So I took a closer look at a few parts tonight to get a better idea of their condition. Here is the rundown:

No chatter lines inside of 5th gear, so thats happy news:

http://forums.pelicanparts.com/uploa...1509998600.JPG


The news is worse inside the 5th gear bushing, which runs directly on the pinion shaft. Looks like it got warm in a bad way. Needle bearing that runs outside it looks good though.

http://forums.pelicanparts.com/uploa...1509998680.JPG

http://forums.pelicanparts.com/uploa...1509998680.JPG

http://forums.pelicanparts.com/uploa...1509998852.JPG


The pinion shaft also bears the same indication, and looks like the adjacent bearing and spacers also left their mark. I can't feel any of these marks but they don't look pretty:

http://forums.pelicanparts.com/uploa...1509998680.JPG


5th-R slider. I wouldn't dream of changing 1-4 syncros and not the sliders, but on R I'd really like to avoid a slider replacement. Thoughts?

http://forums.pelicanparts.com/uploa...1509999351.JPG
http://forums.pelicanparts.com/uploa...1509999351.JPG

I can't believe that the blue marks are from wear. If that race was spinning on the shaft, there would be a lot more gauling. I'm not seeing that. Probably oil staining.

Was the nut loose on the end of the shaft?

I would check the operation of that 5 -R slider on a newly installed synchro ring. Compare that to your new slider and synchros before you decide. It looks marginal, but the mechanics I have worked with in the past would probably say it would be OK. We will see what the experts here have to say.

The marks on the inside of the bushing (and on the pinion shaft) are more brown, then blue.

Nuts on both shafts were tight.

Sheesh, we're talking about nuts and shafts here. Censored! :D

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Just a point worth sharing about the synchro friction bands. You don't want to flip one and reuse it. Reason being is because when you flip the worn side to sit in the dog/engagement teeth, that worn surface is obviously thinner and that changes the installed dimension of the band. In other words, there's a specification about what the diameter of the band should be when installed in the dogs. If you're using a worn band, that changes the measurement and also gives you a false indication of the dogs. Because one of the reasons you take that measurement is to check the wear condition of the dogs on the inside. That is, manual says if the measurement doesn't meet the spec, it's said that the dogs should be replaced.

Also note that the synchro band friction surface acts on the inside of the dog teeth in a rotational manner. If you watch how a synchro band functions, it actually rotates a bit inside the dogs as the slider ring is moving onto the synchro band. That's what the "guts" inside (brake band(s) and two other two blocks) are doing inside the friction band- they manage it's rotation and reliably brake/stop it every time. So if you put a worn side of the band in the dogs, that changes how it grips/slides against the dogs. Again, i'm talking about how the friction band interacts with the inner side of the dog teeth and not how the friction band interacts with the slider ring. They're two separate, but related things going on.

Hey Kevin!

Thanks a bunch for the wise words. As you'll notice from the rest of my thread I'm following Peter's tutorial to the letter and your explanation is similarly clear in why the synchros and sliders must get replaced. I'm replacing all the syncros and 1-2 / 3-4 sliders, 1-4 dogs, and maybe the 5-R slider too. All 6 big bearings and the diff bearings are getting changed. The diff case will get sent out for professional help on the machining side. So I'm not leaving much untouched.

The 5-R slider and needle bearings on the free gears are items I haven't decided on yet. See photos above. SmileWavy

I think that 5th gear bushing is fine. The shadowing seems to occur from the tight slip fit between the ID of the bushing and the OD of the shaft splines. I feel like the oil in between the bushing ID and the shaft splines gets "stale" and tends to stain things. The oil gets into the splines and is like a dead end w/out any flow. So it mostly stays in the same place and doesn't get refreshed nearly as much as oil slinging around the gears. I've found some of the oil in the splines can be really black and stanky.

5th gear itself looks pretty good with only minor chatter marks in the ID. Are those dashed areas inside the gear something you can feel with your fingernail? Only reason I ask is because 5th is by far the most lubrication challenged gear and I would be concerned that over time those marks, if rough, would continue to grow and also damage the needles. But I believe the needles are harder so they probably are the ones doing the harm.

