Another 915 rebuild question

[mikedsilva]

I have a 'spare' 915 to be worked on.

It is from a 1985 3.2 .. 915/73. And appears to have the smaller pinion bearing which gives more meat in the case and less chance of bearing sleeve slop. Good news, is that all my bearing sleeves are nice and tight. In fact, I think this box might not have been opened before me. Only reason I say that, is it came apart nicely, and there appears to be no sealant used on any of the paper gaskets. The only part I am not 100% sure of, is that the nuts used to fasten the bearing retainer plates, were yellow zinc and the washers were all split spring style.

Anyhow, the box was taken apart due to 1st having no syncro at all. I haven't used this box in a while. I noticed it was quite "chattery" when in neutral and the clutch was out. But it seems thats a fairly common trait with these boxes. When cold, it is silent, after 10 minutes or so, as the gear oil heats and thins, it starts to chatter a bit. I was not using Swepco, but will do after I put it back together.

Ok, so the question:
Exactly HOW wrong is it to flip a syncro band?
Also, I have a slider between 3rd and 4th, that looks like the 3rd side is a little mangled.

I've taken a video to try and show some of the wear that I have found on the gears too.. 1st (ultra expensive) and 2nd (still expensive) do show signs of wear, but I really don't want to change these.... another question.. can I flip the fixed 2nd gear to use the other side?

My pinion and ring were not whining before hand, so I am not planning to change carrier bearings and assume that using the original shims, will put me exactly where I was before.

As for syncro bands, looking at the video, I assume I will need to change the band for 1st.. but what about 2nd and 3rd? I know the online tutorial says never to flip them, but I don't quite understand why? If the worn side is put against the dog gear and does not need to be engaged by the slider, then how is it detrimental?

[VFR750]

1st and 2nd gears look like they are nearing the end. 2nd especially.

What I also noticed was the spline on the input shaft where the gears slide on. The surfaces have worn which makes the gears loose. That is the reason I replaced mine. Found a good used one.

Gear dog teeth inspection. I think the problem with swapping synchros band is the band outer diameter gets larger (worn on worn) and the slider has more interference that has to overcome.

[VFR750]

New input shafts are $2,800 From Porsche.

Which is a pretty much a non-starter for me. Would have sucked it up for $1500 or so. 2800 was just way too much.

Hence finding a good used one.

[yelcab1]

1st and 2nd gears are shot. I would not re-use those and feel good about the gearbox. Yes, they are expensive.... I have a set of 1st, 2nd, 3rd gears and Reverse from a 915 on a 74. Too bad they won't fit your box, I don't think.

Synchros, I would flip those around since there are not excessively worn out of shape. Replace that slider in question.

[VFR750]

I just replaced all synchronizer bands. But I reused the 1-2 slider and 5-R slider. Those gears shift very nicely

Replaced 3rd gear set with NOS. New 3-4 slider. Don’t you know, 3rd is the one that fights me.

Cadmium plated nuts and split lock rings holding the bearing plates are from the factory

[proporsche]

Mike... a lot of peeps would say do not flip them ..
well, i did flipped them on several trans. jobs with my mentor back in the 1980`s...no harm done...


Ivan

[mikedsilva]

Originally Posted by yelcab1

1st and 2nd gears are shot. I would not re-use those and feel good about the gearbox. Yes, they are expensive.... I have a set of 1st, 2nd, 3rd gears and Reverse from a 915 on a 74. Too bad they won't fit your box, I don't think.

Synchros, I would flip those around since there are not excessively worn out of shape. Replace that slider in question.

Yes, I think I might try to flip the syncros and replace the slider.
If anyone has a good 2nd gear set of a known matched set, I would be grateful to purchase them!

[mikedsilva]

Originally Posted by VFR750

I just replaced all synchronizer bands. But I reused the 1-2 slider and 5-R slider. Those gears shift very nicely

Replaced 3rd gear set with NOS. New 3-4 slider. Don’t you know, 3rd is the one that fights me.

Cadmium plated nuts and split lock rings holding the bearing plates are from the factory

good to hear about the split lock rings and nuts. Shame about my worn gears!
I imagine on yours, the new slider might need to wear in a bit with the new syncro band.

[Jonny042]

No time at the present, Mike, but when I need a break from my own build I'll have a look at your video. Lucky you, having the 3.2 box with tight races, you have the makings of a budget build, at least - stronger side cover from factory, too.

I have a couple of good used second gears that came out of boxes to be replaced by custom ratios.

[VFR750]

I do imagine time will help 3rd get to the point where the slider comes off a little easier.

That and getting my Wevo shifter properly aligned 🙄

[mikedsilva]

Originally Posted by Jonny042

I have a couple of good used second gears that came out of boxes to be replaced by custom ratios.

Matching set? interested in selling?

[kevingross]

Hi Mike,

I hope you enjoy the process of rebuilding your 915.

As you suspect and others have commented, first and second gear teeth are pitted and at end of life. Whether you decide to reuse rather than replace them is of course up to you! If you're okay with the risk of them coming apart, debris causing collateral damage, well it's your call. For a customer, I would certainly replace them, would insist on it.

Your third and fourth gear synchros are heavily worn and in need of replacement. It's hard to be sure in the video (very nice job with the video, btw), but I think I see "root rot" in the fourth gear floating / shifting. Common end-of-life wear to these gears. Look carefully at the root of the gear teeth for erosion. New, the teeth have a fine polished surface where they mesh, and an unfinished appearance at their root (valley). The erosion I'm referring to is neither of these: it is erosion of the polished operating surface close to its radially interior edge.

You're right that the 3rd/4th slider's teeth are worn on the 3rd gear side. Not horrible and could be reused in a pinch. The real issue is judging wear to the slider from the synchros. It's been said before, but still true: the synchros and sliders wear together, and best practice is to replace the slider when you replace the synchro(s). If you get a new slider, you'll see the difference in the spline / tooth profile.

I don't recommend flipping second gear to prolong its useful life. It's done.

I suspect your measurement of the 1st gear installed synchro width may be faulty. Not sure how you get the numbers you were getting. Nominal is 86.37 +/- 0.17 mm. The mic you have is not ideal for taking this sort of measurement. Where you are replacing synchros, measure with the new synchro installed, not the old one. Only then will you get a proper sense of the dog ring's suitability.

Got this from flipping around your video, hope I didn't miss much. Good luck with the project!

[mikedsilva]

Thank for the replies, very helpful information.

Is a gear set from a 74 mag case 915 suitable for use in this 84 ally case 915?
Reason I ask, is I do see some advertised parts from older boxes occasionally.

[Trackrash]

Originally Posted by mikedsilva

Thank for the replies, very helpful information.

Is a gear set from a 74 mag case 915 suitable for use in this 84 ally case 915?
Reason I ask, is I do see some advertised parts from older boxes occasionally.

They will fit, but the ratios are not the same.

[mikedsilva]

Here are some photos of 3rd gear and syncro.. it's shiny, but measures 76.25mm. I plan to replace it regardless.

[mikedsilva]

Here is 4th gear syncro and dog teeth..Pretty sure this is fine to go straight back in.

[mikedsilva]

Here is the 3/4 slider.. it will be replaced along with 3rd syncro.. the rounded teeth on the slider were on the 3rd gear side.

[mikedsilva]

Here are some close ups of 2nd gear...

[mikedsilva]

And here is 1st!

[kevingross]

4th gear synchro is junk; replace it.