CT90-ST90 Auto to Manual Clutch
  • Honda CT90 uses pretty much the same engine as the CL90 Scrambler models or the S90. Except, CT90 from 1969-1979 had dual range transmission. The Transmission from an S90 or CL90 will not fit into a CT90 case with sub transmission.
  • ST90 However, can be converted to 4 Speed and Manual Clutch using parts from an S90 or CL90 engine.
  • The 90 size horizontal engines share the same components except, the Clutch, shift drum, and the Sub Transmission used on CT90 models. The shift drum or transmission cannot be changed. 
  • For the conversion, you will need the Clutch assembly, right crankcase clutch cover with manual clutch lever from a CL90 engine, and a clutch cable. The left handlebar lever on the earlier CT90 is used for rear brake, it will be used for the manual clutch.
  • The later CT90 models did not have a rear brake lever on the left side. For those models you will need a bracket for the clutch lever. 
One on the top is from a CT90 and the bottom one is from a CL90. Notice the difference in the oil dipstick location.
Closer look at the CL90 Manual Clutch Cover. The Clutch Oil passage is exactly in the same place as the CT90 cover. 
The manual clutch lever in place.
The original CL90 clutch cover was painted with the center polished. The cover is one piece, just like the CT90 cover. The earlier S90 had two piece clutch cover, with removable center. 
Closer look at the clutch lever. This cover is powder coated black for my custom CT90.
The manual clutch center is removable with the clutch plates installed. CL90 uses different clutch plates than the CT90.
You cannot use the CT90 auto Clutch in this conversion. 
On the top are the CT90 auto clutch parts.
Everything is replaced by the plunger assembly in the bottom of the picture.

The center bearing and the clutch cover are the same as the CT90. The manual clutch plunger fits in the same place as the CT90 auto clutch mechanism.
This is the shift lever on the CT90. This part has to be shortened, in order to fit the manual clutch cover. To avoid cutting a CT90 Shifter shaft, get a shifter shaft from a CL90 or S90 Engine.
The shorter lever is from a CL90. Almost identical levers.
Trial fit the manual clutch cover and make sure everything fits.
Use a cutoff wheel to cut the shifter shaft.
Leave about 3/8" stub (to be finished later)
Caution: Always wear eye protection while using power tools
Install the shifter in the case and use a straight edge to make sure the cutoff part does not protrude above the side of the engine case.
Install clutch cable with the lever on the left side of handlebar.
Install the clutch using clutch nut tool.
OEM Honda clutch holder shown in this picture. You can use any method to hold clutch in place.