ST90_Page

Honda ST90 1973-75

Converting the 3 speed ST90 to a 4 speed transmission with high / low range (8 speeds)

This picture above shows the updated engine and the ST90 parts that were replaced with CT90 parts.

A common question about this conversion. Why not just swap engines with a CT90?
The ST90 engine is unique because the engine is mounted to the frame with 3 bolts versus 2 for the CT90 engines. The extra mounting bolt is to support the frame of the ST90.

The output shaft seal to the sprocket on a ST90 has a 34mm diameter bore. This needs to be increased to 37mm to be the same as the CT90.

Bore the ST90 case to 37.0mm (1.457 inch)

Because the ST 90 case doesn't have the drain port in the casting for the high low range gear box, a drain tube needs to be added to the CT90 left side cover to allow oil to return back in the case instead of on the ground. A 1/4" thin wall aluminum drain tube was installed.

Clearance The Swing Arm and install the swing arm bolt to have the head on the left side.

The kick start shaft needs to be change to the CT90. Note the differences in the picture below.

Top shaft is the CT90 and the lower shaft is the ST90.

Assembly

Follow the CT90 assembly procedure. In the picture below note the alignment pin. The ST90 case does not allow for this. Loosely assemble the outside cove leaving the screws loose. Assemble the high low range gears and cover. Then lightly tightened the slack out of the bolts. Then torque the outside cover to the case. This will allow everything to align before tightening the bolts.

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03/08/2010