Overhauling the Toyota/Aisin R151F Transmission
http://www.4x4wire.com/toyota/tech/r151f/ Short Cuts

| Toyota Section | Toyota Tech | Toyota Maintenance |

By: Bill Morgan - March 2004 revised February 2008

There are mysteries in life that hold my fascination. Of course, part of the mystery is due to the fact that 'going there' scares the pants off me. That's why it stays a mystery. But if you are like me, you find one of the great things about four wheeling in general is the opportunity to roll back the dark clouds of mystery and, one of the most mysterious things about how my truck works, at least to me, has been the transmission.

Overhaul kit, plus shifter bushing, seat, and new countershaft nut.

That mystery was about to become an area I understood much better when I located an R151F transmission from a 1986 turbo pickup. This poor R151 had had a rough life, as evidenced by at least two different colors of RTV on the seal surfaces of the cases. But the price was right, and if I am ever to fulfill my dream of swapping a V6 into my truck, I am going to need an R-series transmission! Plus, the advantage of the R151 is the way it mates to a V6 like an R150, but also mates to the geared transfer case (as long as you use a 23 spline input shaft) like a W56, my current tranny. And if I never actually do the V6 conversion, an R151 will be a definite improvement with its lower first gear and stronger design. Besides, my W56, though still serviceable, was going to need an overhaul before long, so why put money into a dead end? Roll back the dark clouds of ignorance, here I come! I bought the R151, and brought it to its new home.

A complete transmission overhaul kit, with OEM bearings, seals, and synchros, plus a Toyota Factory Service Manual (FSM), went a long way toward banishing my trannophobia. A complete transmission overhaul kit can be obtained from Marlin Crawler. Contact information is provided at the end of this article.

Oh, you have trannophobia too? But your gears are grinding and your bearings are whining? Well, that's why I am writing this article, to be of some help. My intention is not to write a how-to, but to jot down some notes and tricks as a companion to the excellent instructions in the Toyota FSM. Almost all of what I will describe applies to any Toyota transmission. So let's roll back some dark clouds!


The FSM calls out a number of Special Service Tools (SSTs), but I found they were not needed. However, before beginning, you WILL need the following tools in addition to a normal assortment of mechanic's tools:

Open it up!

I really hate working on grimey things, and the transmission is one of the grimiest given its location in the vehicle. I used cheap brake cleaner and rags to take the worst goo off (be sure to use good ventilation, and avoid open flames!). Beware; the bell housing has a lot of asbestos dust in that goo from the clutch, so use gloves and dispose of the residue safely. The bell housing bolts were overtorqued, so I needed my teenage son as ballast to be able to unbolt the bell housing. Be sure to observe how the clutch release arm is connected to the throw out bearing and pivot, you will need to reassemble it properly. Taking lots of digital pictures in this process is not overkill, and professional mechanics often use digital cameras to do just this.

Three good shots of the R151F transmission: driver's side, front view, and passenger side.

Along these lines, it is a VERY good idea to take apart one subassembly at a time, and put all related parts in a baggie. Mark the baggie with what the parts are. As you clean up the old parts, work on one baggie at a time, and don't assume you will recognize the parts come reassembly time; you won't. Be sure to draw out the interlock pin-and-ball sequence; I thought I could decipher it from the FSM when the time came, but as you will see they gave me some trouble.

Working On the Mechanism

The FSM recommends putting sacrificial bolts and nuts through the intermediate plate and using a vise to hold the plate by gripping the bolts/nuts. This protects the sealing surfaces of the intermediate plate while allowing the vise to solidly grip the assembly, which is pretty heavy. I did this and it worked very well. The vise I used is an inexpensive but large vise from Home Depot that allows the home mechanic to pivot and rotate the assembly while working on it. I have to tell you, for around $60 that vise is one of my favorite tools.

Optional clips? I don't think so! The overtravel and binding damaged the reverse teeth on the shift hub, the reverse idler gear, and the shift fork. The shift hub should no go this far!

