I have just converted my HZJ 105 Land Cruiser from the notoriously weak R151 gearbox to the H151 box and thought I would post the details. There is a lot of good info on the subject on the Net but a lot of rubbish from armchair experts as
well. I spent a lot of time researching so hopefully you won't have to. I have photos of the whole exercise but this
forum wont let me post them, or I don't know how to do it.
If you tow a lot or have an aftermarket turbo fitted to your 1HZ the box will not last long. I know through sad experience. Even if you get yours rebuilt experts say you might only get 30,000 to 50,000k out of it and less if you are hard on it.
Materials:
1x gearbox (with gear lever) and transfer case from 78 or 79 series TURBO land cruiser (very important to get one from a turbo model as the non turbo 78 79 series has the same gearbox as you already have).
Mine was from a 1HD FTE engine.
1x transfer case lever and linkage (also the associated bolts and bushes that hold it on) from 80 series
1 x clutch plate for FZJ 75 series Land Cruiser (not HZJ 75)
1 x 15 x 40mm sealed bearing (spigot). NACHI 15BC04S18SSU
The advantage in buying a 78 or 79 series gearbox and transfer is that the transfer case is already PART TIME 4WD like the one you have. If you get one from a petrol or GXL 100 series, most likely it will be a FULL TIME 4WD transfer case and you will have to modify it to part time at a cost of around $550 parts only.
Another advantage over an 80 series box is you can pick up a low km late model box from a 78 79 series in great shape. I did not want to go through all this trouble and find the old box out of an 80 series was worn.
I purchased a low km 2006 78 series H151 gearbox and transfer case from a TURBO model. The bell housing was the same so no need to swap. It bolts directly to the 1HZ. The input gearbox shaft on the H151 is a larger diameter than the R151 and has a larger pilot shaft (the tip of the gearbox input shaft that locates into the spigot bearing when assembled) so you need to replace the pilot/spigot bearing on the end of the crankshaft. The one in it for the R151 is 12mm inside diameter and 40mm outside. You need a 15mm inside and 40 outside (NACHI 15BC04S18SSU).
The pilot shaft is a little longer on the H151 so I cut 5mm off the tip with an angle grinder (1mm fiber blade) to prevent it from hitting the crankshaft. You also need to chamfer the tip so it will locate easily into the bearing (google 'how to remove spigot bearing using grease' to show you how to get it out easily) . I think it would have cleared by 2mm but wanted to be safe so I cut the tip off.
You need to replace both gearbox and transfer as the output shaft from the H151 is to big for the input spline socket on the transfer case from the R151.
Clutch.
The shaft on the H151 box is a larger diameter and fewer teeth than the R151 so you have to replace the clutch plate.
There are two options. 1. Try and find (good luck) a flywheel from a factory turbo 1HDT and put it on in place of your old one (it will bolt straight on) and install a new clutch and pressure plate for a 1HDT (expensive). 2. keep your existing flywheel (have it resurfaced if it has grooves or is uneven), Keep your pressure plate if it is in good condition and purchase a clutch plate for a FZJ75 Land Cruiser (14 teeth, 275mm diameter). This clutch plate will slide right onto the H151 shaft and mate perfectly with your original 275mm pressure plate. If your pressure plate is worn, you will need to replace it with one for a HZJ105 (same as the original). I would also replace the thrust bearing (for 78 series TURBO Land Cruiser this time as you have changed the box) and rear engine oil seal while you are at it.
If you can get the 1HDT flywheel and clutch it is the better option as it is a larger diameter 300mm vs 275mm and so is better able to handle high loads.
Transfer case linkage
The lever from the 78 79 series is located on the side of the gearbox where yours is located on top and to the rear of the gearbox (so it won't work). The solution is simple. The transfer lever and linkage from an 80 series will fit onto the new H151 gearbox in exactly the right position (ask for the bolts and little bushes that go with it as
well). The top of the H151 box has a place already for you to move the lever to. The transfer lever linkage will fit directly onto the new transfer case now.
You will need to alter switch positions on the gearbox and transfer case. I just move them to match where the existing ones were on the old transfer case. It is hard to describe without pictures (which I have of the whole process but this
forum won't allow me to post pictures here). Put simply, look at your old transfer case and move the switches on the new case to the same positions as the old case. It is easy as the cases are the same to look at. Use your existing wiring loom and plug it into the same
places as the old one. because the new box is twice the size, a couple of wires on top of the box are a little short and you may have to extend them.
Mine were Ok, but tight.
You will need to transfer the cross member rubber mount from the R151 box to the H151 so it will fit the cross member.
I installed the box first and then the transfer case as it was to heavy to do both at once. Also, the new box is MUCH bigger and you may think as I did at first that it won't fit, but it does, you have to wriggle and jiggle it into position, but in the end it fits in fine.
Drive shafts fit perfectly with no mods.
Gear lever form your old box will not fit the new one so make sure you get one to suit the H151 box.
Make sure you get the right oil for the new box and transfer. Ask at you local auto store. It is not the same as the old 80 series box oil. It's an expensive full synthetic and takes 4litres Box and Transfer combined.
I now have a fantastic strong, easy shifting gearbox that I have full confidence in. The only drawback is I have gone from 2400RPM 5th gear at 100 Kmh to 2500Rpm. 1st 2nd and 3rd are similar and I actually like them better.
Good Luck, it is
well worth the effort.