81sc gearbox issues

[ashww005]

Hi

New member here

Got an 81 sc targa with crunchy gearbox issues, see the post from karsten about the spring, unfortunately I can’t message till I’ve posted a bit, so thought I’d ask if there’s anything else I can do, other than..

1. Spring
2. Rebuild

Cheers

[Schulisco]

1. The right gearbox oil. There were several discussions here. Modern fully synthetic gear box oils do not generate enough drag torque which is required in the 915. The drag torque lets the synchros work far better because they get more speed due to the higher viscosity of the old oils. Porsche recommended in the 80ies a SAE90 GL5 mineral only oil, and that's what I filled in. A workshop filled a modern oil in and therefore shifting at operating temp got more and more fuzzy and caused heavy noise when shifting.
2. Correct shifting linkage adjustment.
3. Overhaul all plastic bushings in the shifting linkage from the shift lever up to the rear shift coupler. Then do #2.
https://blog.*******.com/how-to-replace-shifter-bushings-classic-air-cooled-porsche-911
https://blog.*******.com/how-to-replace-the-shift-coupler-boots-on-a-classic-air-cooled-porsche-911

Did the clutch works good and does it separate fully?

Thomas

[ashww005]

Originally Posted by Schulisco

1. The right gearbox oil. There were several discussions here. Modern fully synthetic gear box oils do not generate enough drag torque which is required in the 915. The drag torque lets the synchros work far better because they get more speed due to the higher viscosity of the old oils. Porsche recommended in the 80ies a SAE90 GL5 mineral only oil, and that's what I filled in. A workshop filled a modern oil in and therefore shifting at operating temp got more and more fuzzy and caused heavy noise when shifting.
2. Correct shifting linkage adjustment.
3. Overhaul all plastic bushings in the shifting linkage from the shift lever up to the rear shift coupler. Then do #2.

Did the clutch works good and does it separate fully?

Thomas

hi, thanks for the reply

i'll check out the gearbox oil

the clutch works fine, the gear selection is fine 95% of the time.

ill look atnthe info you have sent

many thanks

[ashww005]

thanks

I'll have a read

the 1st link is the same as the second, but ive found the blog article through search

Thanks

[Schulisco]

This is a video showing on how to proceed with the ahift linkage adjustment, ideally after overhaul of the shift linkage:
https://youtu.be/b3nPONBAaZM?si=00CtekMGT37d7Ybg

Thanks for the hint with the link, fixed it.

Concerning the oil - there a some threads here discussing the oil topic. Porsche itself recommended in the meantime a modern 75W... oil as well. Technically from the specs is this okay, but again - the drag torque is missing. I have reports here from people replacing that modern oils by the old fashioned oils. The only thing you have to know - with the old oils it might be a bit harder to shift due to temp. But until the 915 reaches operating temp all is fine. I had this experience years ago - with the modern oil it shifted well the first miles, but then I always failed to switch to the right gear. With the SAE90 no more issues. Be careful - GL5 is mandatory for the 915! There are also some 85W90 mineral oils now available, they work also good. But before taking such have a look into the tech. data sheet and compare its values with the specs Porsche had given. They used Shell Spirax MB90 as factory filling here in Germany. Data sheet: https://www.omf.com.tr/assets/upload/services/fd951bed-b8ff-42f5-a1e0-958a7a360a73-pdf19112015041555.pdf

Thomas

[ashww005]

Originally Posted by Schulisco

This is a video showing on how to proceed with the ahift linkage adjustment, ideally after overhaul of the shift linkage:
https://youtu.be/b3nPONBAaZM?si=00CtekMGT37d7Ybg

thanks

[911MANN]

Schulisco: A question for you regarding 915 gearbox oils.

I had read 2-3 years ago that some 911 experts in Germany were quite happy with Motul 300 LS in their 915 gearboxes. You probably know it's synthetic, which is supposed to be a no-no, especially for Limited Slip Differentials- too slippery.

I tried it for a brief time before having my gearbox rebuilt, and here are my thoughts. It shifts like butter, especially when the gearbox is cold. It also seems good when the gearbox warms up. But I thought it was too slippery, such that when shifting fast, the gears would slam instead of mesh together. But if someone doesn't know what he's feeling, he will probably think this oil is great. I suspect in time, the synchronizers and other parts would wear more quickly than desired.

