915 parts - reuse or replace? Another gearbox rebuild

[bdonally]

While I had my engine out to replace the IM aluminum gear, I decided to pull apart the gearbox to tend to a few things – difficulty going into first, and the occasional grind downshifting into 2nd and 3rd.
Not undriveable but it was bit unpleasant. Thought a look would be a good idea.
It came apart fairly easily, although the input shaft nut was crazy tight – I think is had permanent loctite and I wound up grinding away enough that it loosened with a chisel.
A bit of history – car is a 84 3.2, mildly tweaked, and sees a fair amount of track time.
A previous owner installed new gears for 2,4,and 5, creating a really nice set of gears for Mosport, my home track. Because of the gearing, I probably do more shifting than with a stock 3.2. I suspect that is the reason for 2nd and 3rd to start being a bit touchy.
I am now trying to decide what needs to be replaced. I want to replace stuff that is clearly finished, but am trying to avoid “as long as I am in there”. Since this is a DIY project, taking the gearbox in/out and dismantling/reassembly doesn’t involve any labour costs. Also, at some point, I will likely retire the car from track use and at that time will pull the gearbox and install the original ratios.

Things that definitely need replacement:
• First gear dog ring
• First-second shift sleeve
• Input shaft and pinion shaft roller bearings
• Synchro rings (some)

Parts that are not so clear:
Brake bands - they typically look good except for small grooves where they contact the inside of the synchro ring. Is this enough to warrant replacement:



Synchro rings – there is some wear on the inside where they contact the brake bands and the activator blocks.the area at the end of the ring is slightly flattened. Same question:



Guide sleeve – there are two marks on one side of each of the three arms, and one mark on the opposite side. Is there any reason these cannot be flipped to change the wear points?



Shift sleeves – if the dog teeth are OK, how can you tell is the inside is worn? It seems to me that wear would allow the sleeve to move further towards the dog ring before it contacts the synchro ring, but I cannot find any dimensions.



With regards to sourcing parts, most things other than the first gear dog teeth are available as either “OEM” or from Porsche dealers. So far it looks like the bundle of parts I need will be about 35% more from Porsche. I think the bearings are all the same, but how about the wear parts – synchros, shift sleeves, etc? Is it worth the extra for Porsche parts?

[Gordo2]

Good, valid questions - similar to the questions I asked when I rebuilt my 915:

Gordo's 915 Transmission Rebuild

Mr. Zimmerman's Wiki / tutorial is an essential reference: Porsche Wiki

The pro's will recommend replacing nearly all friction components - understandably, their reputations are dependent on rebuilding a trans that shifts well / correctly and reliably.

Meanwhile, some components can be inspected and re-used. Brake bands typically fall in this category. Mine looked good (no wear marks like yours are showing), but I opted to buy new bands for 1-2 gears - since they are relatively inexpensive and 1-2 shifting was problematic on my trans. My trans shifted well 3-4, so I reused the brake band / block components, but did replace the sleeve and synchro rings (primarily based on intent to ensure that the trans shifted as good as possible / like new).

Based on my understanding, the brake bands and blocks essentially act as a lever - as such they are not wear prone or dependent on tight tolerances between the other components.

Shift sleeves are nearly always recommended for replacement if you are also replacing the associated synchro rings. The sleeves and synchro rings have tight tolerances to mate correctly, and wear on the sleeve or rings will reduce the friction - negatively impacting how well it works.

I was unable to determine any accurate means to measure shift sleeve wear (I tried comparing new to old - but couldn't come up with any repeatable accurate measuring method).

I inspected and re-used most of the bearings. I did find the differential carrier bearings in my transmission were shot - as such, they were a no-brainer to replace.

Good luck - post your progress, lots of folks on the board to assist.

Gordo

[kevingross]

Bob, here are my thoughts. First is to use Pete Z's tutorial that Gordo linked. It's fantastic. I do a few things differently from Pete, but everyone has their own technique and tricks.

You're an enthusiast who drives his car on the track: good info to help shape advice. The other important questions would be how long you plan to keep your car, and how much tolerance you have for not fixing some things now at the risk of having to go back in there later.

Regarding replacing parts, for synchros you want to do two things. Check for signs of obvious wear: polishing of the moly coating, patches where it's missing. And measure the synchro installed per factory spec using a proper micrometer. Which will also inform you of the dog ring's condition. Inspect the dog rings for wear at their tooth tips and where the synchro rides.

Shift sleeves have to be replaced when synchros are replaced, non-negotiable. They wear together. Look for damage to other parts, as you're doing. I am seeing a lot of 5th/reverse guide sleeves come to me lately with damage to their splines, for example. Pete's note about 5th gear brindling is good to observe.

I don't think flipping your guide sleeve will bury the problem. Get a new or good-used part.

I have had success with Rauch & Spiegel parts, but there are all kinds of opinions out there. I have a new "German OEM" dog ring that measured out of spec with a new synchro, it's going back. Also have a 915 synchro ring that wasn't bent properly, too large an uninstalled diameter leading to too much tension when installed; it too will be returned. Buying from Porsche will get you the most consistent quality, but no one source is perfect.

If you plan to own your car forever, love pounding it on the track, and have better things to do than pull the transmission every six months, err on the side of new everything. If you're priorities are different, adjust to suit. Good luck with the project!

[Matt Monson]

Originally Posted by kevingross

I have had success with Rauch & Spiegel parts, but there are all kinds of opinions out there. I have a new "German OEM" dog ring that measured out of spec with a new synchro, it's going back. Also have a 915 synchro ring that wasn't bent properly, too large an uninstalled diameter leading to too much tension when installed; it too will be returned. Buying from Porsche will get you the most consistent quality, but no one source is perfect.

