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TR4/4A TR4 A Type Overdrive Part mixup


Senior Member
After finding that mainshaft 3rd lost a tooth into layshaft 2nd and mainshaft 2nd, I decided to have my friend the mechanic overhaul the gearbox and overdrive.
I decided to replace all bearings with the best I could find.
In other words no no-name ones. Bought them from Rimmers, TRF and a local bearing supplier in New Zealand.
The costs are building up.
Upon opening up the the overdrive we found that the rear thrust washer had shattered due to no centre thrust washer (part no. 505548).
My friend said that it would be best to replace the accumulator piston rings (part no.505555) which I ordered from Rimmers.
The two rings were 1 1/4" diameter, my piston requires 4x 1 5/8" rings.
Both TRF and Rimmers list part no. 505555 for an A Type TR4 overdive.
The rings look like they will fit a later Triumph 2.5 overdrive accumulator.

I'm confused.
I'm fairly sure that the box/overdrive is the original factory one.
How many rings should my accumulator piston have?
Do A type overdrives have two or 4 piston rings?

Jim and the 1962 TR4


Country flag

I agree with Phil, the smaller rings are used if you have a smaller accumulator as was put in the IRS models (there is also an aluminum housing with the smaller piston). Part 505555 is for the smaller piston setup. It sounds like you have a larger piston accumulator, found in the earlier cars. TR4A was a transition car, so its possible you had a larger accumulator until they switched to the smaller piston. In Rimmers I think the part number is 500605. I recommend also trying Overdrive Repair Services in the UK, here is a link to their spares page: https://www.overdrive-repairs.co.uk/buy-spares.php?cat=A Type Spare Parts List

They show part number 500605 at 1.75", but also part number JA975 at 1.5". Hopefully yours is one of those. You can also find potential spares from Healey providers if they are closer to you.


Senior Member
Thankyou for yopur help, I was thinking that this question might have been too odd ball to answer.

The serial number is hard to read but I think that it's: 22/1374/017627.
It has a double spring.
At worst we will dress up the rings and put the old one back.


Jedi Warrior
Country flag
Jim, when I bought an A-type OD for my TR3 and had to "rebuild" it, the original steel rings on the accumulator piston looked a bit rough. I was advise to dress them as you mentioned. I decided I did not want to take off too much material. So I bought some real fine "lapping paper" and used that. It worked well. I also buffed the inside of the accumulator bore. I bought a cylinder shaped buffing wheel for my electric hand drill from Harbor Freight and used a really fine compound on the buffing wheel. Again I did not want to take off much material. I tried the "air test" as mentioned in the A-Type rebuild articles on the Buckeye Triumph's web site. I was concerned because I could hear the air around the accumulator piston. As I was bench testing the overdrive, I squirted a bunch of 90 weight GL4 gear oil down around the accumulator piston, and suddenly the air noise stopped and the overdrive shifted great. The overdrive has now been working well after installation in the car. I believe it is possible to use the original steel rings on the accumulator, and, especially if there has been some wear, it is best to use 90 weight gear oil.
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