I've been taking some time to research and study the design of the H150F/H151F gearboxes, and have a close look at the specific changes that occurred over the model years. My goal here is to better understand the advantages and disadvantages of each change, so I can determine what the "best" version of this box would be, and build my own up to that spec.
First of all, some fundamentals. It's impossible to evaluate these transmissions without understanding the physics of how they operate, and the engineering principles behind them. I strongly recommend watching this short 3 minute video, which gives a great visual primer:
And optionally watch this longer one for a real-world example and some info about wear:
And if you want to get really nerdy, here's a journal paper which goes into some more detail:
For reference below, here's a simple named overview of a pair of gears synchronized by a single blocker/synchro ring between the hub and each gear, with the same design Toyota used in the H150/H151:
To break things down significantly, IMO the best revisions of these boxes are manufactured from 1992/08 to 1996/08. I'll explain why.
In the beginning, all the the hub sleeves in the H150/H151F had symmetrical, straight feed dog teeth, like this:
The same is true for the teeth on the synchro/blocker rings, and the teeth on the synchroniser cone attached to the gear. This is the classical design for this kind of synchro going back 100 years, and all H150/H151 gears used it in the first iteration in 1990/01.
In 1992/08 Toyota engineers made a tweak. You see, symmetrical teeth give the same experience shifting up or down, or in other words, when the gears needs to be slowed down to match the output shaft (shifting up, say from 1st to 2nd), and the opposite when the gears need to be sped up (IE, shifting down from 2nd to 1st). This isn't what's needed in practice though. In reality, people can, and do, shift straight from 1st to 2nd very quickly with just a flick, to get quick acceleration, and to a lesser degree from 2nd to 3rd. In reality people don't downshift as quickly or as often. Due to gearing as well, the lower gears need to deal with a much bigger mismatch of rotational speeds than the higher gears, because the difference in ratios is more extreme.
I suspect the first gen gearboxes were more prone to grinding gears on very quick shifts, which occurs when the synchro/blocker ring fails to match the gear speed to the hub speed quickly enough, and also fails to "block" the shift sleeve from passing the teeth on the synchro and allows it to grind against the teeth on the synchroniser cone attached to the gear. To address this Toyota tweaked just the 2nd and 3rd gears, adding an angle offset to the teeth on the hub sleeve facing 2nd and 3rd, and the corresponding teeth on the synchro rings and the gears for 2nd and third, like this:
This angle change improved two things. Firstly, the angle reduced the depth of the teeth on the blocker/synchro ring, giving more time for pre-synchronisation to occur prior to teeth contact, which would reduce wear over time due to less blocking action. Secondly, the more aggressive angle gave a stronger blocking action on upshift, greatly reducing the chances of grinding occurring on upshift, at the expense of slightly weaker blocking action on downshift, which is a good trade-off for how people typically shift the gears. All in all, this was definitely an improvement.
Toyota also made a change on the H150F box in 1992/08, going from a single synchro ring to a triple ring setup, like this:
The three peice rings are better. The way they work is the steel ring in the middle locks into grooves cut into the synchroniser cone attached to the gear, so that it moves with it. This increases the surface area for the synchro rings to operate on, as instead of just the outside surface of the synchroniser cone mounted to the gear, you can now brake against the inside and outside of the steel ring too, roughly tripling your braking power. The H151 came with triple synchros on 1st, 2nd and 3rd from the start. The H150 added it for 2nd, and 2nd only, in 1992.
From here, the boxes stayed the same effectively, until 1996/08. What happened at this point, is that Toyota started making the boxes cheaper to manufacture, but not better. Worse in fact, and it started with what they did to the gears. This was due to tech changes from the 90 series "light duty" Landcruiser Prado, which had been released just 3 months prior, and an understandable desire from Toyota to consolidate their production processes.
First of all, understand that the gears in the H150/H151 are not a single peice. They're technically a gear assembly, consisting of the gear itself, and the synchroniser cone pressed onto the gear. You can separate them, like I've done here for the 5th gear using a press:
This two peice design is the traditional way to do it, and other manufacturers sometimes offer the synchroniser cones as a separately orderable part, which makes sense, since very often your gears themselves are immaculate after a decade or more, while your synchroniser cone may be glazed or grooved, and the teeth worn. Toyota don't offer this part separately unfortunately, but they are manufactured separately. In an attempt to reduce costs and consolidate production, they brought over a machining change from the Prado line about how they made these synchro cones. Earlier versions looked like this:
If you look, you'll see they appear to be forged, and they have an inset, reducing weight and allowing for a deeper cone with more friction area. With the developments on the prado though, Toyota wanted to simplify the machining of the synchro cones and the synchro rings, so they changed the cone design to this:
If you look closely, you'll see the significant differences with the synchroniser cone. While the original had a recessed top surface above the dog teeth, this one has a flat surface. The cone itself is also shallower, with a corresponding loss of depth in the synchroniser rings that fit with it. Although the cone itself is very slightly a wider diameter, there's still a net loss of surface area. The manufacturing process also appears to have changed, switching from a forged part to a cast part, reducing costs, but also lowering the strength of the material, which will lead to it wearing more quickly. Basically, it's a loss all around.
Toyota didn't stop there though. Shortly afterwards, they also revised the 1st and reverse gears, switching from separate gear and synchroniser cones to a single-piece (cast?) gear with an integrated synchroniser cone, like this:
This reduces costs for Toyota, going from a 5 step process involving two separate pieces that need to be formed, tooled, and then combined, to a 2 step process with one piece. There are no mechanical advantages to this though, quite the opposite. It's now impossible to swap or replace the gear and synchroniser cone separately. Additionally, there was a recess with void space between the two in the two piece design, which reduced weight. The single piece gears are heavier, increasing the load on all the synchroniser rings, effectively making the synchro rings wear faster and become less effective.
They also made changes to some of the hub sleeves, cutting away some of the teeth to consolidate parts/tooling to with their new Prado gearboxes, which had a tweak to the way the synchro rings interlocked with the hubs. You can see the missing teeth here:
There should be three cutaways with a tooth in the middle for the hub keys, but in this design there are six, with an extra 9 teeth removed from the sleeve. This offers no advantages to the H150/H151, especially because the synchro rings didn't even change to match the sleeve change, so there's simply less tooth engagement, as you can see here if you look closely:
Since the teeth on the hub sleeve are the only thing that transfers power from the gear to the output shaft, removing these teeth makes the box weaker, and puts more load on the remaining teeth, accelerating wear. It's a loss all round.
The same is true of all other changes to the original H150/H151 boxes for the 80 series, there are no benefits to any of the later changes around the shifting system, its all about tooling and parts consolidation towards the end of 80 series production.
In terms of parts interchange, there are actually a lot of options. The rules are basically this - if you're switching from a single synchro to a triple synchro, you need to switch the synchro cones. If you're switching between straight and angled feed dogs, you need to switch the synchro cones, rings, and hub sleeves.
The "ideal" thing for a 1HZ if you want to keep the shorter 1st gear ratio, is to take a H151F from 1992/08 - 1996/07, and use a press to remove the triple synchro cone from the taller 1st gear in the H151 and press it on the gear from the H150. You can then swap the counter gear, 2nd gear and and the input shaft from the H150 to the H151, or you can swap the 3rd and 2nd gears and synchros from the H151 into the H150. That gives you triple synchros on the first three gears.
Anyway, I'll leave this post here for now. I've got more to write about the revisions of this box in later Landcruisers, such as the H152 in the 70 series, and related changes in synchros, and quite a bit to write about bearings. I'll post that info sometime soon.
