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The_Doc

A-Type vs J-Type Overdrive on a TR4A

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Hello (again) All

 

I have a choice between putting in a J-Type or A-Type overdrive in my restoration project. I was just wondering what peoples thoughts on are this are. I know the A-Type is the correct OD, but is the J-Type a better unit to have? I will keep both units with the car so the next owner (if I ever do sell it!) can change it if they so wish.

 

I have heard that the J-type is the better one to have in your car and use; the A-type can be slower to engage/disengage as well as being more tempremental. Of course, the J-type will give a higher drop in RPM, but I'm guessing going from 22% to 28% doesn't make too big a difference.

 

Any help would be greatly appriciated as I feel I'm in the middle quite a dilemmia.

 

Thanks in advance!

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I have run both types for normal road use and favour the J type as it has a much smoother engage/disengage action with less stress on the rest of the transmission ( I never use the clutch with overdrive operation just adjust foot on gas pedal ) O/D 4th for cruising and 3rd for up and down the mountains.

A type has cross operating shaft (solenoid side to lever side) prone to oil leaks either end and needs setting up. Requires a relay in electrics to avoid heavy current thru operating switches and the solenoid with its two coils and internal switch is more complicated and more prone to failure than the single coil low current draw J type.

J type has rectangular sump plate and gasket which is less likely to leak than the brass screw plug under the A. If you fit the J from a saloon you have to re drill the holes in the rear flange and m/c the locating shoulder for the prop shaft connection. I have a 3a with a J type ex saloon car and bought a mounting kit from Rimmers.

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I fitted a saloon J type to my TR4a using a hybrid shaft fitted into my original 4a box new bearings and seals everything else was in good condition. Made my own rear mounting plate working from photographs of the ones for sale from moss and Rimmers bought an engine mounting for a land rover same as mounting on conversion. Rear flange needed redrilling as holes didn't match TR propshaft and output flanges weren;t interchangable due to splines on output shaft to flange differences. It is a 28% overdive and goes in smoothly, as you say it isn't original but it works and it is about using it rather than keeping it factory, we would have no filler and be painted in cellulose, and all the other things like unleaded petrol.

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Very interesting to see the comments about a saloon J-type fitting. I have a saloon box with J-type OD in the shed and plan to install it on the TR4. I do not mind making my own mountings but if there are mounting kits already available.

 

Peter - what do you mean by "m/c the locating shoulder" ?

 

As for the drilling of the rear flange - can the flange be removed from the gearbox for the drilling?

 

I do not mind making my own mountings but if there are mounting kits already available.

Sean - do I read correctly that a Land Rover mounting can be used?

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There are a number of things to think about when fitting a saloon box to a TR

 

Rear mounting

Clutch type (input shaft to gearbox has different splines)

box is bit longer, so mountings have to move

Speedo drive is a different ratio

Rear drive flange holes are in different place to TR, although a TR flange will fit the box if you have one.

 

Bob.

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I have a saloon gearbox and j type overdrive in my TR6, I have a saloon clutch plate and had 4 extra holes put in the rear flange to suit the TR propshaft. From memory the gearbox mounting is the standard TR6 item but mounted back to front.

The one down side is to get the propshaft out you have to remove the dif and access to the gearbox flange is very tight from under the car. I have just replaced my propshaft and did consider fitting a saloon yoke but in the end didnt bother

 

George

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The flange on the Tr gearbox has a recess 2 5/16" dia to locate the ridge on the propshaft universal joint and the saloon flange has a recess of 2 1/4" dia. As stated the saloon also used different hole centres but easy to drill new holes between the existing. The speedo on my 3A reads 42mph at 100kph as could not be bothered to strip out and change pinion and drive gear in o/d. I use gps for speedo.

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I have a choice between putting in a J-Type or A-Type overdrive in my restoration project. I was just wondering what peoples thoughts on are this are. I know the A-Type is the correct OD, but is the J-Type a better unit to have? I will keep both units with the car so the next owner (if I ever do sell it!) can change it if they so wish.

 

I have heard that the J-type is the better one to have in your car and use; the A-type can be slower to engage/disengage as well as being more tempremental. Of course, the J-type will give a higher drop in RPM, but I'm guessing going from 22% to 28% doesn't make too big a difference.

 

 

This is incorrect.

 

If you have an A -Type overdrive in good condition the operation is absolutely instantaneous, it will have engaged the overdrive before your finger has even left the switch.

The reason the J type were fitted for the IRS cars is the shock loading with this very crisp operation caused cracking of the diff mounts because of the instantaneous change in operation. The A type has a large commonly called "jam pot" piston which delivers all of it's fluid in one hit, the overdrive thinks it's great and instantly operates. The J type has two smaller pistons which operates the fluid delivery which isn't enough in it's single flow amount so as the fluid builds the overdrive sssllllluuurrrrsss into engagement feeding the shock loading in more gradually. This helps preserve the more problematical diff and chassis connections at the rear of the IRS cars.

 

Your car is a live axle TR4a and so was originally specified (I believe) with the A type overdrive which should not have problems with the rear axle and chassis connection, it doesn't have the separate diff and chassis problems of a normal IRS car and hence the J type overdrive is not necessary for the ssssmmmmmoooothhhhererr overdrive change.

When I raced with an overdrive I used the A type because when you need a change in gear (and that's what it is) using the overdrive I needed it NOW and not when the J type would decide to engage it maybe some 80 yards (at 70 mph) later.

 

Mick Richards

Edited by Motorsport Mickey

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