I've got a mildly modified .040+ 400 small block that's out of my 69 Malibu for a little work. The two block drain plugs (down near the oil pan) were rusted in place. I drilled them out to replace with brass radiator-style petcocks. One came out fine but I damaged about half the threads at the top of hole #2 (actually got the threads at the top of one side of the hole with the drill bit).

My question is this - I've got about 4 good threads at the bottom of the hole to thread the petcock into. Not sure if this is enough thread to secure the plug and prevent a leak. I've considered drilling the hole to the next size up, tapping new threads and using a reducer bushing in the new hole (would be a 3/8" bushing for the 1/4" plug).

I've never drilled or tapped cast iron. Anyone had any experience with this? I looked into using heli-coils, but a kit for 1/4" pipe thread is $80. What is the preferred fix here?

Four good threads on an NPT sounds like plenty to me. I'd put some of that white teflon tape on the petcock threads and try it. If it tightens up, you should be okay. Because you are dealing with an NPT, you may want to tap the hole a little deeper since the remaining good threads are towards the bottom. See if the petcock will thread in first, if not, tap a little deeper. If you have to tap deeper, be sure to use an NPT tap, and not an NPSF tap. I believe 1/4-18 NPT is the size you are working with.



[This message has been edited by Wooderson (edited 07-25-2002).]

Like I thought - making more of it than was there. Thanks for the response. Had plenty of room at the bottom to cut new threads. Got all I needed and still some to spare. Thanks again!