Friday, January 13, 2012

Overdrive Modification

As one or two of you may know I was having overdrive problems just before the last 10CR and tried (unsuccessfully) to fix it at the time.

After the run when it gave up completely I started to look around for a replacement unit and found this info on wikipedia,

"The first production vehicle to feature the Laycock system was the 1948 Standard Vanguard Saloon. The first unit to be created was the A-type overdrive, this was fitted to many sports cars during the 1950s. Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC and Armstrong Siddeley. Later the A-type overdrive was to be fitted to the entire range of TR cars.

In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962-1967 MGBs (those with 3-synchro transmissions).

From 1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 1968-1980 MGBs, the MGC, the Ford Zephyr, early Reliant Scimitars, TVRs, and Gilberns.

The J-type overdrive was introduced in the early 1970s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and Ford Transit vans.

The P-type overdrive marked the last updates and included both a Gear Vendors U.S. version and a Volvo version. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier. The Gear Vendors U.S. version uses a larger 1.375 outer diameter output shaft for higher capacity and a longer rear case."

Suggesting that the last and possibly the best unit made was a P/Type overdrive for the Volvo fitted to the 740 turbo of around 160bhp and 175 ft/lb torque.

Looking through ebay I spotted this unit and thought it looked very similar to the J/Type fitted to my GT6.

ebay pic 1

ebaty pic 2

ebay pic3

It was described as a new old stock unit that had been in storage for some time and possibly kicked around the garage floor a bit too!!!

OK as you can see there is no provision for a speedo drive output, (the volvo speedo is driven via a propshaft sensor) I would cross that one when I had too.

I hoped that it would all just bolt onto my existing GT6 gearbox and with a change over of the rear propshaft flange be a straight forward fit. (Arn't I the optimistic type :) )

Once the overdrive arrived I took off the rear housing and had a look to see if there was a speedo worm drive under the spot where the casing was not machined for the speedo drive spigot, no such luck only a spacer between the bearings, however on the bright side of things the inside of the unit looked brand new and still had traces of some sort of red test fluid (possibly ATF).

All I had to do now was take out the GT6 gearbox and get cracking................................................

That was the beginning of October,

Ok I'm a lazy Git I admit it !!!

It only took me 3 months to get around too it................................................. (Thanks Gaz)

Around too it

Here are a few pics of the gearbox and the new overdrive side by side for comparison.

Start Comparison

Start Side By Side

Before Swap

Start Side by Side

Thankfully the swap over was just as straight forward as I had hoped, I took off the rear housing and propshaft flange from the new P/J Type overdrive.

Then removed the old unit from my gearbox and removed the propshaft output flange, speedo worm drive and speedo drive spigot.

Still follow me :-/

Then it was just a case of fitting the old rear cover and associated speedo bits to the back of the new unit and fitting it back to my gearbox.........................

eerrrrrrrrrr not quite but only a small problem of the studs in the volvo nit being to long to be able to get everything bolted up.

A quick swap over of the studs and all was well, even the rear propshaft output flange is a perfect fit.

Here are some pics of it all swapped over,

Swap Complete

Swap complete Side by side

Swap Complete

Next I ran it up on my test rig to see if it was working.........

Don't Laugh too hard it works.

Test Rig

I used ATF to run the pressure test thinking that it is relatively thin when cold and would simulate gear oil when warm (and clean out my gearbox too).

Result of the pressure test was that it was running at 380 psi, I then shimmed the relief valve and it upped it to 490 psi.

I believe the Stag J/Type runs at around 550 psi and will set it to that eventually if that is correct as I have also modified the gear linkage to allow me to use overdrive in second gear and the extra pressure should help the overdrive not to slip in second gear. ( I hope )

Here are some pics of how you get to the relief valve and what it looks like.

Sump removed showing primary filter.

Sump Filter

Primary filter removed showing access to, Top main filter, middle is the pump/piston assy and removing the bottom plug reveals the relief valve.

Sump 2

Pressure relief valve

Relief Valve

I have tried to show the relief valve parts laid out in sequence. The little washers in the middle are in fact the adjusting shims to increase the pressure.

Simply add shims to up the pressure or remove them to decrease pressure if you go to far. it only takes a thin shim to up the pressure considerably, I put one in that at a guess was about 12 - 15 thou and that upped it by around 100 psi.

Relief Valve Detail

Here is a close up of the part number/ID Tag showing it to be ( I believe ) a 27% overdrive.

P/J Tag

In the end not a bad result for a new overdrive unit (providing it all works as expected on the car) for a punt on Ebay of 81 beer tokens.

Now I just have to strip the box and sort out a possibly bearing related grumbly first gear ;D

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