rebuilt 915 trans needs to "break in"?

[cwitt]

Hi all,

I'm considering purchasing a Carrera 3.2 that has 107K on the clock, and had engine and trans rebuilt ~4K miles ago. I drove it and, to me, this 915 trans seemed to require a lot of effort to engage all gears. No grinding or anything, and no unusual issues in finding the gears, just very slow shifting and a lot of effort required to engage the gears, even when you're giving it plenty of time to shift. I know the 915 trans isn't a slick shifting machine like the G50, but this one requires much more effort than my buddy's well worn 915 trans in his SC.

It's got a new Porsche factory short shift kit in it. I doubt that the decreased throw/leverage could account entirely for how it feels, but it's probably a contributing factor. I know that fluid could also be a factor here, but I'm not sure what fluid is currently in it.

I asked the owner about it, and he said the trans still needs to "break in". The local shop that rebuilt it (a reputable place, by the way) is also claiming the need for break-in, and added that they think the short shift kit and it's adjustment could be part of the problem. Another reputable local shop said that the break-in theory is at least plausible.

To me, this is just counterintuitive. Shouldn't a rebuilt trans shift best right after the rebuild, and then slowly degrade again as the new synchros get milled down by the sliders? Or is there truly a break-in period after a rebuild? If so, how long should break-in take? (it's already got ~4K post-rebuild miles on it)

Your thoughts/opinions are appreciated.

Thanks!

Curtis

[Flieger]

The shifting is stiff if you have new synchros and sliders. As they wear in, shifting gets easier. At some point shifting gets too easy and there is not enough synchronizing force to engage gears smoothly without double clutching.

[Roby466]

Hi,
I just had mine rebuilt. It shifts better than before the rebuild, that's for sure but I cannot say that it shifts like a modern transmission. It does require a bit of effort to shift my 915 but if I rev-match as I double clutch while downshifting, it does feel pretty good. Maybe the new operating sleeves and synchros are a bit tight and need to loosen up a bit... not sure.
The only other experience I have with this would be with a VW transmission. After replacing all the shift bushings, it did require more effort to shift through the gears for a while and then got smoother. That could have been called a "break-in effect" but I don't know if it applies to the 915.
I hope this helps and hopefully other guys with more transmission rebuilding experience will chip in...
David

[kidrock]

I wonder if it has Swepco 201 in it?

BTW---we'd all like to see your new pride and joy. Can you post any pix?

[john walker's workshop]

the installed, outside diameter of all the new syncro rings needs to be measured with calipers and be within spec. the inside of the pressed on dog teeth can wear oversize from the rough surface of the syncro rubbing on it during it's lifetime, and even though the teeth look nice and sharp still, they often need to be replaced if they allow the the new syncro to expand too much. the shift sleeve has trouble passing over a "fat" syncro. use a standard gear oil like kendall 80/90 GL-5 and hike the short shifter.

[Wil Ferch]

+1 on hiking the short shifter....

......causes more problems then it solves.

You lose considerable leverage with the shorter stroke. Really only necessary for the extreme seat bolsters that some cars have...and the shift knob interference that then results in the 2nd gear position. Original poster doesn't mention the year. Stock 1984 had the standard, "long" throw....stock 85 already came fitted from factory with a "semi-short shift" kit installed ....reducing throw by 10%. "True" short-throw shift kit reduces throw by 20% over normal....and effort goes up at least this much as its an outfall of the lever-rule.

[cwitt]

It's a 1984 Euro spec Carrera, so I'm not sure what those came with originally. I do know that the Porsche factory short shift kit was recently installed, but I don't know if it's the semi-short or full short kit.

Sounds like I would want to ditch it either way.

Thanks!

Curtis

[Wil Ferch]

You either have stock or the 20% reduction kit if you have an 84. Notice I said the 85's came from the factory with a "kinda" kit as standard fitment...10%....but only as factory fitment. What you buy afterwards as as a kit is the 20% version.

[cwitt]

Originally Posted by Wil Ferch

+1 on hiking the short shifter....

......causes more problems then it solves.

You lose considerable leverage with the shorter stroke. Really only necessary for the extreme seat bolsters that some cars have...and the shift knob interference that then results in the 2nd gear position. Original poster doesn't mention the year. Stock 1984 had the standard, "long" throw....stock 85 already came fitted from factory with a "semi-short shift" kit installed ....reducing throw by 10%. "True" short-throw shift kit reduces throw by 20% over normal....and effort goes up at least this much as its an outfall of the lever-rule.

Okay, so consensus seems to suggest getting rid of the factory short shift kit. Considering that I have the sport seats, what are my options? My understanding is that the bolsters on the sport seats require a short shift kit of some sort, to prevent the shifter from hitting the seat in gears 1 and 2.

Thanks!

[Jdub]

I would approach this holistically - have you adjusted the clutch cable at the trans? How about the shift coupler - did they get to all four bushes (coupler x2; shaft bush; shift end cup bush) replaced when they went with the short shift kit?

When JohnW rebuild my trans. I had already repaired the pedal cluster, replaced the clutch cable and (sadly) the horseshoe spring. As the new clutch was breaking in, went in to check and adjust the clutch clearance (just once - held the setting).

