Reverse gear failures

Steve Hoelscher

Steve Hoelscher

True Classic
I figured I should probably post this as a public service message. Especially with consideration that I have seen two of these instances in the last few months. And given that this is a known failure that I have been preaching about for more than a couple of decades...

Please don't try to roll start your X1/9 (or any car for that matter) in reverse. Or maybe I shouldn't post this because it would reduce my transmission business ;)

So I tore down a customer core the other evening in preparation for another build. I knew the transmission had a tooth broken off reverse, likely from trying to bump start the car in reverse. But there is no way to know the extent of the damage. I was pretty sure that the input shaft was the one with the missing tooth due to the frequency of the "bang" when backing up.

And to deter you from thinking this is something you might get away with once in an emergency let me advise you that you certainly won't and the results are pretty catastrophic.

This is the input shaft. The missing tooth was attached to the magnet.

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The reverse gear on the input shaft is machined into the shaft. There is no way to replace or repair the shaft. The otherwise perfect input shaft is now expensive scrap metal.

Of course the missing tooth and the bang you hear when backing spread its cancer to the reverse idler gear and the 1-2 slider. Both of which are also now scrap.

As is a typical of this kind of damage the reverse idler shaft was bent, which is also destined for the scrap pile. You can see the contact point at each end of the shaft and the gap down the side of the straight edge. This is a 14mm ('81-'82) transmission.

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I often find the reverse idler shafts bent in transmissions with recent rebuilds and completely shredded reverse gears. Because the person doing the rebuild didn't know to check to see if the shaft was true and if not replace it. The bent shaft then substantially weakens reverse and thus shreds the new parts in very short order.

So this transmission needs; an input shaft, a reverse idler gear and shaft and a 1-2 slider. That's about $500 in parts before I even get to the gear stack, that will likely need a gear or two, a set of synchros, a 3-4 slider and at least a couple of shift forks. But we're not through yet. When that shaft bent the end with the o-ring sits in a boss in the aluminum case. That case boss will not accommodate the bending of the shaft. So the boss is cracked and the case is now scrap. :(

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It is important to note that when evaluating one of these transmissions for rebuild there are 5 major "components" that are necessary; the case, the input shaft, the gear stack, the diff and the shift rails/forks. Diffs and shift rails/linkages are not usually damaged and I have lots of spares/parts for these when they are. Gear stacks are wear items and replacing components there is just part of a typical rebuild. So when evaluating a core for rebuild the for my purposes the critical components are: The case, the input shaft, gear stack. In this case the case and input shaft are unuseable. As is a major component in the gear stack (the 1-2 slider) and the reverse idler. Basically, the core is 75% scrap and it would cost less to buy another core than to buy the replacement parts.

So keep that in mind the next time you select reverse in your X. You'll save us both a lot of headache.
 
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Good to know Steve. Those pictures certainly drive home the point and I will certainly treat reverse gently. What about an instance where you are reversing, let off on the throttle, the engine balks and almost stalls? It's hard to explain but the reverse gear is alternating between being driven by the engine and being driven by the wheels?

Brian
 
Previously mentioned.
https://xwebforums.com/forum/index.php?threads/replaceing-the-tranny.34173/#post-291089

Bump starting an exxe in reverse has a very high risk of bending the reverse idler shaft, chipping a gear tooth, cracking the reverse idler shaft in the case or all. No idea where the idea of bump starting in reverse came from, but is a good way to destroy the reverse gear in these gear boxes. The other detrimental action to the reverse gear is slamming into reverse while the gear box internals are still rotating. There has has been previous discussion about this and rounded reverse gear teeth being OK. This might be true for other gear boxes, in the case of the exxe, shifting into reverse while the internals are still rotating is just not a good thing for the gears involved.

*Have a few input shafts that are scrap due to chipped or broken reverse gear teeth and scrap cases with cracked reverse idler shaft bosses (will try to get these repaired, but the process is involved due to oil soaked aluminum. Essentially a possible repair involved grinding out the cracked idler shaft area, baking and cleaning out the oil contamination, weld up the area as needed, then setting the case up in a milling machine then precisely re-bore that 16mm idler shaft boss.

