



While you're in there, be sure to check the fit of your 2nd gear on the shaft. Being that 2nd gear is the next highest loaded gear (1st is the highest), it takes quite a bit of abuse. Especially when you consider the hard 1st-to-2nd shifts that 2nd is often subjected to by speedy street driving. With age and abuse, that splined area on the main shaft can get "shagged." The gear should have a tight fit on the splines and not have excessive rotational play. Your 2nd gear does have a nice looking friction surface of the synchro ring and the dog teeth are fairly sharp. So that's a plus!

5th to 4th can sometimes be just a minor adjustment to the shifter. If you're catching the tip of the reverse lockout pawl/teardrop (assuming you have a factory shifter) on the shifter lever, it can make the engagement of 4th a bit like being locked out of reverse. Also need to make sure there's some grease on the pawl and shifter lever to avoid the pawl tip from digging into the lever and scoring it. Seems that the pawl is much harder metal and the softer shifter lever metal takes the damage.

[mmasse]

Thank you

I will definitely make a deep inspection as I disassemble ever
One question though
The reason I am tearing down the tranny is because at my last track day I suddenly couldn't downshiftbfrom 5th to 4th anymore. During my first look at the gears I could identity anything that would justified this!! The 4th gear synchro has some wear but it doesn't seem to be that bad. Any ideas on where to.look for this?

[Matt Monson]

Try the selector “fork” on the bottom of the box. It guides the main shift rod in selecting the individual shifting rods. The tines break and/or the mounting bolts strip. It’s a 4 bolt trapezoid to remove and access.