Hex Head Cap Screws

Hastelloy Hex BoltThere are several differences between hex head cap screws and hex bolts. One of the main ones is that cap screws have a tighter body tolerance than bolts. For example a 1″ diameter cap screw has a maximum body diameter of 1.000, while a 1″ diameter bolt could range as high as 1.022 and may be as large as 1.060 directly under the head. Hex heads in nickel-based alloys tend to have a body diameter larger than the nominal bolt diameter for two reasons. First, the raw material is generally oversized. Second, the forging process can create a swell under the head.

A bolt can be turned or ground to achieve cap screw tolerances. However, this adds to the cost and in the case of a non-stock item, will add to the lead-time. Our experience has been that bolt tolerances are sufficient most of the time. Therefore, we supply bolts unless a customer informs us the cap screws are absolutely necessary.

Nickel Systems maintains an inventory of hex bolts in Hastelloy® (Alloys C276, C22, B2), Carpenter® 20 (Alloy 20), Duplex 2205, 18-8 Stainless and 316 Stainless in stock at all times. Other alloys that we currently have hex bolt inventory include Nickel 200 (Alloy 200), Inconel® 600 (Alloy 600), AL6XN® and 330 Stainless.

Heavy Hex Bolts

Heavy hex bolts are generally specified for high temperature or high pressure applications, particularly where bolt heads will bear against flanges. Standard diameters are 1/2″ and larger. The main difference between a heavy hex bolt and a regular (or finished) hex bolt is the distance across opposite flats of the hex. Heavy hex bolts are 1/8 of an inch larger than a finished hex bolt of the same diameter. The formula for determining hex size is one-and-a-half times the diameter plus 1/8″.

ASTM (ASME) A193 is one specification that is usually sited in connection with heavy hex bolts. Several different materials are covered by ASTM A193. Nickel Systems can supply bolting to A193 B8 (304 Stainless), A193 B8C (347 Stainless), A193 B8M (316 Stainless), A193 B8R (Nitronic® 50), A193 B8S (Nitronic® 60) and A193B8T (321 Stainless), as well as Strain Hardened ASTM A193 B8 Class 2 (304 Stainless) and A193 B8M Class 2 (316 Stainless).

Nickel Systems is not limited to materials covered by ASTM A193. Currently, we have heavy hex bolts in Hastelloy® C276 (Alloy C276) and Carpenter® 20 (Alloy 20) in stock. Bolting in other materials can be provided.

Tap Bolts

Tap bolts are hex bolts that are threaded to the head as close as possible. Just because a bolt is fully threaded, it does not mean that it will take a nut all the way to the head. In the manufacturing process, a clearance is needed between the thread rolling dies or the thread cutting chasers and the head of the bolt. If contact is made between the bolt head and the threading tools, one or both would be damaged.

Nickel Systems maintains an inventory of tap bolts because many customers require fully threaded parts. Additionally, this allows us to cut parts to non-standard lengths on short notice or cover requirements for standard lengths which may be temporarily out of stock, and still provide an adequate thread length.

Most of the smaller diameter (1/4″ through 3/4″ and MM sizes) bolts we stock in Hastelloy® C (Alloys C276, C22, B2), Carpenter® 20 (Alloy 20) and Duplex 2205 are fully threaded. Inventory of larger diameter tap bolts varies with market demand.