The reverse gear is softer than the reverse gear on the input shaft. Possible the intent of the design is to impose wear on the reverse gear instead of the input shaft gear, in real world running and operating conditions, have seen more than a few input shafts with a destroyed reverse gear on the input shaft with a destroyed reverse gear. Destroyed being more than just rounded gear teeth on the engagement end, destroyed means missing teeth on the reverse idler. Adding to this a cracked case. Any wonder why the gear box jumps out of reverse or not engage reverse at all?

Reminder again, good quality OEM Fiat parts for these gear boxes are becoming more and more difficult to get and pricy. Do use the very best gear oil possible, Do treat reverse gear with care and consideration when shifting into reverse. DO NOT BUMP START IN REVERSE...
Do NOT shift faster than the syncros can work by applying increased force. This will not make the shifts faster, it will only destroy those precious gear box parts.


Bernice
 
With all this discussion about all the stuff that can go wrong and die in these gear boxes, it could easily leave the impression they are fragile.
Not really true. We have 24 Hours Lemons raced the 5 speed gear box for years with few problems until the power and traction was significantly increased. At which point third gear was digested. Perviously to this, the gear boxes were very problem free and reliable.

In comparison, the current PP rotary powered has a habit of digesting some or most of the Porsche 901 gear box per event. Granted the power levels are higher, but the running cost of these Porsche gear boxes is not a good value. Hint, 1st-2nd slider is $500.. and this is just the beginning.


Bernice
 
beezee said:
What about an instance where you are reversing, let off on the throttle, the engine balks and almost stalls? It's hard to explain but the reverse gear is alternating between being driven by the engine and being driven by the wheels?
Click to expand...

Good question. And I will give my standard answer. Use reverse as little as possible. Never back the X up hill. Ever. If you are parking the car and the space points downhill, back the car in. Hold the shifter in position when using reverse. When backing, never fully engage the clutch. Instead, raise the rpm and feed in clutch until the car moves but let the clutch slip instead of fully engaging. Yes, I know this is counter-intuitive but clutches are easier and cheaper to replace than reverse.

Bernice is correct in her assessment however, I am sure the reason for reverse on the input shaft being harder is the input shaft is "hardened" for durability and there really isn't any need for the idler. Also, the reverse gear on the input shaft is only about 1/3rd the diameter of the idler gear, which itself is half the diameter of the 1-2 slider. Thus the teeth on the input shaft have the least amount of engagement and are therefore the most heavily loaded.

Rod, a reverse that jumps out can be a range of problems, only one of which is the idler shaft is bent. The teeth on the idler and its mates (input and 1-2 slider) can get chewed up to the point where there isn't sufficient engagement to hold it in gear under load. And every time it pops out it chews the gears up that much more. I will see if I can find an old reverse idler that has this damage for a photo. I don't have one right now. Typically, a bent idler shaft reduces the idler gear's engagement depth and thus the teeth run on just the outer edges. This greatly increases the point load at the end of the teeth and causes them to break.

And thanks Paul. :)
 
Thx Steve for your amazing insights, especially parts/quality info...priceless.
Seems to my limited exposure that reverse idler is near always chewed up.
Wondered if design is as "sacrificial lamb"?

Pics of 4 idlers, in 3 stages of wear, & a NOS gear for your commentary.
Sorry for poor images.
Thx again for insights, real glad to see you more active on Xweb!

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olerascal said:
Thx Steve for your amazing insights, especially parts/quality info...priceless.
Seems to my limited exposure that reverse idler is near always chewed up.
Wondered if design is as "sacrificial lamb"?

Pics of 4 idlers, in 3 stages of wear, & a NOS gear for your commentary.
Sorry for poor images.
Thx again for insights, real glad to see you more active on Xweb!
Click to expand...

All are typical of what I find in the average rebuild. Its rare I find them better than these.

This reverse idler and 1-2 slider came out of a recent rebuild and is typical of what I find.

Reverse idler.