I would leave the 5-R slider if the engagement teeth on the 5th side look OK. They always look to be heat-affected and I think that's because when the part is manufactured and heat-treated, that area of the part has little mass and changes color? Just a guess on my part. If you see any of those tiny teeth that are slightly mushed on the edges, I wouldn't hesitate to put a fine tooth file on them and knock away the disturbed edge of the tooth.

Also take a good look at the bullnose teeth of the reverse side on 5-R slider. Sometimes people have no mercy and always grind reverse. Over time the face of the teeth get chunked away and that dumps debris if the chunked areas keep shedding metal. This picture below is an extreme example of the dual gear that's used to reverse the direction of gear rotation, so that the car actually goes backwards. You can see on the gear in the background that nearly all of the rounded faces are all chunked.

http://forums.pelicanparts.com/uploa...1446065739.jpg

Replacing 5th/R 915 Components
 

I opted not to replace the 5th / R components when I rebuilt my 915 based on the following rationale:

- Prior to rebuild, trans shifted into 5th without issue.
- It's a relatively minor step (speed differential) going from 4th to 5th, so the action of the synchro assembly doesn't add much "synchronization." I've never tried shifting without the clutch (rev matching, prior to shifting), but willing to bet I could do it reasonably well from 4-5.
- Never had a situation where I've been compelled to shift from 4 to 5 in anything other than a deliberate, slow-paced shift movement (which allows the speeds of the input / output to synchronize).
- The 5th slider is expensive (considerably more than 1-2 / 3-4)
- Reverse is not synchronized, but does need to mesh well with the slider. Best bet is taking your time getting into reverse - not forcing it (sometimes re-clutching if it isn't moving freely).

Bottom line - I didn't see much point in replacing the 5/R synchro components.

For consideration, good luck.

Gordo

Any thoughts on the needle bearings and evaluating these?

Thanks and cheers! SmileWavy

Thanks Gordo! I'm thinking a bit along the same lines, also taking into consideration that to use 5th in Norway at a speed that isn't harmful to the inside of the gear (pr Peter's tutorial) one must be well in excess of the speed limit. I'll make a closer inspection tomorrow and go from there.

I think those needles look fine. As long as there's no discoloration on them and the cage is a nice concentric fit into the gear and around the bushing, they're good to reuse.

Another thing to keep in mind is that the bearing and the bushing/gear contact surfaces have run-in over the years and have reached sort of a happy place with each other. Not saying they're like new and haven't worn. Just saying that they've been working with each other all these years and still have a happy coexistence.

If you introduce a new bearing and use the same old bushing & gear, you've got new vs. old surfaces. I feel like it's similar to the front wheel bearings. When you want to change the actual bearing, you install both the new tapered bearing and its paired outer race to have two (actually three) new surfaces running together. Or it's like chains and sprockets. If you're going to install a new chain, you should do new sprockets too. Otherwise the old sprockets cause the new chain to wear faster and what have you really gained? Just my opinion, for whatever that's worth.

I made a bit more progress during the weekend. The main shaft is empty of gears and under the 3rd gear bushing doesn't look that great. The inside of the busing is also the same brown and included some brown residue. I cleaned up the parts and there doesn't seem to be any scoring, just discoloration and a slightly less smooth surface. The needle bearing is not discolored and everything turned smoothly prior to dissassembly. I think I'll be able to clean it up but would like some input.

Thoughts?


http://forums.pelicanparts.com/uploa...1510518752.JPG
http://forums.pelicanparts.com/uploa...1510518752.JPG

In other news it was time to make some tools. In his tutorial Peter Z wittily comments that many homemade tools could go to a character breakfast at Disneyword as a character. This one:

http://forums.pelicanparts.com/uploa...1510519497.PNG


I can't promise my homemade tools aren't goofy, but the ones that are made to handle torque are pretty robust. No one ever said a P252a needed to be round!

http://forums.pelicanparts.com/uploa...1510519941.JPG
http://forums.pelicanparts.com/uploa...1510520036.JPG

Those shafts look good.

