Bolt Blank

What a bolt blank is for

A bolt blank is a bolt with the head and shank made but no thread cut. Hobson's Did You Know sheet for its class 10.9 hex bolt blanks sums up the idea: you decide the bolt length by cutting the size and length of thread you need, and you can create coarse, fine or special pitch threads. Its sheet lists them where class 10.9 strength is required, and for machinery, trucks and mining equipment. They are not structural bolting: Hobson's AS 4100 article notes that bolts to AS 1110 (series) are not suitable for full tensioning, and these blanks are AS 1110.1 (see below).

That makes a blank the answer when the bolt you need is not a standard item: an odd length, a long thread or a fine or special pitch, especially in the big diameters, up to M48 and 1-1/2 inch here.

Bolt blank types

• •Class 10.9 hex bolt blanks (AS 1110.1): plain, heat treated to class 10.9 and head stamped (Hobson's Did You Know sheet). M16 to M48.
• •Class 8.8 structural bolt blanks: plain, to the AS 1252 (1983) pattern. Hobson's AS 1252 sheet notes that AS 1252:2016 K0 8.8 HR assemblies replace the 1983 bolts, so structural bolting to current standards is a matter for the engineer's specification. M16 to M33.
• •316 stainless hex bolt blanks (DIN 931): Hobson's descriptions mark the M16 and M24 blanks A4-70 and the M27 to M48 blanks A4-50. A4 is 316 grade stainless.
• •Grade B7 heavy hex bolt blanks (ASME B18.2.1, Table 7): plain, in inch sizes. Hobson lists them in its petrochemical range with ASTM A193 B7 stud bolts and A194 2H heavy hex nuts.
• •B8M class 2 heavy hex bolt blanks: 316 stainless, ASME B18.2.1 Table 7, in 1 and 1-1/4 inch.

Bolt blank sizes and lengths

Every bolt blank we carry, grouped by type. Select a row to open that type and size, then pick the length. Every blank is sold individually.

Class 10.9 blank head sizes

Head height and across flats (the spanner size) for the class 10.9 hex bolt blanks, from Hobson's Did You Know sheet. Hobson's spanner selection sheet gives the same across-flats sizes for AS 1110 bolts. The Did You Know sheet does not list M48; the spanner selection sheet gives 75mm across flats for an M48 AS 1110 bolt.

Before you thread a blank

• •The grade belongs to the blank as supplied. Hobson publishes no data for a bolt after it has been threaded, so the thread form, thread length and anything the finished bolt has to carry are for your engineer, the machine maker's specification or the drawing.
• •Coatings after threading: Hobson's AS 4100 article quotes the standard's note that post-manufacture treatment of high strength bolts may adversely affect material properties. Check with the engineer before plating or galvanising a class 10.9 blank.
• •Structural steel: Hobson's AS 4100 article notes that bolts to AS 1110 (series) and AS 1111 (series) are not suitable for full tensioning. The class 10.9 blanks are AS 1110.1, so a threaded blank is not a substitute for structural bolting; structural 10.9 bolting uses EN 14399 assemblies or TC bolts, to the engineer's specification.
• •Nuts: match the nut to the grade and the thread you cut. For B7, Hobson pairs its B7 stud bolts with ASTM A194 2H heavy hex nuts.
• •Stainless: Hobson's galling sheet says galling (threads cold welding and seizing) is most common in stainless, and that rougher thread flanks make it more likely; cut threads are generally rougher than rolled ones. Its suggestions: a slower installation speed, a lubricant with plenty of molybdenum disulfide on the threads, and a nut in a different stainless grade from the bolt.

Plain steel or stainless

• •Plain (class 10.9, class 8.8 and B7): no coating. Hobson supplies plain products lightly oiled to inhibit rust in the packaging and assist installation; they have no corrosion protection of their own. Use them dry and indoors, protect them, or have them coated with the engineer's approval (see above).
• •316 stainless (DIN 931 and B8M): A4 is 316 grade, which Hobson's stainless sheet lists with 2 to 3% molybdenum. Stainless does not rely on a coating outdoors, near the coast or in the wet, but it is much lower strength: A4-50, A4-70 and B8M are far below class 10.9 or B7 (Hobson's petrochemical sheet gives B8M a minimum tensile strength of 515 MPa against 860 MPa for B7). Use a stainless blank in place of a 10.9 or B7 one only on the designer's, engineer's or machine maker's specification, and watch for galling when you tighten it (see above).

Bolt blanks in Adelaide

Allmat supplies bolt blanks, hex bolts, heavy hex nuts, stud bolts and threaded rod to engineering workshops, maintenance crews and machinery repairers across Adelaide and South Australia. Send your list and we will price the lot.

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