My first thought when I opened the cases was, "Gee, that was not bad at all". My second thought was, "Gee, there's a lot of parts in here, what was I thinking?" Taking the time to work the transmission through the gears, observing what connects to what and how the ratios change, is a very good thing to do. This will not only help you with reassembly, but help you understand what is important in the tranny and how to troubleshoot any problems. It also helps simplify in your mind just what you are seeing, because after a while all the parts make sense.

Additional damage discovered after disassembly.

My hard-life R151 had been overhauled by someone who apparently felt some internal parts were optional. The limit clips were missing for 1st gear engagement, which allowed an overtravel condition, sending the 1st/2nd shift hub into the reverse idler gear. This chipped the tips of the shift hub teeth and burred the reverse idler. Not only that, once the shift hub hit the reverse idler, it had gone too far and was VERY difficult to return to neutral. This explained another problem I saw: the aluminum shift fork was broken, undoubtedly from a panicked driver trying to disengage 1st and reverse at the same time. Ouch.

A call to Marlin about the chipped teeth answered some questions. After hearing my description of the problem, and seeing a picture of it, Marlin felt that dressing the shift hub reverse gear teeth ends with a die grinder or similar would be just fine. That was worth saving in the neighborhood of $150 for a new No.1 shift hub!


Taking the transmission apart was pretty straightforward. The rear output shaft sleeve (coupler) required using a two-arm puller and bolt stuffed inside the coupler to take up space, but came off easily. The bearing separator was handy to use to press off the output bearing, as I awkwardly finagled the gear cluster into the press. The timing gear puller (with metric bolts instead of SAE bolts) was perfect for pulling the 5th gear countershaft shift hub; it is threaded to accept the bolts. Aisin designed this transmission to be easily serviced!

I wondered about the phrase in the FSM: "Engage the gear double meshing." Huh. I didn't know I HAD a Gear Double Meshing, much less how to engage it. Ah, I get it; engage several gears at once by locking two or all three of the shift hubs. That's how you keep the countershaft from rotating as you break the torque on the countershaft nut. By the way, the torque on that thing was considerably more than the 90 ft.-lbs. spec'ed out in the FSM, and I thought I was gonna break something. I had some choice descriptors for the last fella who graced this gearbox.

A Pressing Situation
Simple press jig with the bearing separator made life easy. Jig and gear cluster in the press.

Removing the 1st gear/intermediate bearing/5th gear presented a slight problem. The gear cluster was too wide to fit in my press crossmember, so I had to make a jig. I had some Ό" plate and angle steel sitting around, so I cut two pieces of angle about 16" long and welded them to the plate just far enough apart to clear the gears. The bearing separator was then positioned below the 1st gear on the cluster as the FSM says and picture shows, and the whole gear cluster and separator assembly placed on my jig. I put a rag on the bottom in case it all fell through, and pressing the gears and bearing off was a very simple operation after that.

Lay out all parts carefully! Then bag 'em!

Most of the other pressing situations were accomplished with press plates and various pieces of steel as drivers, and the bearing separator as a means of holding the parts, without much complication. Take your time, make sure you follow the excellent FSM instructions, and don't force anything – you may have forgotten to remove something like a C clip first!

Be sure to lay out all parts in order of disassembly. You don't want unnecessary mystery at this point!

Secrets from the Master

Make sure C clip play is minimal. Verify mainshaft runout is within spec.

Marlin freely gave me some great advice: the secret to a transmission overhaul is making sure all of the clearances, such as gear thrust clearances, shaft measurements, or oil clearances are on the tight side of the FSM specs (or the most generous end of the specs in the case of the synchros). Part of this is controlled by the C clips that hold the pressed-on shift hubs, bearings, and 5th gear in place, and part of this is the synchro ring clearance and synchro wear. Marlin has been known to HAMMER tighter C clips in place, he makes sure they are that tight.

Chances are, your C clips have too much play, and here you get to make a choice: Though Marlin can get you C clips if you need them, he is not really set up to do this and can't afford that kind of time. He will sell you a very expensive assortment which may or may not meet your needs. You really need to order specific thickness clips from the dealer, who most likely does NOT stock them. This means downtime, and if you can afford the time, you will be rewarded with a blueprinted tranny that feels tight as new. It's your decision to make. I could afford the downtime, so I went whole hog and ordered and replaced the C clips as needed. If your dealer is willing you could order a full assortment of all thicknesses ($2.50 or so each) ahead of time and return the ones you don't need, but your dealer may not want to be stuck with them.