As mentioned, and experienced mechanic friends have told me, if the needle bearings look good and the cages are good, let them be.

Here are a couple of threads on home built transmission tools.
http://forums.pelicanparts.com/porsche-911-technical-forum/283301-need-901-transmission-tool-sizes.html -includes 915
http://forums.pelicanparts.com/porsche-911-technical-forum/746264-915-tool.html

Those tools look excellent. Nicely done!

Regarding the shafts, I agree with Gordon. I have five 915 mainshafts in my collection and all of them have some semblance of discoloration upon that smooth area of the shaft. So I would tend to call that normal.

Be sure to take a close look at the splined area of the shaft where 2nd gear is placed against that thin sleeve. This spot is where splined 2nd gear can slightly hammer that sleeve (if the nut on the end of the mainshaft is not fully/properly torqued) and also the splines can get distorted. I have a few shafts that are questionable and both have 2nd gear splines that are a sloppy fit. I can place a gear on those splines and it has noticeable rocking back and forth. I can also see on mine where the splines are not uniform and have been distorted by the rocking of the gear.

Also I think worn bearings (the large ball bearings that are held in the retention plate) can contribute to the pounding on the second gear location. I found this fractured race when I took apart my racecar transmission. http://forums.pelicanparts.com/porsche-911-technical-forum/941069-gordos-915-transmission-rebuild.html#post9435103 Remember that all the pieces stacked on the shaft are a "sandwich" of items that are tightly held together with that nut on the end. If the stack becomes loose, the pieces can move along the shaft and hammer on each other. By the way, Gordo's transmission rebuild thread is a really good read. I highly recommend going through it and i'm sure you'll gather some good information from it.

It doesn't surprise me that this is where the splines are most susceptible to damage because 2nd gear has a considerable amount of torque multiplication (only 1st has more) and is the only gear splined to this side of the shaft. The other gears are either integral to the shaft (1st gear) or ride on needle bearings (3rd, 4th). 5th is splined on the opposite side of the shaft but 5th gear is an overdrive ratio and as a result the torque multiplication is much less.

Thanks guys! I'll clean up that mainshaft and move on then. It was torqued down with everything tight in the stack when I undid the nut, so I think I'm OK there. I will double check the splined 2nd gear fit though, to be sure.

I've read Gordo's thread cover to cover several times (I referred to it further up), great stuff there!

LSD test
 

I've been wondering what the state of my factory ZF limited slip differential is. Peter Z was kind enough to share the correct procedure for bench testing one of these units as shown in the Carrera workshop manual. Basically, you fix one flange end of the diff in a bench vice and put a torque wrench on a homemade jig on the opposite flange. The LSD should tolerate between 7-25 lb-ft of torque before breakaway. I tested with 3 different torque wrenches, and with each one it would hold at 25 lb-ft, and slip at 26! Talk about on the money.

Here is the tool I made. I added a center bolt and will weld a small extension on this same tool when it comes time to measure backlash, so it can serve two functions:

http://forums.pelicanparts.com/uploa...1510694667.JPG


Two bolts on the opposite flange allow the diff to go in the vice:

http://forums.pelicanparts.com/uploa...1510694847.JPG


Torque wrench on, testing slip torque between 7-25 lb-ft. Slipped at 26 but not 25! :)

http://forums.pelicanparts.com/uploa...1510694667.JPG

Way to go; Evan!!!

I'm in parts cleaning mode while waiting for boxes of goodies to arrive from our host. This is the 3-4 shift fork bit that the dongle pushes on to select those gears-- is this normal wear? The same part for 1-2 shows no wear.

http://forums.pelicanparts.com/uploa...1511037236.jpg

I looked at a few of my shift rods in the spare parts box and they show some contact at those dented points in your picture. But not nearly as significant as your picture shows. If you'd like I can take a picture of the ones I have for you to compare. One set is from my racecar trans, which worked really good prior to disassembly (I opened it up to check for wear and haven't gotten back to racing) and the other is from a trans that was run without gear oil (wasn't me, I promise!) and some parts failed. But it shifted well up to the point where it didn't like running without oil any longer.....