View Full Version : Engine Drain Plug Thread Size

east tx skier
10-12-2005, 11:53 AM
I've got the OEM plugs with petcocks on my 351W engine. The petcocks are siezed and I never use them anyway because of the likelihood that the will clog. Each year I just take the whole plug out anyway, but it can be tricky to get the wrench around the petcock.

So I want to replace them with plain old brass plugs. What thread size do I need for the engine drain plugs?

Kevin 89MC
10-12-2005, 12:04 PM
Might be easiest to take one to your local hdw store, and find the right nut that spins on, then match that nut to the plug. I think Home Depot has a board with threaded holes and studs that are labelled for sizing. not sure the'd have the brass plugs though. I bought the brass sill cocks from Skidim. So far they work great!
Good luck,

east tx skier
10-12-2005, 12:36 PM
That's my backup plan, but I just don't want to drain water onto my garage floor if I don't have to.

I drained my father-in-law's boat this weekend. He's got petcocks. The ones for his block are surely clogged. Only a drizzle. So I've decided to go all brass plugs and spend a little more time unscrewing (since I'm anal and would unscrew them anyway just to be sure).

10-12-2005, 01:58 PM
You probably wont be able to use the thread gage boards at Home Depot Im guessing. You drain petcock is more likely pipe threads instead of bolt/nut threads. There is a difference in the thread design. Having huge issues in the past with my O2 sensor on my Jeep (long story), I learned that most things like this are probably pipe threads. You should check the plumbing area if you dont find anything that works as Kevin noted

Done quite a bit of pipe thread stuff at work and got all the ASME codes. The plug would be about 0.54 in actual diameter, the 3/8 plug would be .675, and the plug is .84. In all cases, the thread diameter will be a little smaller than these numbers. Aint the English system great.

east tx skier
10-12-2005, 02:19 PM
Well, I pulled the plug over lunch and let the river run out of my garage. Really made me realize how blocked my father-in-law's petcock was (a mere drizzle). I'll be heading to the usual suspects after work today to do a size up. I also emailed indmar in case they feel like giving me an answer before I leave.

10-12-2005, 02:24 PM
I'll bet 50 cents that the plugs are 1/4 NPT.

east tx skier
10-12-2005, 02:37 PM
NPT = __ Pipe Thread?

10-12-2005, 02:44 PM
NPT = __ Pipe Thread?
National Pipe Thread

10-12-2005, 06:46 PM
I just ripped the ears off of mine & use it as a plug :firejump:

10-12-2005, 08:01 PM
I'll bet 50 cents that the plugs are 1/4 NPT.

GM engines are too.
....and 1/4" male NPT taps cost $5.50 (damn knock sensors).

10-12-2005, 08:27 PM
I think they're 3/8" NPT.

east tx skier
10-12-2005, 10:52 PM
Ski King, someone owes you $.50. 1/4" NPT. Jim, you'd be proud of me. I went straight to NAPA. Cost me about $4, but they sure are pretty and shiny.

04-17-2007, 11:20 AM
Any suggestion on getting out OLD rusted Petcock plugs?

east tx skier
04-17-2007, 11:27 AM
Have you tried PB blaster?

04-17-2007, 01:45 PM
What is PB blaster?

04-17-2007, 03:00 PM
That's the best penetrating oil I have used. NAPA, Wal MArt and a lot of other places sell it.

east tx skier
04-17-2007, 03:03 PM
PB Blaster (http://www.blasterchemical.com/)

04-17-2007, 05:32 PM
okay thanks I'll give it a shot and see what happens!!!