915 5th gear "chatter" marks and tall nuts

[kevingross]

So... got a 915 on the bench, lovely transmission. I've found that the 5th gear area has the dreaded chatter marks that Pete describes in his tutorial.


The marks on the bushing can be felt with a fingernail dragged across them and it will be replaced, along with the needle bearing. The marks on the gear's interior surface cannot be felt -- and I am wondering whether it can continue to be used. Opinions? Anyone got a TM gear pair for sale?

Oh, the nuts. Perhaps a foolish question. The DIN spec M8 nuts commonly available today are shorter than what Porsche used, roughly 6.3 mm tall versus 7.4 mm. I am wondering whether there is a source for the taller nuts. I ask in part because the car is a '73 Carrera RS, it's a type 915/08 transmission, and it would be nice to keep everything looking as original as possible. Not my car, not sure how particular the owner is! Cheers,

[Mark Salvetti]

Bumping this back to the top as I was looking forward to the answers. My 5th gear has the same marks as Kevin’s, and I was under the impression that if you couldn’t feel them then the gear could be re-used.

Mark

[kevingross]

Mark, thanks. Yes, current plan is to reuse the gear, replace the bearing and bush. Cheers,

[Matt Monson]

Hone and polish the gear. It will be fine. Replace the bearing and race.

Email me the part numbers on the nut(s) you need. I may be able to help.

[72911]

https://www.belmetric.com/metric-nuts-c-3/?zenid=91cn0v2ir13utbr9dbtnd6okg6

[kevingross]

Matt, thanks, will do.

72911, thanks yes, that's where I get the regular ol' DIN 934 nuts. The short ones! Cheers,

[Matt Monson]

I just wanted to add a comment. I recommending still running that gear because it's a TL and superhard to find. It's your best option given that he's trying to keep the car original. TL's have gotten rare enough that Autofarm has me making them for the CarreraRS cars they do in house. They can't find them anywhere.

If it were a later box with an SN? I'd say take a pass through the classifieds and find one for $200-300.

I felt like my comment needed some context. It's not a universal answer or solution.

[kevingross]

Matt, agreed, thanks again! (Actually it's a TM. But point made.)

[Jon B]

Originally Posted by kevingross

So... got a 915 on the bench, lovely transmission. I've found that the 5th gear area has the dreaded chatter marks that Pete describes in his tutorial.

The marks on the bushing can be felt with a fingernail dragged across them and it will be replaced, along with the needle bearing. The marks on the gear's interior surface cannot be felt -- and I am wondering whether it can continue to be used. Opinions? Anyone got a TM gear pair for sale?

Oh, the nuts. Perhaps a foolish question. The DIN spec M8 nuts commonly available today are shorter than what Porsche used, roughly 6.3 mm tall versus 7.4 mm. I am wondering whether there is a source for the taller nuts. I ask in part because the car is a '73 Carrera RS, it's a type 915/08 transmission, and it would be nice to keep everything looking as original as possible. Not my car, not sure how particular the owner is! Cheers,

Kevin, the technical term for the chatter marks is brinelling...
https://en.wikipedia.org/wiki/Brinelling

I'd agree that if it cannot be felt on the gear surface, then the gear should be fine to re-use. You should obviously replace the needle bearing and bushing.

The M8 nuts you describe are the same found on most 914 transmissions, so finding good used ones shouldn't be difficult.

[kevingross]

Jon, thanks. Any ideas on what causes this to the 915 fifth gear and bushing? The transmission seemed to be well maintained, fluid fresh.

[Matt Monson]

It’s a lubrication issue. Because of the rear engine design coupled with the 3 piece tranny case, the oil tends to pool to the ring and pinion end of the box. On cars that see a lot of highway cruising in 5th this is a common issue.

[72911]

Originally Posted by kevingross

Matt, thanks, will do.

72911, thanks yes, that's where I get the regular ol' DIN 934 nuts. The short ones! Cheers,

I don't know if you saw the page, but they have class 8 high type self locking nuts listed there.

https://www.belmetric.com/class-8-high-type-c-3_752_1754/?zenid=abah348pm2ds8uq2pkomfuf753

[Trackrash]

Originally Posted by 72911

I don't know if you saw the page, but they have class 8 high type self locking nuts listed there.

https://www.belmetric.com/class-8-high-type-c-3_752_1754/?zenid=abah348pm2ds8uq2pkomfuf753

Are nylon self locking nuts what you guys are referring to? When I search class-8-high-type-c-3 I get no pictures on that web site or from that link. How are those different from what is used on the valve covers?

Both of my old transmissions, '70 and '72, had the locking nuts that had what looked like a nut with a heli-coil inside. Which is best to use?

[Bill Douglas]

Originally Posted by Matt Monson

It’s a lubrication issue. Because of the rear engine design coupled with the 3 piece tranny case, the oil tends to pool to the ring and pinion end of the box. On cars that see a lot of highway cruising in 5th this is a common issue.

Makes sense. Maybe that means Swepco gear oil is a VERY good idea as it "climbs" moving parts. There was a demo at one time where it worked it's way up an egg beater.

[Trackrash]

+1 on Swepco.

FWIW, after having to replace a worn fifth, I keep my trans oil level topped off. If you look at the side view of the trans you will notice that bottom of fifth gear sits higher than the other gears.

[Matt Monson]

^ yes. Swepco is a good oil and always keep your box properly filled to level.

[Peter Zimmermann]

Difficult situation, without doubt. Lubrication, or lack of, could be a contributing factor, but because the bad 5th gear sets I've found have no evidence of heat it leaves me to wonder. I've had to replace many 5th gear sets, including the bushing and bearing, in 915s, and based on information obtained from customers I think much of the problem is caused by drivers.

Bear with me here. I think that if 5th gear is used properly, and always at speeds above about 60 mph, that the problem becomes less frequent. When my customers told me that they don't downshift to 4th, or 3rd, until they have slowed to speeds far below, say, 45 mph, I have almost, to the trans, had to replace 5th gear.

I believe that a vibration of sorts, at lower revs, sets up inside the transmission, and when 5th gear, a gear meant for higher speed cruising, is engaged, that vibration in the gear stack works against the gear set - especially if one were to accelerate from 40-45 mph. Our cars are well enough insulated so the vibration can't be heard, but through experimentation I can "feel" it.

So, is the problem caused by oiling, or by driver miss-information? Got a coin to flip?


BTW; 8mm nuts with helicoil threads were used primarily on magnesium trans housings.

[kevingross]

Pete, thanks for the info. I also found the 5/R guide sleeve's splines had some damage to them, and it will be replaced. I don't know the driver, the transmission was sub'ed to me by a shop, and so I cannot editorialize on his/her treatment of the car!

[Geary]

I think you're absolutely correct, Peter .. false brinelling is a function of lugging and/or lack of sufficient lubricant in the gear bore. In my hobby world of Syncro Vanagons (many of which are grossly overpowered with 6-cylinder or Tdi engines), we've taken to gundrilling and oil-feeding our mainshafts ... and we now insist that vehicle owners select the proper 4th gear ratio for their typical cruising speeds (so as to avoid lugging for long distances). https://en.wikipedia.org/wiki/False_brinelling

[Matt Monson]

Interesting theory. Not being argumentative, but then why doesn’t it happen on 3rd? Guys often shift up to 3rd and then lollygag around town at 25-30mph. Does better lubrication save 3rd from the same fate?