A518 overdrive transmission installation

A518 overdrive transmission installation
A518 overdrive transmission installation

Lately I was finally ‘mentally’ ready to start with the transmission-swap in my ’73 Dart. I went from a 904 to an A518 transmission. The A518 is a ‘bigblock’-transmission with a smallblock bellhousing and overdrive. Later tranny’s also have a lockup-function, but mine didn’t have that. This means I can use a regular ’60s stall convertor.

Because an A518-transmission is more bulky than the 904, some considerable modifications are needed in the car like the driveshaft-tunnel and rear transmission crossmember. This is because these overdrive transmissions weren’t available on these cars and years yet.

The original 904-transmission got removed, which had developed some slow 2-3 shifting issues, while its replacement A518 overdrive transmission was waiting patiently.

The top half of the transmission-crossmember had to be completely removed, otherwise the transmission couldn’t be mounted high enough in the car.
The 518-transmission also come stock with a deep oilpan, so to keep enough ground-clearance, a proper transmission height was mandatory.

To restore proper stiffness, the top of the crossmember will be welded from inside the car later on.
The bottom half of the crossmember was modified to accept a simple Chevy mount which I happen to have laying around in the garage.

After a lot of trialfittings with the transmission, it was finally time to install it properly.
It was then time to renew the coolerlines, as these transmissions have 3/8-lines, as opposed to 5/16 used by the 904.
Then it became apparent the stock speedometercable had become too short now, so a new, longer one was ordered.

With connecting the ‘kickdown’-linkage, I had to lengthen it about an inch to get the proper travel again.

Also the console-shifter linkage had some issues now.
Because of the bulky body of the transmission, the linkage could not go straightdown through the tunnel anymore. But with a slight tilt of the linkage I managed to get alongside the transmission again. Another small modification was done before I was finally able to shift through all the gearsettings again.

Finally it was time to shorten the driveshaft (again). I had shortened it before when I swapped the rearaxles.
The A518 is about 5.5″ longer then the 904 transmission. So out came the pipecutter again.
After measuring twice, I cut the driveshaft also twice :o)
I cleaned up the yoke on the lathe and tapped it back into in the driveshaft tube again, were, after some slight correctional taps, it was welded into place again.

My main concern was to get the car drivable again as soon as possible.
So I’ve only just managed to (electrically) get the overdrive unit working when I found a suitable electrical plug.
I rigged up a simple switch on the console where I can (for the time being) control the overdrive myself.

I must say it was a pleasure feeling that overdrive-unit kick in for the first time during driving in this car. Also because I didn’t know anything about this transmission other than the PO said it should work alright.

Even with the 2.94 gearratio in the rearaxle, turning on the overdrive at cruisingspeeds above 35-45 mph makes for a nice and quiet driving car.
The motor doesn’t turn a lot of rpm’s anymore at these speeds, so getting on the throttle is not advised ofcourse.

It was a lot of work, but seeing how much it has improved the ride-quality of the car, it was well worth it.

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