Marlin's machined retainer, versus the stamped OEM unit. Much stronger, and tighter. Should I remove the seals? They didn't keep all the muck out! But NO! don't do it.

Marlin also sells an intermediate bearing retainer that is much stouter than the stock Aisin stamped piece, which also ensures minimum slop. I ordered one, and you might want to also, for $35 or so. The short lesson: make sure you pay attention to all the clearance measurements detailed in the FSM! Don't take shortcuts here.

Nasty, dirty No.2 shift hub.

Another question came up: what are sealed bearings doing in an oil bath? Not all of the bearings are sealed, just the input and center bearings. Why? Shouldn't I remove the seals? My curiosity would not settle for "just because, now shut up and get to work". Some folks have removed the seals to get better lubrication of the bearings, but Marlin was adamant: do NOT remove the seals. How does he know? He tried it. They failed far too soon. Marlin figures the seals are not perfect seals and oil does get in, but the seals retain the oil in slow speeds (that's me!) and keep out gear and synchro shavings. My curiosity quenched, I left the seals in, per the master's advice.

What about the gear needle bearings (inside the gears themselves, on the mainshaft)? Brian Cobain and Joe Risavi (good mechanics and very competent 4x4ers at Burt Toyota, Englewood, CO) confirmed they are almost never needed (unless the tranny is run out of oil, in which case you have far bigger problems). Marlin said so too, but he might replace them in a high mileage vehicle and it couldn't hurt. Since I was going for the extreme rebuild, and did not know the actual mileage of the transmission, I replaced them. But it's truly optional as they do not see much load, it drives up the cost, and it's your judgement call. Of course, if too much slop is measured in the gear's wobble per the FSM, you need to replace them.

Clean out all crevices, grooves, and lands. Take it apart and clean it thoroughly! But remember how to put it together!

Obviously cleaning all parts is critical. Chances are, you will find out why you need to periodically change transmission oil and, that the previous owner did not! Get all the goo, grime, and sludge out of all assemblies.

Stupid Synchro Tricks

Gently, lightly lap the synchro with its mating cone.

Marlin does not always replace the synchros, but does 'remanufacture' them (and the details of this process remain shrouded in the clouds of mystery – hey, he's gotta keep SOME things secret!). He has had people request new synchros, and almost always gets complaints when he puts them in, until they break in. Redline MT-90 GL4 synthetic transmission oil really helps with synchro engagement, and when you disassemble the tranny you will see why. The brass synchros grip the gear's cones, which are polished steel, spinning up the shift hubs to match speed as they engage. The inside of the synchros have ridges which displace oil and allow brass-to-steel contact. Put oil in your transmission that lubricates too well (like GL5 gear oil) and… well, you get the crunch effect because the synchros can't grab, they slip.

A mechanic in our local 4x4 club had a suggestion on synchros. Here's an old-school trick to help new synchros break in. Use a little bit of valve lapping compound and LIGHTLY turn the synchros against their mating cones to break any glaze and to remove ONLY the high points on the inside of the synchros. Don't overdo this or you will shorten the synchros lifespan – but doing it lightly truly works. You do NOT want to remove the interior ridges on the synchros; they are critical for operation. You also don't want to remove material as this will reduce the synchro clearance needed for long life. Be sure to wash off any valve lapping compound with solvent and a toothbrush; it will instantly eat your bearings if you don't remove it completely.

The old synchro, with the bright new one. Note the design change.

While on synchros, I might mention that Aisin had a design change on the 3rd, 4th, and 5th gear synchros (the smaller ones). The picture shows the difference in the engagement teeth profiles. This really makes a difference in 3rd/4th/5th engagement! It's worth the price of new synchros to get this change.


Were you careful to bag and mark all the parts like I told you? Now you get to say "thanks!" Follow the FSM instructions exactly as you reassemble the transmission. I used BG Pre-Lube assembly oil, on all parts, to ensure no dry operation. Use the assembly lube of your choice.