Do you have any thoughts or experiences with this oil? Were the articles accurate by saying German 911 owners like this oil? I'm just curious to hear your perspective. Just for the record, I am using SWEPCO 201 in my gearbox now, and I do have LSD. Thanks for any thoughts or experiences.

[kevingross]

I don't understand where this religion about mineral versus synthetic oils in 915 (and other) transmissions comes from. Every one of my customers uses synthetic oil, street and track, and none has ever had a problem with Mobil 1 LS 75W90 or Motul 300. It is just fine for LSDs, too.

[Schulisco]

@911MANN

Had an pretty similar experience on mine: Workshop, former Honda dealership, race experience, etc. but not specialized on the old 911's, filled a modern green oil in the 915. Consequence: Hard shifting when 915 was warmed up. Cold pretty smooth. When warm, I could´n't find the gears any more.
Came across the tech spec book recommending the SAE90 GL5. Porsche used Shell Spirax MB90, but this is avail only in 20litres canisters...need only 3litres. So I choose Castrol EPX90 which has pretty indentical viscosity values as the MB90. After oil change: Cold harderto shift indeed, but when being warmed up pretty smooth. I find the gears now anytime! Downshifting into 1st when slow rolling is also possible again.

Here are some discussions in a german forum (machine translated):
https://www-pff-de.translate.goog/thread/2808992-shell-spirax-mb90-verkauft-hier-jemand-eine-getriebefuellung/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp

https://www-pff-de.translate.goog/thread/2764223-getriebe-915-revision-ja-oder-nein/?postID=155346799&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp#post155346799

In 2nd link user "pold9000" had an overhauled 915, no improvement. Almost same behaviour on shifting. Was supposed to overhaul it again. Could be prevented by a simple oil change. Was a day to night experience - only by changing the oil. His final post was this: https://www-pff-de.translate.goog/thread/2764223-getriebe-915-revision-ja-oder-nein/?postID=155349586&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp#post155349586:

October 1, 2020
#30
Hello

I drove about 150km today - you wouldn't believe the difference compared to before - only because of an oil change.

In any case, Castrol is ok, I can't say whether Shell is better.

LG

wolfgang

Another argument: Before I overhaul a 915, first I chase for the low hanging fruits and try the cheap and simple things: shift linkage overhaul and adjustment if never been done before, and changing the oil with a proved and recommended oil (following the original specs). If this all stills fails - ok - then I also start to think to an overhaul. But only then.

My suspicion: Porsche moved to the modern oils to save money and space in the workshops to reduce the number of different oils to be stored. Nothing to say against. But: The more worn a 915 is, the more it needs the drag torque to perform normally. Also I do not say that no modern oil will work in a 915. There are some for sure. But I talked in the last years to some people about this. Most of them has been struggeling with their 915. And they could solved it by an shift linkage overhaul and an oil change for 50bucks.

On the other hand - a damaged/totally worn gearbox cannot be fixed by a simple oil change, no question. But in the beginning if an overhaul is in question, I do the simple things first...

No personal experience with the Motul 300LS. You can use a LS oil in an non LSD gearbox of course. But I won't expect any benefits about the LS spec. The viscosity values of this are at 100°C quite similar to the MB90/EPX90 which is pretty good for the 915. 40°C viscosity is far lower than the MB90/EPX90. Technically this looks not too bad and could also work.

Thomas

[porschedude996]

Swepco 201 seems to be high on my 915 Lubricant List.

[PeteKz]

I used SWEPCO for a long time. 4 years ago, I couldn't get it very quickly, so I took someone else's suggestion and tried the Valvoline gear oil you can get at any FLAPS. It works as well as the SWEPCO in my 915. And I can get it anywhere, anytime, for a lot le$$.

[911truther]

well just happen to have what your looking for

[Bill Douglas]

Hi Ash, my SC gearbox was getting a little funky with me. A lot of things didn't make sense. I did the Swepco thing and changed the engine and gearbox mounts to Sport. the Sport was a little firmer than usual but didn't let vibrations come through to the car body. It kept things in line better with the coupler and connections when shifting gears.

Not saying its the answer but it's an easy try before you start doing expensive stuff.

Also check the clutch and throw out bearing and fork.