In this installment of Hagerty DIY, Davin Reckow is working on a 389-cubic-inch Pontiac block fresh from the hot tank. In order to properly clean the block, the plugs blocking the ends of the lifter oil gallery had to come out. Otherwise oil sludge may have remained that could later block a passageway and lead to oil starvation. It’s an easy upgrade for guaranteed peace of mind, since tapped oil passage holes make sure that pressurized oil stays where it should be.
The first step to tapping is to ream the edge of the oil gallery hole to remove any damage done from when the pressed-in plugs were staked in the factory or during a previous rebuild. By the looks of it, Davin’s not the first one to tear apart this 389.
After a chamfer has cleaned up the top edge of the hole, Davin used a 3/8-inch NPT tap to cut threads in the block. He started with a drill press to make sure the threads were plumb and finished with careful, patient application of a wrench to turn the tap and then back it off.
Because NPT threads are tapered (NPT stands for National Standard Pipe Taper; we’re not sure what happened to the S), the hole has to be threaded quite deeply for the plug to sit flush. This plug in particular is right behind the timing gear, which makes it doubly important to be nearly flush and keep out of the way.
This is a relatively simple job, but be sure to check for any intersecting oil passageways in your engine and make sure that adding a plug won’t disrupt oil flow. In some applications, including Pontiac V-8, some plugs use a tiny hole, around 0.030-inch, to provide a small stream of oil to help distributor gear or timing gear longevity, so make sure your replacement matches the factory oiling.
Finally, as Davin mentioned at the end of the video, be sure to thoroughly clean the block and all of its passages thoroughly before assembly to remove any metal shavings created by the ream and tap.