So go around the entire trans - consider the bits that help the 915 shift better. I would also drop the trans fluid, have a look at your magnetic plugs just to have peace of mind and fresh fluids.

[Marc Bixen]

As usual there are many different thoughts and theories.
1) always check the basics. as stated clutch adj, shift bushings, trans and motor mounts etc.
2) as usual John Walker is spot on with measuring the assembled syncro and slider, unfortunately this can only be done when the trans is disassembled. We've returned a number of these parts that where out of spec. Many of these parts are no longer available from Porsche and must come from the aftermarket. In general this will effect only certain gears, those repaired. Typical repair is 1st and 2nd on a low milage car like yours.
3) This can also be a Clutch pilot bearing issue. If the trans input shaft isn't turning freely it will cause hard and or notchey shifting.
4) If it has synthetic trans oil in it GET RID OF IT. 901's and 915's don't like synthetic. We use ONLY Kendall 80-90 and if you have a limited slip Kendall LSA additive
5) Apples to Apples, are the other cars your driving equipped with the short shift kit? There is also an adjustment, pinch bolt on the shift tower that if over tightened makes it difficult to move left and right. Is the shift tower properly lubricated?
6) and last, are there aftermarket parts in the shift linkage. Many times we've seen "rebuilt" shift couplers with poly or bronze bushings that don't have sufficient play in them. O.E. couplers are not "tight" they have some play.
Marc Bixen
Red Line Service
Porsche Parts & Accessories | **************.com

[cwitt]

Originally Posted by Marc Bixen

As usual there are many different thoughts and theories.
1) always check the basics. as stated clutch adj, shift bushings, trans and motor mounts etc.
2) as usual John Walker is spot on with measuring the assembled syncro and slider, unfortunately this can only be done when the trans is disassembled. We've returned a number of these parts that where out of spec. Many of these parts are no longer available from Porsche and must come from the aftermarket. In general this will effect only certain gears, those repaired. Typical repair is 1st and 2nd on a low milage car like yours.
3) This can also be a Clutch pilot bearing issue. If the trans input shaft isn't turning freely it will cause hard and or notchey shifting.
4) If it has synthetic trans oil in it GET RID OF IT. 901's and 915's don't like synthetic. We use ONLY Kendall 80-90 and if you have a limited slip Kendall LSA additive
5) Apples to Apples, are the other cars your driving equipped with the short shift kit? There is also an adjustment, pinch bolt on the shift tower that if over tightened makes it difficult to move left and right. Is the shift tower properly lubricated?
6) and last, are there aftermarket parts in the shift linkage. Many times we've seen "rebuilt" shift couplers with poly or bronze bushings that don't have sufficient play in them. O.E. couplers are not "tight" they have some play.
Marc Bixen
Red Line Service
Porsche Parts & Accessories | **************.com

Thanks everybody -- especially Marc and John.

I know that the trans rebuild included synchronizer rings for gears 1 thru 4, and new dog teeth for gears 1 and 2. No sliders were replaced from what I can make of the parts list. Flywheel was resurfaced and Sachs clutch kit was installed at the same time. I don't see a clutch pilot bearing on the parts list, so that's a possibility.

I see that a new ball cup bushing was put in when the new factory short shift kit was installed (replacing the factory short shift kit that the car originally came with). I don't see any evidence of a new shift rod bushing or any other new shift linkage parts, so I think I'll throw in a complete bushing kit and adjust the clutch cable and pedal. I prefer this to the more drastic options, such as a $700 Wevo shifter.

All of these repairs were done ~5,000 miles ago (along with engine top-end).

I definitely plan to dump the trans oil and replace with Kendall. I have a quick question about the Kendall...

Should I get the NS-MP Hypoid Gear Lubricant in the 80W-90 weight and add the Limited-Slip Axle Additive Concentrate to it? If so, how much of the LSD additive should I add?

...or...

Should I get the Special Limited-Slip Gear Lubricant, which has the LSD additive already in it? I'm not certain if this starts from the same base oil as the NS-MP or not, or what the additive ratio would be.

Thanks!

[javadog]

Originally Posted by cwitt

My understanding is that the bolsters on the sport seats require a short shift kit of some sort, to prevent the shifter from hitting the seat in gears 1 and 2.

The problem is only 2nd gear and only if you are super short and have the seat way forward. If you are of normal size, it isn't an issue.

JR

[KTL]

It should have smoothed out after 4000-5000 miles by now, if it's internal. The three 915s that i've helped people rebuild got better within 1000 miles.

Regarding the Kendall special limited slip oil, i've used it this year in my racecar's trans and it works nicely. Definitely better than the Redline pink goop that was in there before! I've got a Robotek shift kit on top of the factory short shift. Shift effort is firm side-side, due to the Robotek's spring action, but fore-aft action is not too bad. Gear engagement is fine. No forcing it into gear whether cold or hot. I'm happy with the stuff thus far and will continue using it.

