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Testing an A type overdrive


Jedi Trainee
I have the opportunity to purchase an A type OD unit from one of the guys in my club. It was "running when removed" but has been sitting for a number of years unused. It has been drained and fluid was clear with no "chunks". How do I spin it up to test it before I make an offer? I understand you can use a flexible coupling and a 1/2 in drill?? How many RPM and what direction do you spin it in? Any and all advice would be greatly appreciated. I know these boxes are considered "bullitproof", but I do NOT want to make a costly mistake on something as important as this.

Geo Hahn

Country flag
I used an old Maytag motor but most any motor might do including a stout electric drill...


The coupling is just a piece of hose. Direction is the same as engine direction (clockwise when viewed from the front). Haven't tried it on my OD box but see no reason why it would not work.


Great Pumpkin - R.I.P
I'd be worried about torque; it does take some torque to turn the OD & pump and your Maytag motor might not cut it. But probably OK.

Any rpm up to 5000+ should do. I use an 800 rpm 1/2" drill motor. There is a piece of radiator hose in my scrap bin that just fits onto the nose of the drill chuck and the splines on the input shaft, with clamps.

"Running when removed" covers a lot of ground with these units. They frequently will go on working for a long time after a failure like broken thrust washer or whatever. No disrespect to your club member, but I would plan on doing at least some disassembly for inspection. If it does have a broken thrust washer, better to catch it now than to wait until it's ruined the gears and so on.


Luke Skywalker
Country flag
I couldn't spin my rebuilt OD with a 1/2 drill or any other small motor.

Unless you're lucky, I'd suspect you'll need a more powerful motor to do the job the way you'd want, and you'll need some kind of bench to hold the motor and transmission firm.

I know I'm going to generate some disagreement, but won't you be tempted to tear it down, inspect it, and rebuild it anyway? My experience tells me that bench testing will assure the pump, valves, and other essential functions are working, but won't tell you if the bearings need replacement, whether the clutch is in bad shape, whether thrust washers are worn, whether clearances are poor, etc. I'm only saying from my experience, but I'd tend to judge it on appearance, the story and history, gut feel from the owner, market value, the privilege of owning a good one, and use a bench test only as a backup, expecting some necessity for an eventual overhaul or maintenance.


Country flag
I'd never expect a 50 year old tranny to work. I always plan to rebuild them after popping the shifter to check for obvious broken gears etc. , to make sure it is rebuildable.


Luke Skywalker
Country flag
I could spin it with my 3/4 drill but could not get enough speed to operate the OD. The good news, we were so confident that it was perfect (ha, ha), we put it into the car and tested it along with the engine. It worked. I sure would like to be able to test them outside the car. I have a spare electric motor, perhaps like the motor attachment above. Jerry


Jedi Trainee
The full method is given in here:


A cheap 1/3 HP, 250W single-phase motor will JUST do this on a belt drive. When cold you need to slip the belt a bit or the motor over-current detector will cut-in.

A 1/2 HP motor, 400W, motor would do it fine.

You want a run speed of about 1500 rpm or so and set the belt pulleys to get this.

You need to get a pressure gauge because this will show exactly whats going on in there.

The great benefit of doing this rig-testing is that you can FIX LEAKS easily. There WILL be some.
Its worth doing just for this.


Senior Member
Country flag
I use a 9amp,1000 rpm drill to test the a or j type overdrives,I use a liq filled pressure gauge to test,put the trannie in third or 4th.spin her up till you get pressure and manualley press the lever to put in overdrive and hold the lever,your gauge will tell you if its going in overdrive,should have 400 lbs of pressure and when you put it in overdrive the pressue will drop to about 125-150 and then build back up to 400.when you let off the lever it takes a couple of seconds to shift out of overdrive and then you can do it again,and again till you feel its working,when you stop the drill the pressure will drop some ,but will stay and bleed off slowly and then stop between 50-100 lbs and stay there for awhile.hope that helps


Jedi Trainee
9A at 110V is best part of 1000W so this drill will do it for sure.

However driving on this end means you need to be careful not to bend or mark the spigot on the input shaft.

Driving with a pulley on the rear-flange avoids this risk.


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Jedi Trainee
I claim no originality for this. Its all on Buckeye.

I just bought a cheap alloy pulley and put a coulple of holes though and bolted it on.
You can just see em in the first photo. They dont show in the other photo because its actually running!