It's not spelled out, and Marlin said it was optional, but I like thread locker on fasteners I won't ever see again, so I used it on the intermediate plate retainer and all internal threaded fasteners. Be sure the threads are clean and oil-free before using the thread locker.

Despite my best care, I ended up with a steel ball left over. Not good! Where does that dang thing GO? Now is when your working of the mechanism earlier might pay off. I noticed there was nothing to make reverse and 5th work! I could engage one or the other, but as soon as I did I was there forever. This is not something you want to find out after you install the tranny! Yep, I found the home for the missing ball, it was in the interlock stack and doubled as a 5th/reverse shift rail lock to the actuator. It really helps to know how everything works – you do NOT want leftover parts, and I can assure you there are no optional parts in the thing!

After you make sure everything works as designed, you are ready to put the front and rear cases back on. I am picky about such things, so I had the local auto parts store dip the cases for $20. They came back looking brand new. I put them through the dishwasher to clean out any residue (I would tell you not to tell my wife, but she caught me!) and even coated them with clearcoat (except the inside, and the sealing surfaces).

To drive the seals in, I used the old seals and/or my hardwood dowel since I haven't gotten around to buying a seal driver set. Make sure they are square to the housing, and installed exactly like the FSM details.

I prefer Form in Place Gasket (FIPG) and Toyota RTV products in particular. The orange FIPG is for transaxles and differentials, the black is for locations that could introduce chemicals into the intake and affect your O2 sensor. Remember, when using the orange FIPG the same amount oozes on the inside as oozes on the outside, use only enough to seal. Too much is not better, and can break off on the inside. Just enough is just right.

While you are at it, be sure to replace the hard rubber shifter seat and nylon shift tip locator (bushing). These are inexpensive and are likely a stocked part at your dealer, and will go a long way to ensuring crisp and positive shifts. One other optional thing; I have no use for the hex-head fill and drain bolts, they round off far too easily. I use Toyota part number 90341-18021, which should be about $2 at your dealer. Use new aluminum sealing washers on the transmission.

The Results and The Bottom Line

I won't go into the gyrations of converting from a W56 to an R151 transmission, but I do need to answer your question: how did it turn out?

I was nervous of course, how would it shift? When I first started out there was a clunk and I suddenly had very little clutch. No worries, reduce heart rate, something just needed to seat. I pumped it up and had a clutch again; but man, this first gear is hard to engage! Second too! Third and fourth are a breeze though, from the very first shift.

Calm down, Marlin was right, even with the valve lapping trick, new synchros need a little time. Several days of driving yielded better shifting results every day. Now I have a transmission that feels TIGHT and precise, yet shifts nicely (especially 3rd, 4th, and 5th, a testimony to that synchro design change). One thing that may be responsible for this slow improvement instead of instant performance was my use of BG assembly lube, which takes a while to wash away. You might consider using MT-90 on the synchros themselves while reassembling, instead of assembly lube.

A few days later? Wow, the original tranny never ever felt so good, and even it wasn't bad. Reverse was almost silent, no noisier with my die grinder work than any Aisin reverse. Put 500 miles on it, change the oil with fresh MT-90, and forget it for 30,000 miles. Keep on like that for 200,000 or 300,000 more.

So should I have had Marlin do it, or did I save a bundle? Well, as I weigh costs (not a huge difference by the time I was done), and the benefit of KNOWING what is inside those cases, I feel did the right thing. It might not be the right decision for you though, and no one has ever gone wrong going to Marlin in my opinion. But overhauling a transmission is very possible for anyone who has decent mechanical knowledge, and I now have an essentially new transmission save the gears themselves, which are totally fine. And the knowledge and troubleshooting skills gained? Priceless.

A like-new transmission, and I did it myself; it doesn't get a lot better than that. Not only that, there is a little less mystery in my life!

Related Links: Contacts:
  • Marlin Crawler
    Dept. 4x4Wire
    1543 North Maple Suite B
    Fresno, CA 93703
    Phone: (559) 252-7295

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