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I am in the process of bringing a 73 Gt with an AT back to life. It had lived most of it's life in Ohio where too much salt is used on the roads. The car was undercoated and the body is solid but the brake lines and automatic transmission cooling lines are rotted. I was able to get replacement brake lines from Opel GT Source but not the transmission lines. During removal of the engine, the cooling lines were destroyed and the fittings were rusted to the tubes and were damaged. I searched the forum for size information but it is not there. After a lot of searching, I have found the data and thought it might be of value to someone else with leaky auto trans cooling lines.

The lines connecting the GM TH180 transmission to the flexible hoses coming off of the radiator of the Opel GT are 5/16 inch (8mm) steel. The short line on the passenger side is 26 ½ - 27 inches long. The longer one that crosses over to the driver side is 49 – 49 ½ inches long. The tubes have double inverted flares at the ends. The tubes will need to be bent to match the original lines. The tube fittings are 14X1.5 metric thread with no threads at the tip of the fitting (see picture). The lines are attached to an adapter at the transmission with 1/8 male pipe threads into the transmission and 14X1.5 metric flare to the tubing. The tube fitting end of the short adapter is not threaded fully in which is why you need the unthreaded end on the tube fitting.

The flexible hose attached to the radiator is 10 ¾ inches long and has ¼ male pipe threads going into the radiator and 14X1.5 metric flare threads attaching to the steel tube.

All measurements are based on a 1973 GT.
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Just Some Dude in Jersey
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EXCELLENT information! Thanks for posting it. It's darn near impossible to find that info written anywhere. It's so hard to cobble together the right fittings that I did all sorts of rig jobs over the years.

As an FYI, I dreaded this search for the right fittings at the radiator so much that, when I was building my GTX car, plus the fact that it has a hotter running, chromed and painted, engine, I decided to go with a separate tranny cooler and braided rubber coolant lines with AN fittings. I also didn't like that the passenger side coolant line ran very close to the exhaust, so I routed both the braided lines down the drivers side, cut a hole in the radiator wall, and ran the lines to my tranny cooler in the front of the car. I also installed quick disconnects. Here's some pics:

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