The GMC CCKW

Does anybody know what an 'Octane selector' is? I have just put all new points, condenser, rotor, cap and HT leads on my 235 Chevy engine. In the manual it mentions setting the 'octane selector' to zero after checking the tension of the points spring and also that it is set to zero before adjusting the timing, but I can find no more info about the 'octane selector' in the manual. What is it? and where can I find it?

Hi Pop
The octane selector is that brkt that is mounted between your dist. and the engine block.
Basically the dist. base is held on the selector by a clamp the other end of the selector is fastened to the block with a screw.
The screw hole is elongated to permit you to rotate the dist a few degrees in either direction ( advance or retard )
This will permit the driver to adjust the timing of the engine to compensate for low or poor octane fuel ( to stop ping or knock )
I hope I didn't confuse you
Depending on weather BUBBA left the brkt there or not of course

John

Excellent thanks John.

I did wonder if it was some sort of base setting of marks on the base of the distributor where it went into the block, but I never cleaned off the area as I had made a score mark when I removed the distributor and didn't want to wipe off my marks before I re-fitted it. The selector is still in place as I remember seeing the screw holding it to the block and just assumed it was the distributor clamp and not an adjuster.

At least I can set the timing properly at the weekend now

Complementing information about this subject:1)Each line of the distributor manual advance is equivalent three degrees. 2)In the center is when the octane is 75. 3)Moving in direction of the letter "R" when the octane is 70-72. 4)Moving in direction of the letter "A" when the octane is 80.