stage block repair?

[87action]

Was going to purchase a 1 1/2'' tap(straight) to tap all 4 freeze plugs to eliminate further cracking of the engine block. Someone talked about this before and seems like the only logical task left to do. My question is, do I run a course tap or fine thread? I'm guessing course. And where do I get the cast iron plugs to replace the freeze plugs. Plumbing store? I'm assuming cast iron?

 

[ronsgn]

yep

 

[84BuickGNYorkPA]

Here is a chart for course thread @ 1 1/2" Diameter Tap Chart UNC/UNF Threads - provides tap sizes, drill sizes, pitch, (threads per inch) basic major diameter, basic effective diameter, basic minor diameter of external threads, and basic minor diameter of internal threads for UNC/UNF threads
1 1/2" - 6 NC (national course thread) = 1 11/32" Diameter of drill before tapping(minor diameter).

I would rather see you use a 1 1/2"-12 thread tap, this takes a 1 27/64" diameter drill (minor diameter), before tapping the threads in there. That will give you 12 threads per inch instead of 6, 12 threads will be stronger for cast iron.
Just so you know that the standard depth of thread for any diameter hole is
"hole diameter X 1.5"" so a 1.5 hole for standard use should have a thread depth of 2.25".

Of course for your application a couple of threads should work, you can buy a 1 1/2"- 12 standard bolt from any bolt supplier in your area.
http://www.mpindustrial.com/pdf_files/tap_drills_chart.pdf
Good Luck,

Chuck