Just wondering what size hole a drill to tap a 1/8 BSP thread.

I bought a 1/4" fitting for my wastegate line and it is a threaded 1/8 BSP. I couldn't find a tap for it anywhere. Got home and my metric tap and dye kit has a 1/8 BSP tap and dye. Miracles never cease to amaze me.

I'm sure I can work it out but I'd rather just know the rule of thumb up front before I start drilling into my turbo.

 

I believe it's a "R" in a letter drill.

 

Sheez, I've heard of Metric, Imperial, NSP, BSP and many others, but never "Sesame Street" this drill bit brought to you by the letter "R".

 

Sorry, I was just looking at a drill chart tonight, from tapping a 1/8 NPT EGT bung.

Letter drills are farly common is Machine shops in the Good Ol' USA.

I don't have the chart in front of me, otherwise I could tell you the fractional size.

 

I found this:

http://www.engineersedge.com/drill_sizes.htm

R appears to be 0.3390. I assume that is inches so about 8.5mm. And that is pretty close to the diameter of the threaded section.

Cheers

 

Just did a test and 9mm is about right.

 

9mm is probably borderline When you tap it there isn't much bite so is probably bigger than you should use. Will be fine for a pressure tap but not for anything structural. I'd say try 8.5mm and let the tap do the work.