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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?

My guess is that it some sort of ignition timing adjustment. It would be on the distributor mechanical or vacuum advance circuit.

Lower octane fuels would require less ignition advance due to their susceptibility to detonation.

My limited research put this as a 1930's era item on Chevy motors.

I have found out what it is now thanks. The clamp bracket which holds the distrubutor into the block is the Octane Selector. It is mounted to the block using a slotted hole, and has timing marks to change the timing by 10 degrees before and after TDC.

I have now setup the engine and she is running like brand new, I couldn't believe it was the same motor, so smooth and quiet

Kind of obsolete in this age higher octane fuels. Vacuum gage works better.