I cannot figure out the bolt thread size for my 66 shovel for the 4 bolts that attach the lifter base cover to the case.
took bolt to store, and it appears to be a 1/4" width, but not "coarse" or "fine" thread ( 20 or 28 thread). Its in between. does anyone know for sure if its a 1/4"x24?
need to know for tapping a hole.

 

yes its 1/4 X 24 thread and do not cross it with somthing else as it will leak - in my shop we have taps / dies and heli coils for repairs

 

For reasons only known to Harley they used a lot of non standard threads. The lifter blocks screws as you found are 1/4 x 24. The oil pump studs and nuts are 1/4 x 24. The floor board pivot pins are also 1/4 x 24. Yet at the same time they also used 1/4 x 20 and 1/4 x 28 for other applications.

 

i am glad they stopped using 12 X 24 threads as only a very small number of screw types are available today

we believe it was a carry over from WW1 work shop machinery of the day, as the J series bikes even Indian uses those odd threads 4 X36 - 12 X 24 - 1/4 X 24 - 3/8 X 28 and a few others - they must have hade millions and millions under goverment contracts -- jz