ZF 4th gear breakages

#1
I've got a Roush 427 Windsor stroker 505hp 520ft. lbs. Twice I've broken 4th gear in my RBT, 7 years ago and now. Both times during track days. Once on a downshift, once on upshift. First time all the gear teeth were stripped off, this time just one broken tooth. Reading old posts someone said the input shaft can deflect at high speed and 4th gear is vulnerable being in the middle between bearing supports. Also the weakness of helical cut vs straight cut gears. I realize this is a 50 year old design and perhaps the reason for the T44 development for 500hp engines. Anyone else have this happen especially during track use?
 
#2
Hey Dave,
I remember your MKII from the Cruise Colorado event a few years back....

Sorry to hear about the broken ZF, but this information is good to know. Although I haven't done as many track events as you have, I have about 6 track events in 13 months and 5000 miles so far through my ZF with no issues. Last dyno visit had me at 503tq, however, I'm sure that's not corrected for our atmospheric pressure here (60-78kpa), so the ZF is not seeing the same loads as yours.

FWIW, I run a chip detector in my ZF connected to an acft PTT warning lamp that goes off once the magplug detects enough metal to complete the circuit. An early warning system of sorts....

What's the turnaround time to fix your ZF? I've been seriously considering buying a spare so I can swap quickly if something was to break.
 

Julian

Lifetime Premier Supporter
#4
Surprising to hear, I thought the RBT was designed around the later BMW M1 ZF with additional ribbing etc. What does Lloyd have to say on the failures?

What are you using for a pilot bearing on the input shaft?
 

Mike Drew

Active Member
#5
Lloyd has shown me some destroyed gearboxes, but all of them could be attributed to operator error. Gregg Esakoff has about 750 hp in his race Pantera and his car has a Dash-1 variant which isn't as strong as the Dash-2, never mind the new ones. But Gregg drives a car like 'Animal' from the Muppet Show plays the drums. Slam-shifting it, not using the clutch etc so it's no wonder he blows them up frequently. (He is super-fast and an excellent driver, but extremely hard on machinery).

Some of his other customers use them in dune buggies, and they beat the hell out of them too.

I honestly have never heard of a ZF failing under 'normal' use to include track use. Use the clutch, shift like a gentleman and it will outlive your grandchildren. The fact that you have had two failures of fourth gear suggests to me that perhaps you are being overly aggressive, would you agree?

Reverse is the only somewhat weak link. I broke reverse gear by impatiently jamming a freshly rebuilt gearbox (in my wife's Pantera no less) into reverse when it didn't want to engage smoothly. (She was Not Amused).

Remember that Ford didn't have faith in the original (and admittedly weaker) first-gen ZF gearbox for high-torque applications, so developed the toploader-based T44 four-speed transaxle for use in the 427-powered cars. Those cars are weak as a kitten compared to yours.....
 

Ron McCall

Bronze Supporter
#7
I would tend to agree with Mike. I have built close to 150 ZF transaxles and have never replaced 4th gear in any of them. These include quite a few 650-750hp track only Panteras/Lolas/GT40s. Curious, is it breaking the drive gear , driven gear or both? Who performed the repair after the 1st failure?




Ron
 
#8
I've never broken a gearbox except the ZF. Have driven formula cars with FT200 and Mk9 Hewlands and Staffs boxes hundreds of times. Also synchromesh boxes in Lotus Europas and Datsun 280Z IMSA racers. I never power shift and always use the clutch. I don't know which of the two 4th gears broke. I will try to find out when I get the box out of the car. After the first breakage I bought a new RBT so the current box is not a repaired unit. I have the McLeod clutch with reversed hub that Olthoff sells. To Julian's post my pilot bearing is the bronze bushing Roush supplies. I haven't checked wear on that yet and do wonder if that is a factor. I'm really being honest when I say I don't abuse gearboxes. These 2 ZF breakages are the only ones in my 51 years of driving. I'm not saying ZF failures are common, but here is a ZF gear failure from a post on this forum:
http://www.gt40s.com/forum/attachment.php?attachmentid=18226&d=1173294293
 
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Jac Mac

Active Member
#9
Given your statement above on driving habits I would suggest your problems are due to transaxle housing flex, essentially the housing is twisting lengthwise, this causes the gear pairs to load at either end of the teeth rather than along the full tooth length, in doing so any built in clearance/backlash is removed and the teeth act like pry bars against one another, failure is not immediate but gradual as you continue to drive it, when you strip it down examine the tooth contact pattern carefully under magnification.
 