Porsche replacement dog rings are unfinished. They require a 9 degree backcut to make them correct for the curve of the synchro. This is something we do on our new gears during manufacture. We only use German OEM unless we make the parts ourselves.

The R&S replacement teeth have had the backcut on them for a couple of years now. They previously didn't. They are the only option for the DIYer unless they've got a lathe and want to backcut their own teeth.

R&S synchros are junk. They are too soft and wear really quickly. This is true of the 901/915 synchros, as well as the G50/GT3 steel ones. I have no idea about their just released carbon G50 synchros. But for 901/915 I wouldn't use anything but German OEM or Genuine Porsche.

[bdonally]

Thanks for the input.
I am using PZ's guide, a;long with the factory and Bentley manuals.
Everything has been measured and inspected; the only things not clear are the ones I mentioned in the original post.
I'll be pulling the gearbox in a couple of years to reinstall the stock gearing; I'll take another look at things then. I can take a risk on some stuff as long as it doesn't lead to a disastor.

Are the marks on the brake bands enough to warrant replacement?

[Gordo2]

Originally Posted by bdonally

Are the marks on the brake bands enough to warrant replacement?

I would replace that band - primarily because it appears to have unusual wear (at least compared to mine that showed no wear) and it's relatively low cost component ($35-$65 depending on which one it is).

Here's a quick video I made attempting to explain how the synchro components work together. May help you understand how the brake band components interact and determine if you want to replace:

https://youtu.be/BbBxR8stYRE

Gordo

[bdonally]

The wear on the brake band is typical of all except 1st and 2nd.
I understand how they work but am less clear on whether the marks are significant. Since the bands contact the grooved inner surface of the synchro ring, the contact pressure would be high and some marking would seem inevitable.

[Matt Monson]

I will give some input on the shift sleeve question. If you look carefully, you will see that these have a little divot in them. This is where the synchro slots in once you engage the gear. If the teeth are still good, the concern is the hump the synchro slides over and it makes it's way into the divot gets worn down. This picture shows one side worn more than the other.

I use a pretty simple test on used sliders. I take the newly "rebuilt" gear with the fresh synchro in place and slot the slider onto it. It should have some resistance and drop into the divot with almost a click. If it slides on easily, I consider the slider done. I've got piles of good used sliders that I wouldn't hesitate to use on a gearbox I was putting together.

On the guide hub (tripod as I call them) you are seeing where ALL the drive force goes through the thing. I would be curious to know if those marks on yours are on the drive side or if they are the result of hard down shifts and on the braking side. You may be able to reverse it and continue to use it, but it really depends on the cause for the wear. You don't want any slack in it at all against the slider. Ultimately, you will want to put it together with the slider and see how tight it is and see if it has gotten sloppy.

[bdonally]

Yeah I see the divot. That's a 3-4 slider, with 4th on the right.
Thanks for the inspection suggestion - I'll use that.
The marks on the tripod are on the drive side, and line up with the contact point on the slider when the slider is engaged.
As far as slack against the slider, the outside is a pretty nice fit in the slider, but all three sliders have side clearance against the tripod. It doesn't look like wear, so I wonder if its a designed in clearance.

[Matt Monson]

The teeth on the 4th side look pretty hammered. They kind of look like someone previously filed them down. Replace it.

[Gordo2]

Originally Posted by bdonally

The wear on the brake band is typical of all except 1st and 2nd.
I understand how they work but am less clear on whether the marks are significant.

Appears to me that you've made up your mind to re-use the bands. No problem - it's all a judgement call based on how much you want to invest, the outcome you desire and willingness to re-address if the outcome doesn't meet your expectations.

I ran into similar dilemmas with my rebuild.

Good luck,

Gordo

[bdonally]

Originally Posted by Gordo2

Appears to me that you've made up your mind to re-use the bands.

Nope; just trying to understand how worn they are - trying to avoid replacing stuff unnecessarily.

[R&S Admin]

=The R&S replacement teeth have had the backcut on them for a couple of years now. They previously didn't. They are the only option for the DIYer unless they've got a lathe and want to backcut their own teeth.

R&S synchros are junk. They are too soft and wear really quickly. This is true of the 901/915 synchros, as well as the G50/GT3 steel ones. I have no idea about their just released carbon G50 synchros. But for 901/915 I wouldn't use anything but German OEM or Genuine Porsche.

Good evening Guys! I just got a chance to look into this thread and thought I'd respond to answer any questions or add some clarification. We consistently strive to manufacture the absolute best quality products for our customers, all made right here in our facility in Fort Collins, CO.

I see here that Matt Monson believes our synchros to be "junk" and are "too soft" across multiple product lines.... I guess I have some questions for Matt to help determine if there is actually a problem, or perhaps a misunderstanding in regards to material mechanical properties. We sell many thousands of our synchros every year in the U.S. and in Germany and if you feel there is truly a problem, we would very much appreciate your substantiating data.

Question 1. Matt, how did you arrive at your assumption that our materials are too soft? Are you refferring to the part substrate or the molybdenum coating? What testing instruments did you use and what HRC readings did you observe relative to the hardness?

Question 2. Do you know what the material substrate hardness of a Genuine synchro is?

Question 3. If your comments are about the coating hardness rather than the substrate hardness, what methodology did you employ in determining that? We have gone to great lengths and expense to ensure our products are made using the same quality materials, hardness, geometry and tolerance that the Genuine products are comprised of.

We intend to continue following this thread through to its resolution.

As always, we care about our customers and are here to answer any questions or concerns they may have. Please feel free to reach out to us by calling (970) 482-0602 or visit our website for further information @www.rauchandspiegel.com