Overdrive Tune Up

By Olin Kane

Originally published in the Austin-Healey Magazine, Vol. 28 2a


Ever look at an overdrive unit and wonder what’s in there? Ever wonder what condition it was in even if it works? Tempted to look inside but afraid because it looks complicated? Turns out that an overdrive isn’t all that complicated or difficult to tear apart. You can tear it down almost completely with a few simple hand tools, inspect all moving parts, replace O-rings, gaskets and rear seal and put it back together in a few evenings for less than $40. This is assuming that it doesn’t need anything other than a good cleaning, inspection and replacement of a few inexpensive bits and pieces. If it does need more, you can probably do that too. By the way, several of the more common overdrive maladies can be fixed without even removing it from the car so don’t remove it and dig into it unless you really want to or have to.


My purpose is not so much to tell you how to tear into you overdrive as it is to simply convince you that it’s not that mysterious or hard to do. After having gone through my transmission twice and the overdrive once, I’d have to say that they are about equally complicated but not beyond the owner/mechanic who has a decent set of tools and average mechanical skills. Although the manual identifies several special tools that would make the job easier, none of these are required. Tearing down either the transmission or overdrive is not for the faint of heart, however. These two boxes are about the most complicated gadget on your Healey.


To start, get some documentation. I have the Robert Bentley manual that explains how the overdrive works and how to take it apart. I also have Norman Nock’s Tech Talk which details an eat way to put it back together again (page 39). A couple of parts catalogs with an exploded view of the overdrive are nice also.


The overdrive is divided into three sections easily identified from the outside (going from front to rear):

  1. An adapter which is about 1 inch thick which contains no moving parts and doesn't do anything except provide a way to mount the overdrive to the transmission.

  2. The hydraulic section that contains the hydraulics (pump, pistons, etc.) that power and operate the clutch.

  3. The rear section that contains the clutch and gears.


Separating 2) and 3) is a cast iron ring, the brake ring, which you can also see from the outside. Remove the overrides from the transmission. The first step in tearing down the overdrive is to break it into the three sections described above. A simple ½” wrench is all that is required. First remove the adapter. Remove the four corner nuts first then the two nuts on the long studs. As you unwind these latter two nuts, you are relieving the compressed clutch springs so do one a couple of turns then the other a couple of turns so that the adapter comes off straight. Once off, note that the clutch springs are different lengths and go different places. Make a not of that and clean and bag the spring, nuts and washers.


Next, remove the solenoid to get it out of the way. Then the nuts that hold the two remaining two sections together and separate them.


Important: When separating the rear housing and gears from the main case or hydraulic section, the brake ring stays with the hydraulic section. So split the two section behind the brake ring, no tin front of it.


At this point, you’ll have three chunks of your overdrive sitting in puddles of oil on your work bench. Take them apart as per your Bentley or other manual. Disassemble everything except maybe removing the ball bearing races and the oil pump if you’re sure they’re OK. Go slowly. Take notes and mark parts if the diagrams in the manual aren’t abundantly clear. Note which piston goes in which cylinder, which nut on which stud, etc. It may be several weeks before you put it back together again and little subtleties will quickly be forgotten unless you take notes or pictures. Clean thoroughly, inspect and bag all parts as you go. Put the parts from the three sections in three different boxes.


At this point, you’ll discover one of the overdrive’s weaknesses – steel pistons in aluminum cylinders. If these cylinders have mild scoring, you may be able to clean them up. If the accumulator cylinder is gouged up badly, just replace it and its piston and rings. Examine the bronze thrust washers for excessive wear. Replace the O-rings and if nothing else is needed, you’re ready to start putting it all back together again.


Assemble the hydraulic section and attach the adapter plate to it. Use the sealing material of your choice between the main casing and the brake ring as no gasket goes here but something needs to. If you removed the pump, leave it out for now if you have the pump installed, you need to compress its spring so that its roller will clear the cam when you reinstall. I use a fine grade of aviation safety wire to do this which I cut and pull free once the overdrive is reinstalled.


Stand the transmission up on end by supporting it between a couple of concrete blocks. The third motion shaft should be pointing straight up. Put those two big steel washers and the oil pump cam in place and then attach the adapter plate/hydraulic section. So far, so good.


Next lay the transmission on its side so the third motion shaft is horizontal. Assemble all of the sun wheel assembly stuff inside of the annulus as per the manual. Test the uni-directional clutch by trying to rotate in both directions. Look down into it and align the splines and slide the whole thing onto the third motion shaft. The beveled outer surface of the sun wheel assembly should fit snugly into the cone clutch assembly.


Now flip the transmission back up in the vertical position and install the rear housing after heating it (propane torch works fine) and outer bearing as per Nock. Use sealant on the brake ring surface as before. Initially, there’ll be a 1/8” gap or so between the housing and the brake ring due to the clutch springs. Draw this closed by evenly tightening the housing nuts. At this point, make sure the third motion shaft turns freely. You’re pretty much done now. Install the rear seal and driving flange, oil pump (if you had it out), filter and solenoid and you’re done.


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