BSPP & BSPT British Pipe Tap Drill Chart: ISO 228 (G) & ISO 7 (R/Rc) Calculator
British Standard Pipe (BSP) threads use a symmetric 55° Whitworth profile with rounded crests and roots (r = 0.1373 × P). BSPP Parallel threads (G series, ISO 228-1) seal against a bonded Dowty washer or O-ring face, whereas BSPT Taper threads (R external / Rc internal, ISO 7-1) cut on a 1:16 conical taper to seal directly on the thread flanks.
BSPP (G Parallel) & BSPT (R/Rc Taper) British Pipe Tap Drill Calculator
Lookup ISO 228-1 (G Parallel) and ISO 7-1 (R / Rc 1:16 Taper) 55° Whitworth pipe thread major diameter, pitch diameter, TPI, and tap drill size.
What Is the Difference Between BSPP (G) Parallel and BSPT (R / Rc / Rp) Taper Pipe Threads?
Outside North America, virtually all hydraulic manifolds, pneumatic cylinders, coolant valves, and process piping systems use the 55° Whitworth British Standard Pipe (BSP) thread system governed by ISO International Metrology Standard (ISO 228-1 and ISO 7-1). Because engineers and machinists frequently encounter four different letter prefixes on blueprints—G, R, Rc, and Rp—understanding their exact metrological and sealing distinction prevents scrapped hydraulic blocks:
Under ISO 228-1 (Non-Thread-Sealing Parallel Pipe Threads), both internal and external parallel threads carry the prefix G (for example, G 1/4 or G 1/2 A). A G parallel port cannot seal pressure on its own threads; instead, the top of the tapped hole must be spotfaced flat and perpendicular so a captive nitrile/metal Dowty Bonded Seal, copper crush washer, or ISO 6149 O-ring boss seals against the shoulder. Conversely, under ISO 7-1 / BS 21 (Pressure-Tight Taper Pipe Threads), external conical male threads carry prefix R, internal conical female threads carry prefix Rc (tapered 1:16 on diameter, or 0.750 in/ft TPF), and internal parallel female threads intended to mate with an R male taper carry prefix Rp.
| Nominal Pipe Size (G / R) | Threads Per Inch (TPI) & Pitch P | Major Diameter D (Gauge Line) | Pitch Diameter d_2 = D - 0.6403P | Minor Diameter d_1 = D - 1.2807P | BSPP Parallel (G) Tap Drill | BSPT Taper (Rc) Tap Drill |
|---|---|---|---|---|---|---|
| 1/16" BSP (G / R 1/16) | 28 TPI (0.907 mm) | 7.723 mm (0.3041") | 7.142 mm (0.2812") | 6.561 mm (0.2583") | 6.80 mm (0.2677" / H) | 6.40 mm (0.2520" / E) |
| 1/8" BSP (G / R 1/8) | 28 TPI (0.907 mm) | 9.728 mm (0.3830") | 9.147 mm (0.3601") | 8.566 mm (0.3372") | 8.80 mm (0.3465" / S) | 8.40 mm (0.3307" / Q) |
| 1/4" BSP (G / R 1/4) | 19 TPI (1.337 mm) | 13.157 mm (0.5180") | 12.301 mm (0.4843") | 11.445 mm (0.4506") | 11.80 mm (0.4646" / 15/32") | 11.20 mm (0.4409" / 7/16") |
| 3/8" BSP (G / R 3/8) | 19 TPI (1.337 mm) | 16.662 mm (0.6560") | 15.806 mm (0.6223") | 14.950 mm (0.5886") | 15.25 mm (0.6004" / 19/32") | 14.75 mm (0.5807" / 37/64") |
| 1/2" BSP (G / R 1/2) | 14 TPI (1.814 mm) | 20.955 mm (0.8250") | 19.793 mm (0.7793") | 18.631 mm (0.7335") | 19.00 mm (0.7480" / 3/4") | 18.25 mm (0.7185" / 23/32") |
| 3/4" BSP (G / R 3/4) | 14 TPI (1.814 mm) | 26.441 mm (1.0410") | 25.279 mm (0.9952") | 24.117 mm (0.9495") | 24.50 mm (0.9646" / 31/32") | 23.75 mm (0.9350" / 15/16") |
| 1" BSP (G / R 1) | 11 TPI (2.309 mm) | 33.249 mm (1.3090") | 31.770 mm (1.2508") | 30.291 mm (1.1926") | 30.75 mm (1.2106" / 1-7/32") | 30.00 mm (1.1811" / 1-3/16") |
| 1-1/4" BSP (G / R 1-1/4) | 11 TPI (2.309 mm) | 41.910 mm (1.6500") | 40.431 mm (1.5918") | 38.952 mm (1.5335") | 39.50 mm (1.5551" / 1-9/16") | 38.50 mm (1.5157" / 1-1/2") |
| 1-1/2" BSP (G / R 1-1/2) | 11 TPI (2.309 mm) | 47.803 mm (1.8820") | 46.324 mm (1.8238") | 44.845 mm (1.7656") | 45.25 mm (1.7815" / 1-25/32") | 44.50 mm (1.7520" / 1-3/4") |
| 2" BSP (G / R 2) | 11 TPI (2.309 mm) | 59.614 mm (2.3470") | 58.135 mm (2.2888") | 56.656 mm (2.2306") | 57.00 mm (2.2441" / 2-1/4") | 56.00 mm (2.2047" / 2-7/32") |
When tapping blind manifold ports for high-pressure BSPP (G) hydraulic fittings, machinists must machine the flat spotfaced counterbore specified in ISO 1179-1 or DIN 3852-2. The counterbore diameter must exceed the outside diameter of the bonded Dowty seal or ED captive ring by at least 0.40 mm, and its surface roughness should be held to Ra 1.6 to 3.2 micrometers perpendicular to the bore axis so the vulcanized elastomeric sealing lip cannot extrude under cyclic pressure spikes up to 630 bar.
