Here's an odd one for ya:
On my 1970 Honda CB350 -- it arrived from the eBay seller with the screw tops off of the spark plugs. When I replaced the plugs, I attempted to put them in "whole", that is with the screw tops on. I've never seen them installed (on cars) without those screw things. But, the plug wires wouldn't fit with those tops on. Should they be on or off???