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TR2/3/3A Type-A Overdrive Handling Differences

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Tropical TR

Jedi Hopeful
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Gentlemen,

As part of my TS52910L rebuild, I'm looking to add an overdrive. I understand there were perhaps three changes to the A-type used from TR3 to TR250/early TR6, that these dealt mainly with the accumulator piston sizes and that piston size affects how smoothly the OD engages? Someone informed me in the past that for that reason many opt for the later version and that the earlier ones, known to be fast and sometimes harsh shifting, are used more for racing now.

However, I am sure many have original earlier OD and some will have added early and late versions. Never having had the experience of an OD either on my '74 TR6 years ago or my current '59 TR3A (when it was running), I would appreciate your feedback concerning how "harsh" the earlier A-types actually are in their shifting. Is there an advantage in getting one of the later ones? This presumes both are serviced and running to spec. Oh, and I will not be racing. The OD would be used for higher speed highway driving.

Regards,

Walter
 
I have the earlier, large accumulator type on my daily driver TR3. Basically, it can be a bit harsh under certain circumstances; so when I'm using it under those circumstances I also use the clutch. By using the clutch, it becomes the same as any other shift. Using the clutch is probably a good idea anyway, as it reduces stress on the OD and likely extends it's life. Broken thrust washers are a common problem and I think that banging around while shifting under load (without the clutch) is part of the reason why.

It's actually possible to convert to the later small accumulator; you just have to add the reducing sleeve and replace the piston and springs. This is often done to save castings with a damaged accumulator bore. Unfortunately the reverse is not true, the later castings are not machined properly to accept the larger piston.

So, my opinion, I wouldn't put any extra effort into locating a later unit just because of the smoother shifts. Even with the smaller accumulator, they will still go "bang" when shifting under light throttle without the clutch (making it still desirable to use the clutch). But, since the early castings are getting a bit scarce, you'll probably find a later one anyway.

As a side comment, I now use an alloy flywheel which also reduces the harshness when shifting under power. But my comments above are based on earlier experience with the heavy stock flywheel; and I still use the clutch to shift into OD under light (or light trailing) throttle.
 
I have an A type in my TR4 and do not find anything harsh about it. Granted, I generally use the clutch when I engage/disengage but even if I opt for that kick-in-the-pants engagement it merely seems an appropriate part of the OD experience.
 
Just for clarity, Geo, I think he is talking about the accumulator size change when the IRS was introduced; not the difference between the A-type and J-type. Your TR4 would have had the large accumulator originally (as would my TR3) but how many cars are still wearing their original overdrives :smile: (Plus the repair technique I mentioned above.)
 
Thanks, gentlemen. Your comments help a lot. Correct me if I am wrong but I believe all of the A-type OD will fit on a TR3 or 3A, and J-type will work only with adaptor plate and other mods?

Walter
 
I have a J Type in my 3A and bought the new rear mounting bracket from Moss. It seemed a lot better than trying to make one myself. Works just great.
 
No practical way to mate a J-type to a TR3 gearbox; you'd have to get the complete gearbox plus OD from a late TR6 (or sedan). All of the overdrives require a special mainshaft that has to match both the OD and the gearset in the main gearbox. As far as I know, no one makes the shaft to mate a J-type to a 3-synchro TR3 gearbox.

Other problems as well, like the rear mount that Chris refers to; plus the speedo drive ratio is different (meaning you need either a ratio adapter or disassemble the OD to change the drive gears). Not impossible, just extra hassle.

I put a J-type in my Stag, and I'm kind of sorry I did. Although the J-type is simpler and more rugged than the A-type, I find it annoyingly slow to shift after becoming accustomed to the instant shifts with the A-type.
 
So sorry I DID NOT mention that I have a late model gearbox and I changed the worm gear to correct the spedo. No doubt someone who knows all will now want the last say.
I was just trying to help..... Sorry!
 
Thanks again, everyone. I have a much better picture now of how to proceed.

Regards,

Walter
 
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