3rd gear teeth gone! is this common?

[flatspin911]

I have been lurking around these formums for some time and I appreciate the body of knowledge and experience. Now I am brave enough to ask for some direct feedback and comments.

We have had a catastophic failure of 3rd gear in the 915 gearbox in our racecar.

The car:
74 911 Carrera track only

Motor:
3.2L high compression, high lift cams, weber carbs.
HP? not dyno'd but likely high 200's - low 300's? (per DYNO2000 simulation)

Gearbox:
915 stock box with 7/31, facotry LSD, new syncros, dogs, later mainshaft, later 4th and 5th gears to bring ratios closer. 3rd gear is stock. Swepco fluid.

The issue:
The third gear has been rounded off on the main shaft - the Pinion shaft part of this gear is just half ground off.

Situation:
Under load in 3rd gear just before shifting to 4th on main straight at Mosport.

Totally by surpise - to warning - no noises.

Something let go - engine reved to limiter - and no drive.
At the same time - the clutch let go. basically the inner part rotated right over past the springs and poped the springs out and snaped the 4 rivetted posts.

Here are pics of clutch and gearbox.

Any theories, thoughts? comments?
Is this common issue with 3rd gear?

[lane912]

wow!

[euro911sc]

If the clutch went 1st would you get that kind of trans damage? I think not, so I would start by making the assumption that the trans went 1st and the clutch was the result. Is that a Guard billet bearing retainer plate I see there as well? Hrmnn... got me scratching my head! What does the inside wall of the case look like? Could something have gotten jammed between the case and the gear, breaking teeth off?

-Michael

[MBruns]

It looks to me like the syncro on 1 st gear has been ripped out of its spot, that would sure get into the gears and do damage to both clutch/gearbox

Mike Bruns JBRacing.com

[kenikh]

That's some gnarly carnage - a question, then a guess:

Does your track happen to have an ascending acceleration zone where you redline shift from 2nd to third, then carry third through the fattest part of your torque band?

My guess is that your high HP/torque motor finally had its day with third gear. The power pulses from the big part of the torque curve, likely with the wheels highly loaded (e.g. up a hill) likely were pounding free play into the interference zone between the shaft and gear. Once that free play hit a ciritical juncture, all it took was one more hit to third gear to spin it loose.

I believe that this is why the VW guys weld their 3rd/4th gear hubs when their transaxles are used in high power applications - it helps keep things from coming loose and raining destruction on the gearbox (at least for a while).

[Steve@Rennsport]

Third gear is the weakest one in a 915.

I don't know what tires you run nor how tight your LSD was set up to, but this is not an uncommon failure with a big motor like yours under track conditions. Further, any unloading and reloading of the suspension while under full throttle places high loads on gears and that can exacerbate the problem.

Driver technique also plays a role here.

[flatspin911]

Some updates:

Driver technique does play a role
We try to protect against it best we can!

All the syncros are still intact.

The inside of the mid case has witness marks where the gear teeth sprayed against one side.

Tires: We run Dunlop slicks - 265's
LSD: Not sure exactly how tight set - 80% is a number I remember??

Track situation: The back straight at Mosport is uphill coming from a hairpin in 3rd gear and up through the gears to a downshift to 4th and then to 3rd through the Esses. The front Straight is flat again starting in 3rd. The rest of the track is 4th, 5th - lots of elevation changes including in turns.


So if 3rd gear is the weakest - what shall we use for 3rd gear?
Is stock out of the question?
Can Guard Help?
Gary Fairbanks?
Where to find a solution?

There were some postings of how to strengthen the 915.
Anyone have a link or remember the details?
Don't want to keep rebuilding - wanna put in the right parts and make as bullet proof as possible - oh and work on driver technique too!

[Steve@Rennsport]

Sticky tires, I see,.....

A tight LSD, grippy tires, and wheels off and back on the ground under full throttle is a recipe for transmission failure. Those kinds of shock loads are difficult to guard against besides making certain that the rear suspension has sufficient droop travel to ensure the tires remain on the ground during elevation changes.

If the driver keeps his/her foot more or less planted on the throttle when the car settles back after a bump or over a steep rise, this sort of thing will happen and its very difficult to prevent without lifting a bit. Suspension changes may be necessary.

