Fitting a Trimatic three speed automatic

Conversion from Hydramatic to Trimatic is relatively straight forward if ALL the details are known.

Whilst some details were sourced via the web it is important to note the job was made easier due to several discussions with, “Alby”. Without his input the changeover would have been far more traumatic and expensive so “thanks Alby”!

It is also important to note that the conversion was done to a factory original EH Premier with a Hydramatic so it had the larger tunnel and extras to make the conversion so much easier.

There were a few second hand parts that I needed to source before the job was started so a trip to the wreckers assisted me to see from the outside what the Trimatic needed and then gather the parts in readiness to complete the job. Some new parts are also required and you will see further in the text what is new and second hand.

My requirement was to keep the gear change selector linkage on the steering column and this was made a little easier by firstly selecting a Trimatic with the selector linkage on the Drivers Side (DS) of the transmission. The Trimatic in earlier Holdens came out with the linkage on the DS where-as the Commodore series has the linkage on the Passenger side (PS). (Now, just to confuse the issue, the transmission I installed was a reconditioned Commodore unit BUT the housing was from an earlier Trimatic. This enabled the linkages to come out on the DS. The transmission company reconditioning the Trimatic assisted with this).

The trip to the wreckers assisted me to locate a lever with the two flats to fit over the selector shaft and this was sourced from a LX Torana with a Trimatic and the linkage rod was sourced from any vehicle that has an adjustable slide and tighten linkage connection. In my case I also wanted to reuse the existing intermediate linkage connected to the transmission and the EH Y frame. Alternatively, I have seen it possible to connect direct from the Steering Column lever to the Transmission. This was due mainly because that vehicle was originally a manual and did not have the intermediate linkage. Now, there is another method to do this and Alby suggests using cable. I have no experience with this but it would also accommodate using a Transmission that has the linkage on the PS.

The next important point is to consider is the differences between the Hydramatic drive requirement and the Trimatic. The Torque Converter on the Trimatic is attached to the “Drive Plate” which is attached to the Crankshaft, (The Hydramatic has a Damper plate which is attached to the “Flywheel” via six bolts on the perimeter. (The ring gear is on the flywheel in the case of the Hydramatic) The damper plate has a splined centre and this conducts the drive forces through to the Hydramatic Transmission. The “Flywheel” with ring gear must be removed from the Crankshaft and the Drive plate is attached to the Crankshaft. (In my case I decided to source a new Drive Plate with Ring Gear Pt # 7420311 from Motor Traders. I wanted this job to be done once and be a long-term fix so new stuff was purchased where needed). You can not use the same bolts (6) to hold the Drive Plate as it is thinner therefore shorter ones must be purchased. Pt # 7420312 (Don’t skimp here. Get some good quality ones like ARP) You will need to get three new bolts for the Torque Converter as well. Pt # SP2563 It is important to use locktite on the 6 Crankshaft bolts AND TENSION THEM TO THE BOOK SPECS ie 45 foot pounds.

You will also need a Tapered Spigot Bush Pt # 7420738 as it is needed to be gently knocked into the Crankshaft and enable the Drive Plate to be centred.

Before the transmission is offered to the engine. Make sure the Torque Converter is fitted to the front of the transmission properly. If the Torque Converter is not all the way on damage will occur to the Torque Converter & Pump when transmission is tightened to the engine.

All three splines should be picked up ie Input Shaft, Stator & Pump.The easiest way to do this is to stand the transmission up on the extension housing (output end), & gently rotate the Torque Converter until three audible clicks are heard. (Check that the gap between the back of the Torque Converter & Bell Housing is too narrow to fit your fingers in).

Another point to note here is the bolts to attach the Trimatic to the engine need to be longer! There are 4 of them and they are originally 2″ and need to be 2.5″ UNC x 7/16. If you try to use the existing bolts you will find that they will attach by a few threads only and that’s about all!!!!

Another change I did was to the Differential. The Trimatic has different gear ratios than the Hydramatic and it is advisable to alter the Diff centre which in my case was a 3.55:1 I installed a centre with 3.36:1. You should be armed with a new Diff centre gasket Pt # 7435153 plus oil also. The Trimatic has a different speedo cable connection therefore you can source a cable from a HG. (This will save you $60 bucks from having to get one specially manufactured)! The issue you need to address is with the length of the cable. It is really too short to go via the firewall so it is best to drill a 1″ hole in the floor pan and run the speedo cable through a grummet and under the carpet inside the vehicle. It really does fit quite well so do not be too concerned about doing this. To get the speedo accurate you need to get the correct gear ratios.