Julian

Lifetime Premier Supporter
#10
The reason I asked type of pilot bushing is I had shifting problems with a freshly rebuilt ZF, so much so that it went back and was disassembled for inspection with zero findings. I reinstalled with the very same problem, the vendor then asked me the very same bushing question (I was using an oilite bronze bushing), he duly sent the correct needle bearing and I have had zero problems since.

The graphite content of the oilite bushing can vary greatly depending on source.
 
#11
JacMac I think you may be onto something. Here is a photo of the 2 broken teeth. Originally I thought only one tooth broke, but this appears to be the corners of two teeth. The trans would still drive in 4th gear, but I could detect a different sound and stopped the car. Additionally SPF requires that a strengthening fin on the top of the gearbox be cut away to accommodate the cable shifter. See SPF photo on this. Granted the input shaft is on the lower part of the gearbox, but I wonder about case and input shaft flexing too. Any other thoughts on this?
 

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Kelly

Lifetime Premier Supporter
#12
The reason I asked type of pilot bushing is I had shifting problems with a freshly rebuilt ZF, so much so that it went back and was disassembled for inspection with zero findings. I reinstalled with the very same problem, the vendor then asked me the very same bushing question (I was using an oilite bronze bushing), he duly sent the correct needle bearing and I have had zero problems since.

The graphite content of the oilite bushing can vary greatly depending on source.


The bronze versus needle pilot bearing is an old chestnut.Really depends on time exposure and where you think you're operating on the bearingPV curve but will say the journal on the ZF pilot shaft is not suitablehardness to host the needle bearing and would you rather replace a bronze bushingor your pilot shaft? Beyond that, you must have confidence the seals and greasecan be retained in the needle bearing under abuse……but the are many examples ofboth being used very successfully.


The one thing I never seem to see mentioned in the pilotbearing discussion is the use of offset dowels to dial the bell housing and TAshaft assembly to center. The first time I did so was real eye opener for me.If you observe and measure the size (tolerances) of the features and clearances that all stackto center the pilot shaft and then think about the desired bearing clearance ona diameter the size of the pilot shaft, the value of dialing to center becomesapparent. I think many pilot bearing assemblies can become heavily loaded duringassembly.


Best,


Kelly
 
#13
The debate between bronze bushings and needle bearings for crank pilot doesn't seem clear one way or the other. Your point on bellhousing indexing is well taken. Does anyone know how to index a Quicktime ZF bellhousing? Quicktime offers an indexing ring for some of their bellhousings, but not ours? I don't think the inner edge of the bellhousing is smooth enough to get an accurate reading. I've never had a shifting problem though.
On another matter I'm wondering about the lack of support for the rear of the ZF. I believe Panteras support the rear of the case? See the rear support Steve C had fabricated for his SPF. As JacMac said, can the case flex longitudinally?
 

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Mike Drew

Active Member
#14
Dave,

The earliest Panteras only supported the gearbox at the center rear using brackets attached to the studs that hold the cast iron rear plate on.

With rubber motor mounts, there was enough flex to break those studs. Given that the case had provisions to accept side mounts, it didn't take long to abandon the idea of supporting the gearbox from the tiny rear studs, and a beefy pair of side mounts resulted in no further problems.

Remember that original GT40s used not only the bolts holding the gearbox to the bellhousing in a straight-ahead orientation, but also a pair of long bolts splayed at about a 45-degree angle, anchoring the side plates to the bellhousing, and a pair of mounts on the top cover as well. Most GT40 replicas fail to follow suit, which might be a factor.

Having said that, your problem seems rather unusual and unique--you would think that if you were mounting yours like everybody else, then others would have the problem too?
 
#15
Mike, thanks for the Pantera info. The whole reason I originally posted was to see if any others had my problem. Are there other SPF owners tracking their cars with no gearbox problems? I believe 2 others do more track days than me, but they're RHD and presumably don't cut off the strengthening rib on the case as illustrated. As additional info the first breakage was after 11 track days and the second after 9.
 
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