These aren’t particularly structured; they’re actual questions being asked — not that we post every question we get, either:

Q: I see how you’ve stressed the importance of leaving well enough alone concerning trying to rebuild the internals of the transmission.  What would it take, in your opinion, for someone or some group to become proficient at complete restoration of the transmission?  Do you think it’s even possible?

A: I can’t answer the first question exactly, only approximately. As to the second question, the answer is “yes.”

“Complete” needs to be defined. Assuming all the parts were available new from ZF, then the cost would probably be prohibitively expensive as to parts, and the person assembling the transmission would have to assemble everything, which would also make it prohibitively expensive as to labor, too. I don’t know of anyone who has ever done this. Perhaps  the only such individuals are the engineers who designed and built the first few transmissions. Even at the ZF factory, I’d be amazed if they didn’t use automated or robotic assembly to some extent, plus a production-line approach for the manual labor. So for practical purposes, at most a few people on the planet have probably assembled one of these from core components, However, it’s not a skill set I consider necessary because I can’t imagine anyone ever trying this, nor even being able to, because of the impossibility of getting the new parts so as to attempt it. At best this 100% approach provides an interesting high-water mark for conceptualization.

At a much more practical level, the next tier up, as to realism, is probably when it comes to parts that are still commercially available for that transmission, assuming ZF is the only viable source — and I like working on that assumption. That’s a much smaller subset, and knowing how to to replace an unobtainable part isn’t useful anyway.  As for the parts that are available, trying to replace them all might involve disturbing and possibly worsening parts that are no longer available. So, that’s not a good approach and for me to hypothesize as to how hard it’d be isn’t probably useful either.

The next tier up, as to realism, is when it comes down to being able to source a part from ZF but deciding not to, on the premise of “even if this part doesn’t seem to need replacement we can replace it, so let’s do that.” This runs afoul of my guideline of “unless there’s specific reason to deviate, better to leave things undisturbed as ZF last touched them back at the factory.”

The next tier up, as to realism, is when I follow my own guideline: I replace only what my technical advisers deem prudent in general plus on a case-by-case basis.

Starting with prior experience in working with automobile mechanics and with ZF automatic transmissions in particular, I’ve spent approximately three years to reach my current level of skill.  That’s not at 40 hours a week but still, I have been focused on these 5HP-24A transmissions intensely: studying them, in books and manuals, plus by dismantling them and analyzing their behavior (and misbehavior) when installed in cars.

One of the books I studied is the ZF spare parts catalog, showing the exploded views and parts lists. Another book I studied is the formal repair manual for the 5HP-24 (there is none for the 24A). The assembly process is very complex, and special tools make it viable or possible, but not all of those are available from ZF any more, nor are they all realistically affordable even if available.

Here’s one example: all of the microscopic variations in the thicknesses of the various clutches and drums have an accumulated affect that has to be compensated for, by adding the right thickness of shim before installing the oil pump> Hence, there’s the need for very precise measurement, using a high-quality dial indicator. We bought one of good quality, and it still wasn’t up to the task so we bought a high-quality one.  There are many more examples of subtleties to be managed. Personally, I’m confident up to the “C” clutch. I also trust myself to work beyond that, but I’m not easily confident beyond that point.

As advisers, I’ve befriended several professionals who specialize in this type of transmission, so I don’t feel like I’m thrashing or guessing. Even so, dismantling, fixing and reassembling these transmissions is a complex, high-precision project. Methodical as I am when I work, I nevertheless always wonder what step or subtlety I might be missing.  So while I believe I can do a good job of disassembling, fixing and reassembling the internals all the way to the back of the case, my cautious approach precludes me from classifying even myself as proficient.

The subtleties aren’t just technical. They’re also economical. I should replace enough of the parts so the customer has a good experience; a transmission that lasts through the warranty period and ideally far beyond — but also while running the business in a way that’s profitable. It’s a balancing act.

To one of my advisers, I have recently expressed an interest to attend a formal ZF overhaul school, and he opined that by now, I’m at a skill level where I probably don’t need to go to that school.

Will I ever get to where, by my standards, I’m proficient? I expect so, yes. However, at the current rate of learning it’ll probably take me another three or four years.  So, that answers your first question.

For additional context: I’m very detail-oriented and I’m probably somewhere on the autism scale, so I learn at a different pace than typical people. One difference I observe is that many typical people exude much more confidence than I do, yet when I learn more about their skill level, it turns out that they know less than I do.

Q: [In reference to the 30 points where a service can create problems, so approaching with the “can’t hurt” mindset is overly optimistic.] What 30 points does this refer to? Are they listed within this article? Is this a separate list?

A: When talking about the BMW variation of these transmissions (the ZF 5HP-24) it’s useful to also bring in experience from the Audi Quattro version (the ZF 5HP-24A) which is basically the same transmission.

When I just got started working on these transmissions, I bought a badly abused 2000 Audi A6 Quattro 4.2 V8 in which the person who’d worked on it had made about a dozen mistakes, specific to the transmission, and readily apparent. As I worked on that transmission some more, I added to the list, and then subsequently over the years, I observed more and more.

The amount of “30” is an informal estimate.  I was surprised at how high the number is.

I have a dead car over my pit right now, from one more mistake that an experienced tech had made. This is on one of my own cars, with this type of transmission. The car had been fitted with an awkwardly-positioned oxygen sensor that precluded the transmission oil pan from being removed, unless the oxygen sensor was removed first. This was done, and the transmission service was done. Then, the tech (being rushed) screwed in the oxygen sensor finger-tight, and omitted to mention that in the hand-over report.

The next tech took over, didn’t notice or check the oxygen sensor, and it finally fell out at during a high speed test drive, banging itself into oblivion against the road and the bottom of the car, all the while being powered and shorting out. It’s an obscure mistake but … it happened. It’s one of the 30-or-so items on the list.