Drill and tap thread

[B92 8NW]

I need to drill and cut threads into my inlet manifold and EGR plate (I have a spare plate) to fit a thermocouple and MAP sensor.

The thermocouple is 1/4"-18 NPTF, says to use 7/16" drill bit.
The MAP sensor is 1/8"-27 NPTF, says to use "R" drill bit.

What is an "R" size drill bit?

I think rather than buying all the gear to do it, what sort of shop would be able to do these two jobs?

[Killer]

"R" is a letter drill size equivelent of 0.339 inches. 11/32 is close at 0.334 inches. My tapping chart says a drill size of 8.4mm (0.331 inches) is correct for 1/8-27 NPT.
Most engineering jobbing shops should be able to do the job for you.

Cheers, Mick.

[JDNSW]

R is apparently 0.339" or 8.61mm. 8.5mm would probably work seeing it is, I assume, aluminium.

Beaten!

John

[Bigbjorn]

I need to drill and cut threads into my inlet manifold and EGR plate (I have a spare plate) to fit a thermocouple and MAP sensor.

The thermocouple is 1/4"-18 NPTF, says to use 7/16" drill bit.
The MAP sensor is 1/8"-27 NPTF, says to use "R" drill bit.

What is an "R" size drill bit?

I think rather than buying all the gear to do it, what sort of shop would be able to do these two jobs?

Number drills and letter drills are commonly used in machine shops and manufacturing particularly for tapping drills as the very fine progression from size to size allows for precise hole size according to the percentage of thread depth required. Number drill diameters range from No.1 0.228" to No. 80 0.0135". Letter drills range from A 0.234" to Z 0.413"

The smaller number drills are mostly used in instrument, and orifice work like carburetor jets and fuel injection nozzles.

The tap drill size for 1/8" NPT is 21/64" with reamer, 8.4 mm without reamer. For 1/4" NPT the sizes are 27/64" with reamer, 7/16" without reamer.

As to buying "all the gear" you only need two twist drills, or two twist drills and two reamers to do the job properly.