For tapered BSPT (Rc) female ports in 316L stainless steel process valves and instrumentation manifolds, pre-reaming the drilled hole with a 1:16 taper pipe reamer before tapping dramatically improves thread flank finish and eliminates radial chip jamming. Whereas tapping directly into a straight cylindrical drill hole forces the final teeth of the BSPT tap to remove a heavy triangular wedge of work-hardened metal, taper reaming distributes the chip load evenly across every chamfer tooth and extends cobalt tap life by over 300 percent.
Why BSPP (G) and BSPT (Rc) Require Different Tap Drill Diameters
As shown in our verified calculation table and Machinery's Handbook (Industrial Press), a parallel G tap and a tapered Rc tap of the exact same nominal pipe size require different pre-tap twist drill diameters! For example, tapping a parallel G 1/4-19 hydraulic port requires an 11.80 mm (15/32") drill bit (which sits just above the basic minor diameter d_1 = 11.445 mm to give ~80% thread height), whereas tapping a conical Rc 1/4-19 port directly without taper-reaming requires a smaller 11.20 mm (7/16") drill bit so the narrow front end of the 1:16 tapered tap forms full crest threads at the bottom of the port.
| Nominal Pipe Size | BSP Thread Angle & Crest | NPT Thread Angle & Crest | BSP Pitch (TPI) | NPT Pitch (TPI) | Interchangeability Verdict |
|---|---|---|---|---|---|
| 1/8" Pipe | 55° Whitworth Rounded (r = 0.137P) | 60° Seller Truncated Flat (0.033P) | 28 TPI | 27 TPI | NO — Pitch & flank angle mismatch |
| 1/4" & 3/8" Pipe | 55° Whitworth Rounded (r = 0.137P) | 60° Seller Truncated Flat (0.033P) | 19 TPI | 18 TPI | NO — 19 vs 18 TPI locks after 2 turns |
| 1/2" & 3/4" Pipe | 55° Whitworth Rounded (r = 0.137P) | 60° Seller Truncated Flat (0.033P) | 14 TPI | 14 TPI | DANGER — Same 14 TPI but 55° vs 60° leaks under pressure! |
| 1" to 2" Pipe | 55° Whitworth Rounded (r = 0.137P) | 60° Seller Truncated Flat (0.033P) | 11 TPI | 11.5 TPI | NO — 11 vs 11.5 TPI galls threads |
Danger Warning: Why 1/2" and 3/4" BSP and NPT Share 14 TPI But Leak Under Pressure
The most dangerous cross-threading hazard in global fluid power maintenance occurs between British BSP and American NPT (ASME Geometric & Mechanical Standards ASME B1.20.1, audited by NIST (National Institute of Standards and Technology)). In sizes 1/8" (28 vs 27 TPI), 1/4" and 3/8" (19 vs 18 TPI), and 1" to 2" (11 vs 11.5 TPI), the pitch mismatch causes an NPT fitting in a BSP port to jam after 1.5 turns. However, in 1/2" and 3/4" pipe sizes, both BSP and NPT use 14 TPI! A 1/2"-14 NPT plug will thread all the way into a G 1/2-14 port by hand, but because NPT has a 60° sharp-flat flank and BSP has a 55° rounded Whitworth flank, the threads only touch on a razor-thin line that can blow out violently under hydraulic pressure. Always compare American pipe specs on our NPT Pipe Tap Drill Chart, check O-ring face glands on our SAE AS568 O-Ring Groove Chart, program helical port interpolation on our CNC Thread Milling Calculator, and verify lathe taper angles on our Lathe Taper Per Foot Calculator.