3/8-19 BSP at a Glance
This is the large-bore member of the BSP grease fittings family. The thread runs 19 threads per inch, a 1.337 mm pitch, cut in the 55 degree Whitworth form. Thread major diameter measures about 16.662 mm, so the 3/8 figure is a nominal size, not a measured diameter. These fittings suit large-bore lube ports where grease volume and sturdy threads matter on British and Commonwealth built machinery. Both forms exist: tapered BSPT and parallel BSPP. For smaller ports, see 1/8 BSP grease nipples.
Specifications
Dimensions below apply to the 3/8-19 BSP body in all three materials. Torque and pressure figures are the same across the range.
| Parameter | Value |
|---|---|
| Nominal size | 3/8 inch |
| Threads per inch | 19 |
| Pitch | 1.337 mm |
| Major diameter | 16.662 mm |
| Overall length | 18 mm |
| Threaded shank | 7 mm |
| Hex | 17 mm |
| Minimum engagement | 7 mm (5 full threads) |
| Working pressure | 5,000 PSI (345 bar) |
| Burst pressure | 10,000 PSI (690 bar) |
| Installation torque | 15-25 Nm |
| Weight | carbon steel 8.5 g, brass 6.2 g, stainless 8.8 g |
BSPT or BSPP
BSPT, the R form, tapers at 1:16 and seals on the thread flanks as the male thread wedges into the female. BSPP, the G form, runs parallel and seals on a face O-ring or a bonded washer at the shoulder. Match the form to the tapped hole, not to the hose end. A BSPT male threaded into a BSPP hole binds after 2-3 turns and never reaches a seal. A BSPP male in a BSPT hole wobbles, sits loose, and weeps under pressure. Check the hole with a thread gauge before ordering. The two forms share the same 55 degree flank angle and 19 TPI count, so a caliper alone will not separate them; only the taper tells them apart. On a BSPT hole, the gauge should sink flush at hand torque, while a BSPP hole stops the same gauge proud of the face.
Materials and Temperature Ranges
Zinc-plated carbon steel covers general industrial use from -40 to +120 C. Stainless 316 handles marine decks and chemical washdown, rated -40 to +200 C; see the full range of stainless steel grease nipples. Free-machining brass suits moderate duty from -40 to +150 C. Plating is the only coating option; there are no other finish choices. Material selection follows the environment, not the pressure rating, which is common to all three.
Head Styles
Straight head gives open access where the coupler approaches on axis. Button head passes more grease per shot, which suits large bearings that take a full charge. Flush head sits almost level with the surface for covers and guards that must stay clear. All three share the same 3/8-19 thread and the same ball-check. Choose by access and grease volume, not by thread. Where a coupler can only reach at an angle, an angled head saves a swivel adapter and keeps the grease path short. On recessed ports, measure the clearance from the shoulder to the nearest obstruction before picking a head; a button head needs more axial room than a flush head.
Installation
Clean and inspect the female thread before fitting. On BSPT, apply thread sealant tape or pipe compound to the male thread, hand-tighten, then torque 15-25 Nm with a 17 mm socket. Stainless threads gall, so use the low end of the range, 15-18 Nm, with an anti-gall compound. Brass is softer; keep it under 20 Nm. Aim for about 5 full threads engaged. Never apply sealant to BSPP O-ring faces. For volume orders, send a quote request. Two common field errors are over-torquing a tapered fitting to stop a weep, which cracks the female boss, and backing a fitting out to align the head, which breaks the thread seal. If the head must face a set direction, seat the fitting first, then add an adapter rather than turning the nipple back.
Frequently Asked Questions
What is a 3/8-19 BSP grease fitting?
It is a large-bore BSP threaded grease nipple with 19 threads per inch.
Is 3/8 the actual thread diameter?
No. The thread measures about 16.662 mm across.
BSPT or BSPP, which do I need?
Match the tapped hole. Tapered and parallel forms do not interchange.
What happens if I mix the forms?
BSPT binds in BSPP holes after 2-3 turns. BSPP wobbles and weeps in BSPT holes.
What pressure do they hold?
5,000 PSI working pressure and 10,000 PSI burst pressure.
What torque should I use?
15-25 Nm. Stainless toward 15-18 Nm, brass under 20 Nm.
Which material for marine use?
Stainless 316, rated -40 to +200 C.
Do they fit standard grease guns?
Yes. The ball-check head works with standard hydraulic-type couplers.
Is thread sealant needed?
Yes on BSPT male threads, never on BSPP O-ring faces.
Who supplies these fittings?
IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, builds and supports every unit. Email inquiry@isohitech.com for specifications and a quotation.
To confirm material and form for your ports, reach us through the quote request page.
