Hydraulic fittings
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Frequently asked questions
How do I tell BSP, NPT, JIC and ORFS threads apart?
BSP (British) threads have a 55 degree flank angle, in parallel BSPP and tapered BSPT forms. NPT is an American tapered thread at 60 degrees that seals on the thread. JIC seals on a 37 degree cone, while ORFS seals with an O-ring on a flat face. Measure the thread outside diameter, count threads per inch and inspect the sealing face to identify it.
How do L and S series 24 degree DIN fittings differ?
Under ISO 8434-1, L (light) series is for medium pressure and S (heavy) series has thicker walls for high pressure. For the same tube diameter, L and S nut threads differ, so they do not interchange. A marking like 12L means L series for 12 mm outside diameter tube.
Should I use PTFE tape on hydraulic fitting threads?
Do not use PTFE tape on parallel threads such as BSPP, JIC or ORFS, since these seal on a cone or a seal ring, not the thread. On tapered NPT or BSPT threads, use a dedicated liquid sealant if a sealant is needed. Tape fragments entering the circuit can jam valves and block filters.
What happens if a fitting is overtightened?
Overtightening cracks flares, deforms sealing faces or cracks the port in a pump or valve body. Undertightening leads to leaks or blow-off under pressure spikes. Tighten to the recommended torque or by the flats-from-finger-tight method given by the manufacturer.
How can fittings be kept from leaking?
Use the right sealing method for each thread: tapered threads seal on the thread, while parallel threads need an O-ring or a bonded washer. Clean the threads and put a little hydraulic oil on threads and O-ring before assembly. Tighten to the specified torque or turns, and if it leaks, take it apart and inspect the sealing faces rather than adding force.
How is a bite type compression fitting assembled correctly?
The tube must be cut square and deburred inside and out, then pushed fully against the fitting body's stop during preassembly. Tighten the nut the number of turns specified by the maker so the ring bites into the tube, then open it to check that an even collar of material is raised around the tube. Thin wall tubes should have an internal support sleeve.
Should I choose zinc plated steel or stainless fittings?
Zinc or zinc nickel plated steel is the economical choice for dry workshop conditions. 316 stainless suits wet, marine, chemical or food washdown areas, but watch for thread galling during assembly. Avoid combining stainless fittings with plain steel tubing in damp conditions because of galvanic corrosion.
Do removed fittings and lines need protection?
Yes, every open port and line end should be closed with a clean plastic cap or plug as soon as it is removed, to keep dirt out of the system. Do not stuff rags into openings, as fibres become contaminants. New fittings in stock should also keep their caps until installed.