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1-2 slider. The teeth on the outside of the gear is the reverse component on the output shaft.

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This and the pictures you posted above are typical of the most common reverse failures. It starts when the driver engages reverse cold. That is; without first engaging a forward gear like 1st or 2nd. Reverse is not synchronized so selecting it cold causes a slight grind and that starts the process of chewing up the teeth as you see in the above photo. The reason for engaging 1st or 2nd before reverse is to use the synchros on 1st or 2nd to stop the rotation of the input shaft. Then select reverse (without cycling the clutch) and it won't grind.

The photos you posted just show the stages of wear and damage and its progression. If it gets bad enough and starts popping out the driver is tempted to hold it in reverse and yet it still pops out. The result is the idler acts as the bit on a lathe to cut the teeth right off the three gears. I have found reverse idlers and 1-2 sliders that only had about 1/8th of an inch of the teeth left all the way at the opposite side.

I have also seen the reverse idler, sometimes the 1-2 slider, and input shaft gear, worn down from the outside. As if reverse was being driven solely by friction instead of the gear teeth. This is the result of a severely bent idler shaft. I wish I had more examples but I have to throw the scrap away. Maybe I'll do more photo documentation in the future.
 
As memory serves, these types of reverse gear issues are common for many transaxles of that era, regardless of origin. The lack of a syncro and the location of the components within the box were often the same.

All this talk of the X trans and what not to do (e.g never reverse uphill) reminds me of the old Ferrari days. The conventional wisdom was to never utilize second gear for the first 20-30 minutes of driving. The box needed to get fully warmed up for the 2nd syncro to work, otherwise no matter how you tried to shift you'd get nothing but grinding. Problem was the cars were often only driven for short distances, so 1st to 3rd was the norm.
 
I do a lot of work with Porsches too. Older Porsche transmissions have the same "Porsche Type" synchros that the X uses on 3rd, 4th and 5th. You have to baby those, be graceful with your rev-matching and always wait for the transmission to warm up.
 
Steve Hoelscher said:
Use reverse as little as possible. Never back the X up hill. Ever. If you are parking the car and the space points downhill, back the car in. Hold the shifter in position when using reverse. When backing, never fully engage the clutch. Instead, raise the rpm and feed in clutch until the car moves but let the clutch slip instead of fully engaging.
Click to expand...

All this talk about reverse being fragile and what not to do in reverse gear made me think of this video.

 
Hey steve -folks, this is my first post but seeing this as a new member is making me cringe, as this has happened to me, not from my own doing, but that's another story... Just a ballpark best to worst case, how much best case worst case would you say a typical shop would charge to repair the damage done.
 
Wow, that video makes me cringe. :eek: Clearly the driver isn't the one who will be making the repairs. ;)

mr80X19 said:
Hey steve -folks, this is my first post but seeing this as a new member is making me cringe, as this has happened to me, not from my own doing, but that's another story... Just a ballpark best to worst case, how much best case worst case would you say a typical shop would charge to repair the damage done.
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Well, I am currently building these transmissions for $1750, plush shipping and a $350 core charge (assuming the core is usable it is refunded). This is given the current parts availability

If I make some assumptions about the job I can give you a range of pricing. Assuming to drop the car off at a shop that actually knows something about X1/9s and is competent to do the work; R&R (remove and re-install) labor can run $800~$1000+ depending on the local economy (obviously labor rates vary from places like major California cities to a small town with a low cost of living). Assuming that shop also did the rebuild figure $1000+ in parts, plus labor at $500~$1000. I would think that a reasonable rate for the job would be $2500 to $3500 depending on the parts needed and the local overhead. Parts costs could easily run the price up if the transmission needed more parts than just reverse and I am betting it will. That's pretty much the way I would price it out to a customer and while it that number sounds really high it is legit.

These days I would expect that most shops would either reject the job or price it very high to make sure they cover their butt's if the customer says yes and they find themselves in over their heads. I have bailed out a few shops over the years that found themselves unable to find parts or fix a problem and had to send me the unit to fix.
 
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