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TR4/4A A-type overdrive pump


Senior Member
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I have an early TR4 A type laycock overdrive that suffered a pump roller lockup. Roller appeared to bind and flat spotted in several places before completely seizing. I have broken the trans completley down and cleaned and inspected everything. I have found only the pump plunger roller damaged and the cam is discolored from heat but I have replaced both. I have recently reassembled the trans and found a binding when rotating the output shaft at one point. I also found when mating the trans to the overdrive that I could not insert a ss lock wire to pull and hold the pump down as there was not enough room beneath the roller.

I'm thinking that the diameter of the pump roller has been increased or I have a pump roller from another laycock overdrive. I don't remember who I purchased the pump from but I plan to call John at Quantum to see if he has experienced this issue.

At this point I either break down the unit to replace the new pump plunger or install a copper washer beneath the pump body bottom plug.

Any other ideas?

Graham H

Jedi Warrior
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I had the same issue having purchased the pump from Moss UK and it appeared to be poorly made with the piston longer than the original. I filed the end of the plug until there was a small gap with no resistance when the shaft was rotated. I eventually replaced it with a new one from ORS in Sheffield UK, you could try a copper washer to get the clearance you need.



Senior Member
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Very interesting. I messaged John at Quantum about the issue and I'm waiting to hear back before I do anything. I did see ORS does have the pump plunger in stock. I believe I purchased the pump from Rimmer and the cam from ORS due to availability issues.

It is good to know that this is not a one off issue and that you were able to get your overdrive sorted.
Last edited:


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You could try this tool to hold the pump roller down when you have the size issues sorted out.

It seemed to work well.

Pump tool size.jpg
Pump tool s.jpg


Senior Member
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I did and it worked great! I could not get a stainless lock wire under the roller like I did last time so I used a coat hanger and fabricated this tool.

I ordered a new pump plunger from ORS. I haven't heard back from John at Quantum but I think we have identified the issue as a part quality one.
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