[Matt Monson]

Originally Posted by john walker's workshop

the installed, outside diameter of all the new syncro rings needs to be measured with calipers and be within spec. the inside of the pressed on dog teeth can wear oversize from the rough surface of the syncro rubbing on it during it's lifetime, and even though the teeth look nice and sharp still, they often need to be replaced if they allow the the new syncro to expand too much. the shift sleeve has trouble passing over a "fat" syncro. use a standard gear oil like kendall 80/90 GL-5 and hike the short shifter.

I have an additional comment about dog teeth. In recent years the replacement dog teeth that have been available in the aftermarket, including those through OEM sources, have frequently been a tad too tight right out of the box. Back in the old days the factory put a slight backcut on the teeth allowing the synchro-ring to properly seat within the dog. What we have seen is that this backcut is not being done. It's something we do on all of our teeth at the point of manufacturing the gears. It's something many rebuilders know to do with a dremel at the point of assembling the gearbox with the new teeth.

However, if they just pressed the dog teeth on the gears, installed the synchros, and put the thing back together assuming all the new parts measure within spec, then the synchro in this freshly rebuilt gearbox may be getting pinched too tight and isn't getting enough friction from it's engagement with the slider on certain gears.

Just something for the OP to consider if this problem persists and none of the other suggestions in this thread remedy the issue.

[cwitt]

Originally Posted by Matt Monson

I have an additional comment about dog teeth. In recent years the replacement dog teeth that have been available in the aftermarket, including those through OEM sources, have frequently been a tad too tight right out of the box. Back in the old days the factory put a slight backcut on the teeth allowing the synchro-ring to properly seat within the dog. What we have seen is that this backcut is not being done. It's something we do on all of our teeth at the point of manufacturing the gears. It's something many rebuilders know to do with a dremel at the point of assembling the gearbox with the new teeth.

However, if they just pressed the dog teeth on the gears, installed the synchros, and put the thing back together assuming all the new parts measure within spec, then the synchro in this freshly rebuilt gearbox may be getting pinched too tight and isn't getting enough friction from it's engagement with the slider on certain gears.

Just something for the OP to consider if this problem persists and none of the other suggestions in this thread remedy the issue.

So, Matt, if I do indeed have the "tad too tight" dog teeth situation in my gearbox, can I expect it to improve over time, or no? (I'm the OP of this thread, BTW)

Thanks!

[Matt Monson]

Curtis,
I really couldn't say. There's several different brands of dogs out there and some are tighter than others. And it's pure speculation that this is even your problem. The fact that it doesn't grind is in your favour and there's a good chance it's something else, though whether or not that's inside the box or outside of it, I don't know.

You also don't specify what gear(s) it happens in. If it happens in every gear, that points to something else. If it just happens on the 1 to 2 shift then it might be a tight dog.

[Marc Bixen]

Kendall recommends 1oz. per pint, so on a 915 you add 6oz. Don't spill the stuff and don't throw the container in the indoor trash, the stuff STINKS.
We sell the Kendall 80-90 NS-MP Hypoid and the LSA additive. The additive comes in 4oz. bottles so you need 2 bottles. Jegg's sells the 80-90 with LSA pre mixed. I'm temporarily out of stock, but Jegg's price is high.
Good luck.

[joetiii]

Does it make a difference what type of LSD to use the additive with? Plate type or Torque Bias?

[Peter Zimmermann]

Originally Posted by cwitt

I asked the owner about it, and he said the trans still needs to "break in". The local shop that rebuilt it (a reputable place, by the way) is also claiming the need for break-in, and added that they think the short shift kit and it's adjustment could be part of the problem. Another reputable local shop said that the break-in theory is at least plausible.

To me, this is just counterintuitive. Shouldn't a rebuilt trans shift best right after the rebuild, and then slowly degrade again as the new synchros get milled down by the sliders? Or is there truly a break-in period after a rebuild? If so, how long should break-in take? (it's already got ~4K post-rebuild miles on it.Curtis

Curtis, I'm sorry to hear about your difficulty. Extended break in usually occurs when something is overlooked. As others have posted, the installed dimension of every new synchro must be measured, and every new synchro must be mated to a new slider. That's my rule, and my transmissions are often broken in before the end of a 5-mile test drive. They never require more than 20-30 miles to become comfortable.

There is a lot to be learned from the comments made by Marc, John and Matt, and if you have to time, I urge you to read my 915 repair Tutorial at...
Welcome to Red Line Porsche Wiki - Porsche Wiki
...simply locate "Page Tags" and click on "915."

Regarding your transmission, today, your old sliders are carving away friction material from your new synchros. That material loss will shorten the effective life of your synchros, and make your next refresh happen sooner than it should. I, like Marc, have returned out-of-spec synchro rings to a supplier; one can only imagine how difficult shifting might be with a new, egg-shaped synchro combined with a used, worn sliding sleeve.

About your shifter. Try to determine if you have stock 1985 shifter pieces (10% shorter than '84 and earlier original shifters), or if you have a super-short shifter (20% shorter than '84 and earlier original shifters) introduced in 1986. The '85 set-up should serve you well, but I've removed SS shifters because of complaints of unusually heavy motion (not to be confused with poor shifting characteristics).