As far as I know there are two basic types of testers. One with a conical rubber tip that you have to push and hold in the spark plug hole and one with a metal thread that screws in like a spark plug. This kind has a rubber O ring to help seal it. I suggest the screw in type. Dave Delman aka D² & 6530 Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/DMCForum/ <*> To unsubscribe from this group, send an email to: DMCForum-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/