LONG TAP EXTENTION
Chasing threads at the bottom of deep holes in engine blocks can be done with a pulley tap. Pulley taps are made with an extra-long shank. However, if you do not have a pulley tap, you can make a tap extension. Here’s how: Cut off the threaded end of a long bolt and drill a hole in the end of the bolt’s shank 1/2″ deep. To determine the hole size, measure the distance across the flats (AF) of the tap’s square drive. Multiply AF distance x 1.155 to find drill diameter (DD). Example: AF = 0.252″ Drill size = 0.252″ x 1.155 = 0.2916″. In this case, used a letter “L” size drill bit (0.290″). Set the tap’s square drive on the hole in the bolt; mark the bolt where each corner of the tap’s square is located. Using an angle grinder with a 1/16″ wide cutting wheel, slot the bolt where marked, about 1/2″ length. Slots should be just wide enough to let the tap’s square drive to wedge into the bolt’s hole. Slot width is typically 0.196 x DD. Max slot width = 0.392 x DD. Example: typical 0.196 x 0.290 = ~0.057″ and Max 0.392 x 0.290″ = 0.114″. If you wish to permanently secure the tap, silver solder it in bolt. Otherwise use the tap with the wedge fit, or with cured red threadlocker.
Tom Nichols
Automotive Machine & Supply
Joshua, TX
NOV, 2022