If your rear suspension is working properly: proper rebound values and sufficient droop, then you may consider replacing the 915 with a G50. This is after all, the main reason why Porsche eventually replaced the 915 in the 3.2 litre cars.

I'd suggest reviewing your suspension setup, as above as well as feathering the throttle when the car's suspension is really unloaded.

[euro911sc]

Those marks are not what I was expecting to find. My case has a good 2.5" long by 1/8" deep gauge next to the 2nd gear cluster. Not my fault it was there when I bought it... no really!

Steeve/Kenik, I thought 2nd was the week gear? I also would expect the half shafts or CVs to fail on over the hill landings... not a drive gear.

Best regards,

Michael

[euro911sc]

The thing that is confusing for me is the OP says everything was normal and it was just before shifting into 4th... I would expect things to explode just after shifting and when you are applying the throttle not under a constant throttle...?

Best regards,

Michael

[kenikh]

In relative terms to a G50, the ideal solution for this application, every gear is weak. It's just that 2nd and 3rd tend to get the vast majority of the full throttle torque, so they get rap for being 'weak'.

[flatspin911]

So if 3rd gear is weakest - who has the strongest 3rd gear?
DO I get one from a 930? G50? Guard? Wevo?

We have gone over this between me (builder) and the driver a number of times.
The car was driven moderatly at the time - not pushing hard.
This reflected in the times - we usually run a 1:35 at Mosport, that day we were running 1:40, 1:42. So I believe him.

The track is mostly 4th gear.
3rd is used in turn 5, 9, 10 (last turn) so all other turns 1,2,3,4,6,7,8, are in 4th gear.

There was no noticeable wheel spin - not pushing that hard.

We have a race coming up so I want to put this back together but not if it breaks again. I do have a stock 3rd gear that I will x ray but perhaps I should get an aftermarket gear?

We do have a cooler in this case- spraybar from Wevo with tilton pump and cooller.

CV's are from the original 74 so they are the large ones with 4 8mm bolts.

Any recommendations on a "STRONG 3rd gear?"

[kenikh]

Albin's out of Australia is probably the #1 provider of aftermarket gears and can make pretty much anything. If there's room, you may be able to get Albin's to cut you a custom set of 3rd gears, perhaps in a stronger steel alloy or with fewer total teeth on both sides (same ratio). This would result in stronger individual teeth, with the downside being fewer teeth to spread the load over - this last suggestion is just wild shot in the dark though.

Straight cut gears may be stronger, too. Some suggest that cryo treating gears also makes them wear better, as well as some suggesting TFL coating the gears (I don't buy it).

[wevoid]

Have you opened the final drive yet and looked at the CWP assembly?

That size motor, a 7:31 and slicks, you might find that the pinion head started to shed pieces into the transmission.

Regards

Hayden

[Matt Monson]

Hello Flatspin,

Our gearsets would be significantly stronger than that stock gear. Not only is that gear 35 years old just in actual years, but metal alloys and heat treating have come a long ways in that time frame. A brand new replacement would be much much stronger.

And because of the huge popularity of the 3liter spec series in the POC and their stock gear requirements, we have recently started producing the stock NT ratio as one of our gears. I don't think Albins even makes the stock 3rd ratio. I know they didn't when they used to cut gears for me. Of course, I've also got a pile of cheap good used if you want to attempt to repair this on a budget, but that's not the strong way to repair it.

I also agree with Hayden that you would want to really closely inspect your 7:31 R/P. It could be done. This may be a more expensive repair than you are currently thinking and just finding a new core to build on may be the more cost effective approach.

If you do need a new ring and pinion, I can help there as well. I've got good used or NOS as a 7:31 as well as our own proprietary version which is 8:35 instead. It's about the same decimal ratio but significantly stronger.

Anyway, drop me a line if there's any way I can help.

Kind Regards,

Matt Monson
Guard Transmission LLC
303-530-1094
gtgears@yahoo.com

[kenikh]

Matt you are saying the right things regarding the new gears! I thought Albin's was still cutting your gears...nice to know there is a domestic producer. Can you shed light on the OEM steel alloy and if not proprietary info, the alloy and heat treatment you use on your gears and R&P? Moly steel I assume? What grain structure do you find wears the best? Again if proprietary info, I'll shut up, but I'm a materials geek, enough so that I minored in materials science many moons ago.

