FTM 119: Hex Bolts and Hex Cap Screws

Fastener Training MInute 119: Hex Bolts and Hex Cap Screws

This edition of the Fastener Training Minute with Carmen Vertullo was originally published August 31, 2017, as “the difference between a hex bolt and a hex cap screw” during episode 119 of Fully Threaded Radio.

Hi everybody, this is Carmen Vertullo with your Fastener Training Minute from the Fastener Training Institute and the Carver FACT Center here in beautiful. El Cajon, California.

Today, we’re going to talk about a very simple but important and often misunderstood topic and that is the difference between a hex bolt and a hex cap screw, why that matters and the kind of trouble that we get into if we don’t understand the difference. These both come from a very important standard called ASME B18.3.1. We’re going to be talking about the dimensional differences. And if you happen to have the IFI 9th Edition or even the 8th, but you should have it available for reference. Go ahead and get it out because it’ll help you understand what we’re going to talk about when we come back.

Welcome back to the Fastener Training Minute This is Carmen Vertullo coming to you from the Fastener Training Institute and the Carver FACT Center where we study fasteners here in El Cajon, California.

We recently had a client who received an inquiry from a customer for some inch and 1/2 diameter hex bolts as they were called, to Grade 8. Grade 8 comes from SAE J429, and as we know it’s a fairly high strength fastener, but we’re used to seeing grade 8 bolts made as what are called hex cap screws or cap screws. There’s a difference between a hex bolt and a hex cap screw, and if you get out your IFI 9th Edition or if you happen to have a copy of ASME B18.2.1, we’re going to talk about to the very important differences in these standards.

Hex bolts have a larger body diameter tolerance and hex bolts do not have a washer face. To illustrate that, if we look at this inch-and-a-half product from table to ASME B18.2.1, one of the things will notice is that for an inch and a half bolt the diameter tolerance for the full body is 1.470 to 1.531 inches. That means it’s allowed to be 31 thousandths of an inch over the nominal size for an inch and a half. So for an end-user who expects their inch and a half bolt to go into a more or less inch and a half hole we can have an issue. Additionally the standard allows for some swelling even beyond that at the top of the head up to 90 thousandths of an inch. That’s because these large products are typically hot headed as hex bolts.

Now normally, we don’t make high-strength products as hex bolts. The Grade 8 bolt for example would be made as what’s called a Grade 8 Hex Cap Screw, and a hex cap screw does have a washer face under the head and it has a tighter Tolerance on the body diameter. So for example, the inch-and-a-half body diameter for the hex cap screw from Table 6 is much tighter if its 1.500 which is the nominal size on the plus side to 1.488 so it’s only 12 thousandths of an inch undersized. So that is the difference between those two products. Primarily, if you buy a 1 1/2 hex cap screw, you can assume it will fit in a 1 1/2 hole.

Now the other feature is the washer face. This is a raised area underneath the head that gives the bolt a nice bearing surface. So if the end-user intends to screw this into a tapped hole or even if it’s a precision assembly such as a machine or a crane or some kind of automotive or truck application, and it’s a high-strength bolt, they’re going to want that washer face. Many manufacturers in the United States, if you call them and say I need a 1 1/2 inch grade 8 bolt, they’re going to make you a bolt and your customer may not be able to live with the dimensional problems. Do be aware, there’s a difference between a hex cap screw and a hex bolt and that difference can make a big difference to the person who’s trying to assemble their product.

This is been Carmen Vertullo with the Fastener Training Minute. Thanks for listening.

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