The EH I have still has original 13″ wheels so the transmission re-conditioners installed a drive gear (White in colour) from a Gemini Trimatic in the rear bell housing as the Gemi’s have 13″ wheels. The outer cog (which slides on the end of the speedo cable) was coloured Green (Pt # 7430969) and it has 21 teeth. This enabled my speed to be 60 kph when the needle was on 40mph and 80 kph when the needle was on 53 mph. (ie approximately 3 kph error according to a GPS).

The Hydramatic and Trimatic rear mounting bolt holes are the same distance from the engine therefore they line up with the existing bracket BUT the Trimatic transmission mounting needs to be lower as the output shaft is higher. (this puzzles me a bit) From the measurements taken before the job was started it looked like they were identical but there must be some angle difference hence the rear mounting bracket needs to be modified. To do this I took to the cross bracket (which is curved and shaped like a U section) with a friction cut off saw and cut a section out large enough to accommodate the rubber mounting bracket. I added two 5 mm thick x 50mm wide x 170mm long plates into the box section and welded it all together to make a boxed, stepped bracket. In effect, this lowered the mounting position by about 30mm and meant the tail shaft was nearly in line and the transmission housing cleared the tunnel. It is not a good idea to space the bracket away from the floor but better to cut and weld the bracket. Besides, there are 6 bolts that hold the bracket up to the floor and two of them are on an angle therefore as soon as you try to move the bracket down these two bolts do not line up as they have moved from the correct angle.

The Tail shaft in my case was the same length but the yoke needed to be changed to suit the different spline.The lower cover plate on the transmission bellhousing needs to be replaced with one to suit the Trimatic. The Hydramatic has “wings ” extending out from the bellhousing and this is why the plates are different. The top cover is the same on both transmissions so does not need to be changed.

The transmission pipes need to be longer as they enter the Trimatic in different places. (you could use rubber hose to join the pipes but then they can suffer from fatigue or leaks so I put the effort in to have fixed connections). I sourced two pipes from a LX Torana and straightened one front bend to fit the requirements at the front of the car and join to the hose going to the coolers. I ran the transmission oil via 2 coolers rather than via the Radiator as I have a concern that water may enter the transmission if the cooling pipes in the radiator fail. I also have overheating problems at Idle so I figured that the heat load would be best kept out of the radiator and released via efficient transmission coolers. (There are two schools of thought on what is best here and certainly it is known that water helps conduct heat AWAY or even TO the medium passing through it so this is a debate that I can not enter as I do not have temperature facts to argue either way which is best).

The transmission oil filler tube/dipstick also needs to be sourced from a HG or perhaps even a LX Torana. Some not so significant mods were done to make it fit better and if you are not careful you can move the alignment angle where the tube fits into the Trimatic. The tube has little tolerance to alignment and the “O” Ring and pipe WILL NOT go into the casing if any alignment error is created.

You will also need to source a vacuum pipe to go from the rear of the transmission (Vacuum Modulator) to pick up manifold vacuum so you may also need a multi way manifold connector to facilitate (in my case) Brake Booster, Vacuum Gauge and kick down requirements. Note the fitting to the modulator must have a 0:020″ hole to allow the modulator to operate properly. Alternatively a restrictor with a 0:020″ hole can be fitted into the vacuum hose at the manifold end. This is important because the modulator reads engine load & changes gears appropriately. Alby suggests fitting an adjustable modulator, so the desired gear changes can be achieved no matter what engine spark timing is required. Note engine spark timing adjusts manifold vacuum.

The Trimatic also has an electric kick down solenoid which you may like to connect with a switch connected to the accelerator linkage some where and feed 12 volt + to the solenoid when the pedal is put to the floor. I found an ideal spot to attach a bracket I manufactured and it fits neatly to the chassis frame/firewall connection bolt. I used a kick down switch from a Commodore which has a variable adjuster on it and the mechanical accelerator pedal came into contact with it as required.

The gear selector indicator on the steering column will also need to be changed and I am adapting one from a HR as it uses the same perspex type curved display for L, D, N, R, and PARK selection in the correct order. The mechanical lock bar on the steering column also needs modifying to accommodate the differences between the transmissions.

The Reverse/Neutral switch on the steering column also needs replacing to get the reverse lights to function in the correct position and the switch from the HR model is suitable for this job.

Author: Peter Lenthall EH Holden Club of SA