Overdrive OOPS ... Oh No!

By Tom Mason

Originally published in the Austin-Healey Magazine, January 1997


Figure 1 - Tools for the job: left to right O rings; pump puller; test shaft and pressure gauge.

Ever had one of those "Oops! Oh no's!”? You are tightening a bolt and something cracks. I cracked the overdrive-to-transmission adapter bracket and I learned another lesson in repair. In fact I think I have my Masters degree in overdrive now. (That means you have screwed up everything that there is to screw up, at least once.)


The photos show the sequence in dismantling and rebuilding an overdrive. Note the long screws used to guide the oil pump back into the housing. If you don't use these, the pump will not go in straight and you won't be able to reinstall the retaining screws. Take the time to get some long fine thread screws todo the job right.


Figure 2 - Hydraulic half dismantled

When reinstalling the overdrive remove the adapter bracket from the transmission. The overdrive won't go on the transmission shaft with it installed because it will be too long. With the adapter bracket on the shaft, guide the overdrive on the shaft and then slide back the overdrive oil pump cam while holding the plunger down. Then reinstall the adapter to the overdrive.


It will draw easily onto the overdrive, if there is any resistance, stop. Think about how hard it will be to find that adapter.

(Hint: It is not in any of the parts manuals from anybody). Once that is bolted to the overdrive make sure the cam is still on the pump, and then work the overdrive right up against the tranny. It will slide easily in place, you may want to spin the driveshaft end.


Figure 3 - Pulling the oil pump

When the adapter and overdrive are coming together they are fighting eight springs. They don't need the shafts and splines in the problem. I will say that Hemphill's was good about supplying this part to me.


You might also want to be careful about jacking up the engine while working on the tranny. If you are not careful you can raise the engine too much and the fan will

foul the radiator and punch a hole in it. I did not do this, but I was only lucky in this case, someone else did hole his radiator. This adds a day and some cost to the project. Jacking up the engine also can strain the throttle linkage so check that too. Last, you will note in the photos that I made a dummy shaft to go into the overdrive like the tranny. I also ground a cam from plate and the pinned it to the shaft. This allows me to spin the shaft with a drill and to check that the overdrive actually works before it is reinstalled on the car. It was not much work on a lathe. Before I reinstalled the drive shaft and other things, I test the overdrive for oil pressure with the gauge that you can see in the photos.


Figure 4 - Mechanical half dismantled.

All the overdrive tools can be easily made, the extractor for the pump is a fine thread bolt and some washers. The overdrive is a unit that you can fix if you are persistent./

I hope my experiences will even save you some grief. Thanks to Fred Crowley for the tip that a broom handle will withdraw an accumulator piston.


Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.



Figure 5 - Gauge installed. Driving the test shaft with a drill will check overdrive. Note: oil must be poured into bottom half of overdrive prior to test.


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