That 8:35 is the bees knees: 8:31 durability with the quick take off of the 7:31. Sweet!

[Matt Monson]

Hello,

First off, I do want to be honest on one thing. We are still making our gears in Australia. Paul Guard stopped cutting gears with Albins 5 or 6 years ago. I had Albins cutting gears for my old company as recently as 18 months ago. While I am patently against negative selling and cutting down another company's product, I will go on the record as saying that I do consider our current gear cutter to be making the best Porsche gears in the world and there are some significant design differences and production protocols that make that the case. If there weren't Paul wouldn't have made the change in factories.

As for proprietary information? I can share some of that stuff with you. Our 915 gears are made from a vacuum processed 9310 steel to an AGMA (American Gear Manufacturer's Association) rating of 9. Most other gear products on the market are rated between 5 and 7. Our GT3 and Cup Car gears carry a rating of 11.

The actual heat treating process is one of those proprietary things that is closely guarded. Believe it or not, but our gears, Albins gears, and Holinger gears (including Porsche Motorsports stuff) are all treated in the same facility. When I toured the facility last year, the owner explained to me that each company had it's own recipe that he followed based on the input he was given from each gearcutter. He was very proud of the fact that he was like a vault and would never disclose Holinger's process to my guy, or the other way around. I personally don't even know the details of our exact process. I just know that the depth to which our hardening goes is unmatched by the competition.

All of our gears are shot peened after hardening. We do believe in this process. They are also reground after hardening. Not everyone does that. It's part of why certain gear companies have gears that are noisier than others (but not all of it. Part of it is tooth profile as well). Straight cut gears will always be noisier than helical gears.

I personally don't believe in cryo treating gears. It's a waste of money.

I do believe in REM finishing, but on gears like ours that are reground after hardening, there aren't substantial gains to be had. On OEM gears and some of the other aftermarket offerings, it makes a huge difference. On all of our pro-level LSDs we REM the center gears; both spiders and side gears. Inside the differential, where there's a lot of friction generated heat, making the gears run smoother and lower friction is something we think has merit. So we REM those parts.

We have recently started making straight cut 915 gears. They are stronger than helical. Currently they are only offered in 2nd and 3rd gears in low ratios where we've seen strength issues in the past. Ratios like 1.63 (19:31) and 1.60 (20:32) we are making straight cut. A stock 1.26 is something we'll likely never do that for. There's just no demand for it. But the rally guys, who run those short thirds do break gears, often in the way Steve describes with a jump or a bump. In Europe there's a lot of vintage guys starting to run our straight 2nd and 3rd gears in historic rally.

I hope that satisfies your inner geek a little bit. If you want more, you should shoot me an email and we can chat offline so we don't sidetrack this thread too far into the weeds. I like to try to respect the sanctity of the original poster and his needs.

Regards,

Matt

[Flieger]

I read an article in Machine Design highlighting crowning gears. Would a slight crown on a straight-cut gear give a little reduction in noise and vibration through center contact, while maintaining the stronger, perpedicular thrust face?

What is REM? A dry film lubricant coating?

[Walt Fricke]

REM is more like polishing: toss the metal piece into some kind of vat filled with some kind of media, turn it on to shake, rock, and roll or whatever it does, come back after a while and remove piece. Produces, it is said, a super smooth surface. Fewer microscopic level peaks and valleys means less friction.

Something like that.

[Matt Monson]

All of our gears are "crowned". What you see here is an NOS QP 4th gear next to one of our own QP 4th gears. I used an NOS gear so that there would be zero wear and you can see the dramatic difference in tooth profile and design.


We do the same thing on the straight cut gears. This one happens to be a Cup Car gear but 915 would be the same. We do it for the durability. Relieving the tip like that, in conjunction with the rest of our tooth profile gives a gear that is unmatched in how much surface area it contacting on the thrust face.

Lastly, this will give you an idea of what they look like after REM treatment. As Walt explained it's an isometric treatment that gives it a surface polish to a very very fine resolution. You could say that it's blueprinting something at a microscopic level.