AN to BSP Fittings

This is the single most important check before ordering an AN to BSP adapter, and getting it wrong causes the majority of BSP adapter leaks. BSPP (British Standard Pipe Parallel) threads maintain a constant diameter along their entire length. The port will have a flat, smooth sealing face at the bottom of the threaded hole — this face is where a copper washer, Dowty seal, or O-ring compresses to create the seal. The threads themselves do not seal; they only provide clamping force. When you thread a fitting into a BSPP port by hand, it will screw in smoothly all the way until the sealing face makes contact, with no increasing resistance.

BSPT (British Standard Pipe Tapered) threads get progressively tighter as they go deeper. There is no flat sealing face at the bottom of the port — the threads narrow as you go in, and the seal forms through thread interference combined with PTFE tape or liquid sealant on the threads. A BSPT fitting will tighten progressively as you screw it in, similar to an NPT fitting. If you are looking at a fuel pump, oil cooler take-off, or factory filter housing on a European or Japanese vehicle, check for the presence or absence of that flat sealing face. BSPP has it. BSPT does not. If you thread a BSPP fitting into a BSPT port, it will bottom out on the threads long before the sealing washer contacts any surface, leaving an open leak path. If you thread a BSPT fitting into a BSPP port, the taper will bind before the fitting is fully seated, and the lack of a compressible seal means the connection will weep from day one.

This is a common forum suggestion and it is incorrect for any fuel system operating above gravity-feed pressure. NPT threads use a 60° thread angle. BSP threads use a 55° Whitworth thread angle. The thread pitch is different for overlapping nominal sizes — for example, 1/4 NPT is 18 threads per inch, while 1/4 BSP is 19 threads per inch. The combination of different angles and different pitches means an NPT male will only engage the first one or two threads of a BSP female port before the mismatched profiles bind. Adding extra PTFE tape fills the gap between those mismatched threads temporarily, but it does not restore the lost thread engagement. Under vibration, thermal cycling, and fuel system pressure (40 to 70 PSI typical EFI, higher on diesel), that tape-compensated connection will loosen and leak. On a low-pressure coolant overflow line you might get away with it. On a fuel pump outlet, fuel pressure regulator port, or turbocharger oil feed, you will not. A purpose-built AN to BSP adapter costs roughly 9to11 and preserves both the thread integrity of the port and the safety of the fuel system.

Matching AN to BSP depends on flow requirements rather than a one-to-one dimensional match, but the most common pairings in automotive fuel systems are:

4AN to 1/8 BSP: used for low-flow applications like carburetor fuel pressure gauge lines, small bleed ports, and vacuum reference lines. 4AN ID is approximately 0.172 inch, suitable for flow rates under 30 LPH.

6AN to 1/4 BSP or 3/8 BSP: the most common size for EFI fuel systems supporting up to roughly 500 HP on gasoline. 6AN ID is approximately 0.297 inch. Match to 1/4 BSP for return lines and lower-flow feed lines; match to 3/8 BSP for main feed lines where the BSP port is the bottleneck in the system.

8AN to 3/8 BSP or 1/2 BSP: used for high-flow EFI systems above 500 HP, E85 conversions that require roughly 30% more fuel volume, and surge tank feed lines. 8AN ID is approximately 0.391 inch. The 3/8 BSP port is slightly smaller than 8AN ID, so the BSP side becomes the flow restriction.

10AN to 1/2 BSP or 3/4 BSP: used for high-horsepower builds (700+ HP), large surge tank plumbing, and diesel lift pump systems. 10AN ID is approximately 0.484 inch.

12AN to 3/4 BSP or 1 BSPL: primarily for diesel fuel systems, large racing fuel cells, and high-volume transfer pump plumbing.

When selecting, the AN size should be determined by your hose and target flow rate. The BSP size is fixed by the component port you are connecting to. If the BSP port is smaller than your AN line, the port is the flow restriction — upgrading the line size without addressing the port does not increase total system flow.

Yes. BSPP threads do not seal on the threads — the threads only provide clamping force. The seal requires a deformable element compressed between the fitting shoulder and the flat sealing face at the bottom of the port. Which type to use depends on the port and the fitting:

Copper crush washer: the most common choice for AN to BSPP adapters. Place the washer under the fitting hex, thread into the port, and tighten until the washer compresses flat. Copper conforms to minor surface irregularities on the port face and handles temperatures well above what a fuel or oil system generates. Use an annealed (soft) copper washer; work-hardened copper will not crush properly and will leak.

Dowty seal (bonded seal): a metal outer ring with a bonded rubber inner lip. These seal at lower torque than copper washers and are reusable for several install cycles if the rubber lip is undamaged. Common on European OEM fuel and oil system ports.

O-ring: used when the adapter has an O-ring groove machined into the metric or BSPP side. The O-ring compresses into a chamfered or flat-faced port seat. NBR (nitrile) O-rings are standard for fuel and oil. Viton O-rings offer higher temperature tolerance for turbo oil feed and coolant applications.

Do not use PTFE tape or liquid sealant on BSPP threads. If the fitting leaks after the washer is properly compressed, the washer is the problem — not the threads — and adding sealant to the threads will not stop a face-seal leak.

Yes, when the adapter body is 6061-T6 aluminum with a black anodized finish and the sealing element is NBR or Viton. The aluminum body and anodizing are chemically stable in ethanol concentrations up to E85 and are not degraded by the water content that ethanol-blended fuels absorb from the atmosphere. The sealing element on the BSPP side — copper crush washer, Dowty seal, or O-ring — is the component to pay attention to. Copper washers are inert to ethanol and require no special consideration. NBR O-rings are rated for continuous exposure to gasoline and ethanol blends up to E85. If you are running E98 or straight ethanol, upgrade the O-ring material to Viton (FKM) for extended service life at higher temperatures. The AN flare side is a dry metal-to-metal seal and is unaffected by fuel type regardless. For complete E85 fuel system plumbing, pair these BSP adapters with PTFE fuel lines — standard rubber fuel hose will degrade internally from ethanol exposure over time. See the Evil EnergyPTFE fuel line kitsfor E85-safe hose and hose ends.