Bolt tightening torque table

Tightening torque figures below are in newton-metres, for clean and undamaged threads on a plain steel bolt, tightened to 75 per cent of proof load with a plain washer under a rotating nut. Divide by 1.356 for pound-feet.

Lubrication changes everything. Oil on the thread cuts friction, so the same torque produces far more tension in the bolt. Apply a dry-torque figure to an oiled or anti-seize bolt and you can snap it or strip the thread. As a guide, reduce by about 10 per cent for zinc plated, 20 per cent for lightly oiled, and 35 per cent for anti-seize or moly paste.

Bolt grades and head markings

Metric bolt grades are readable straight off the head. On an 8.8 bolt, the first number multiplied by 100 is the tensile strength in megapascals — 800 MPa. The second number is the ratio of yield to tensile, so 8 means it yields at 80 per cent of 800, which is 640 MPa. Every metric grade works the same way.

Going up to 10.9 or 12.9 without a reason buys brittleness rather than safety. Higher grades are more notch-sensitive, less tolerant of shock and corrosion, and they fail suddenly instead of bending first. For most work, 8.8 is the right answer.

Metric thread data — pitch, tapping drill and spanner size

Spanner sizes given are the ISO figures. Older DIN hardware uses 17 mm on M10, 19 mm on M12, 22 mm on M14 and 32 mm on M22, which is why a set of both turns up in most Malaysian workshops.

BSP versus NPT — how to tell them apart

They look identical and they are not. BSP is a Whitworth 55-degree thread; NPT is a 60-degree thread. Forcing one into the other damages both and leaks under pressure. The fastest check on the shop floor is a caliper on the male thread, because the outside diameters differ at every size.

Note that a BSP “1/2 inch” fitting measures 20.96 mm across, not 12.7 mm. The size refers to the bore of the old iron pipe it was designed for, not the thread diameter.

Parallel or tapered? BSPP, marked G, is parallel and seals on a bonded washer or an O-ring at the face. BSPT, marked R, is tapered and seals on the threads themselves with sealant or PTFE tape. NPT is always tapered. A parallel male will screw into a tapered female and feel tight, but it will weep.

BSP dominates in Malaysia, as it does across most of Asia and the Commonwealth — plumbing, compressed air, hydraulics and most gas equipment. NPT turns up on American machinery, imported compressors and oilfield equipment.

Frequently asked questions

What is the torque setting for an M12 8.8 bolt?

About 91 Nm (67 lb-ft) dry, tightened to 75 per cent of proof load. Reduce to roughly 82 Nm if the bolt is zinc plated, 73 Nm if lightly oiled, and 59 Nm with anti-seize. Where an equipment manufacturer specifies a torque, always use theirs.

How do I read a bolt grade marking?

On a metric bolt the first number times 100 gives the tensile strength in MPa, and the second number is the ratio of yield to tensile. So 8.8 means 800 MPa tensile, yielding at 80 per cent of that, which is 640 MPa. Stainless is marked differently: A2-70 is 304 stainless with 700 MPa tensile, A4-80 is 316 stainless with 800 MPa.

What size drill do I need to tap an M10 thread?

An 8.5 mm drill for a coarse M10 x 1.5 thread. The rule is nominal diameter minus the pitch, which gives roughly 75 per cent thread engagement — that is what you want, since chasing 100 per cent engagement only breaks taps and adds almost no strength.

Can I re-use a bolt that has been torqued?

Ordinary bolts, usually yes, if the threads are clean and undamaged. Torque-to-yield bolts — cylinder heads, connecting rods, many suspension bolts — are designed to stretch permanently once and must be replaced. If a workshop manual gives an angle after a torque figure, it is a stretch bolt and it is single use.

Why do stainless bolts seize when tightened?

Stainless on stainless galls: the surfaces cold-weld under load and the nut locks solid part way down. Once it happens the only way out is a grinder. Use anti-seize on every stainless thread, tighten slowly, and never run the nut down with an impact gun.

Is BSP the same as NPT?

No. BSP uses a 55-degree Whitworth thread form and NPT uses 60 degrees, and the diameters differ at every size. The dangerous pairing is 1/2 inch, where BSP and NPT differ by only 0.38 mm and share 14 threads per inch — they will start together and cross-thread within two turns. If a fitting starts easily then suddenly gets tight, stop and measure it.

Where can I buy high tensile and stainless fasteners in Perlis and Kedah?

Bann Edar Sdn. Bhd. stocks high tensile and stainless fasteners, BSP and NPT fittings, bearings, chains and hand and power tools at branches in Kangar, Perlis and Sungai Petani, Kedah. Call 04-976 5570 or WhatsApp 019-470 9929.

Disclaimer. The figures and recommendations on this page are general guidance for planning and set-up only. They are not a substitute for the manufacturer’s data sheet, an approved welding procedure or the applicable standard, and they cannot account for the conditions of your particular job. You remain responsible for checking every figure before you rely on it. Bann Edar Sdn. Bhd. accepts no liability for any loss, damage or injury arising from the use of this tool.

Penafian. Angka dan cadangan di halaman ini adalah panduan am untuk perancangan dan penyediaan sahaja. Ia bukan pengganti lembaran data pengeluar, prosedur kimpalan yang diluluskan atau piawaian berkenaan, dan ia tidak mengambil kira keadaan kerja anda yang sebenar. Anda bertanggungjawab menyemak setiap angka sebelum bergantung padanya. Bann Edar Sdn. Bhd. tidak bertanggungan atas sebarang kerugian, kerosakan atau kecederaan akibat penggunaan alat ini.