Best if you get it concentric but belt drives are tolerant.

I am running a two-step pulley on the motor but top speed will only run when its hot.

The description above of how the pressure should vary is exactly correct. You can tell if the accumulator is leaking of the valve not opening right by watching the gauge.

You are fortunate in the US beacuse stuff like pressure gauges is easy on mail-order. Its all illegal over here for the general public so I imported mine from a guy in the Mid-West somewhere.


Jedi Trainee
All the spares for these overdrives are available in the UK from a guy who worked at the original manufacturer.


He is a helpful friendly guy.

Key items to replace are the big circlip and the thrust bearing. These take quite a bashing and do a lot of damage if they break up.

Dismantling one of these is much easier than doing a gearbox. You need a few "special tools" but these can be made by hand.

Buy this if the price is anything like affordable as these are v.hard to get now.


Jedi Trainee
Oh I remeber now. I drilled four holes. You can see em in the pic. But I failed to realise that they are NOT evenly spaced around the flange.

So only two were in the right place. Works fine with just two holes. The pulley had a boss sticking out which I cut off so it mounted flat to the flange.


Marvin Gruber

Country flag
I just bought a couple of TR4a's with a bunch of parts. There are 4 A type overdrive trans. so looks like will be be doing some testing myself. I have a 3/4 drive drill but I don't think it turns but around 800 rpm. Good info.



Senior Member
Country flag
I like your set up,could you give me more details,makes more sense to test from the rear than the front
thanks Philstr6


Great Pumpkin - R.I.P
I just bought a couple of TR4a's with a bunch of parts. There are 4 A type overdrive trans. so looks like will be be doing some testing myself. I have a 3/4 drive drill but I don't think it turns but around 800 rpm.
800 rpm is plenty to see if an A-type works. That's the equivalent of 1600 engine rpm in 2nd gear, so within the normal usage range (just barely). Spinning it faster will cover up any problems with internal leakage.


Jedi Trainee
There is also some good stuff on doing this in the "Greasy-Hands Guides" on the VTR site.
Mine is just based on the Buckeye and VTR articles.

I just had an old packing box about the length of the transmission. Across the front there is a bit of 1in square tube that bridges across the box and is bolted to the gbox flange. You don't need a box like mine. This has been done other ways.

At the back there is a bit of 3 x 3 timber cut so the transmission sits about level.

The motor came off ebay for $30 or so and is mounted on the side of the box. Remember that in O/D you CANNOT reverse.
So you must KNOW FOR SURE that the motor is rotating in the required direction before engaging the O/D.
Motors can usually be rewired to select the direction of rotation.

There is a wooden wedge so you can adjust the belt-tension. This sits on the 3 x 3 and lifts up the O/D tail.
It useful to be able to "tune" the belt tension. When the oil is cold and thick you slacken the belt and this gets the motor spinning.
Then tap the wedge in and gradually pick up the transmission.

Belt drives can be dangerous. Keep your clothing, fingers and beards out of the way!
It would not be too hard to have an accident if you are careless.

It takes about 1/2 and hour to reach a steady oil temperature. Its amusing to start in neutral and then engage 4th, then 3rd.
Having also rebuilt the gbox it was with some anxiety that I did this for the first time.

If you are running above 1000rpm, engaging 2nd is a touch demanding on the syncro and 1st more so.



Jedi Warrior
Country flag
I just bought an extra trans. with an A type OD. I would recommend disassembling&inspecting both units before installing. The main box had a chunk broken&welded near the top, the clutch fork was welded to the shaft, and the constant gear had a chipped tooth. Everything else in the box was in excellent condition, lay shaft&gear was unmarked, all clearances in spec, and the synchro rings were like new. The OD sliding member was worn down to .030" above the rivets and will require replacement ($225). I built a test rig as shown in the Buckeye article and will test the pressure when everything is assembled. I have used the rig before and it is much quieter than using a drill motor. Anyway, buying a used OD trans is a gamble and it is easier to check it before installing. At the very least, the seals and gasket, and orings should be replaced. I am blessed/cursed with a lot of extra trans. parts, so replacing stuff wasn't a big expense. Berry


Darth Vader
I made a flexable coupling out of a water hose and used a 1/2 inch drill, worked fine. Plenty of RPM's for my needs.

Good luck, Tinkerman
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