SEMs: Pre-assembled Fasteners

If you want to boost the speed and efficiency of your assembly processes, you should look into SEMS, or “pre-asSEMbled” fasteners. These handy devices combine the screw and the washer into a single component, eliminating the need to match the right items up and slip the washer into place by hand. It also simplifies your inventory by giving you one part to track and purchase instead of two. SEMs have been around since the 1930s and come in inch-measure or metric varieties. (Guess which kind we sell!)

SEMs typically come with some form of lockwasher to prevent slippage once the screw is firmly secured. Two popular choices include internal-tooth and external-tooth lockwashers. Internal-tooth SEMs have teeth inside diameter of the lockwasher to bite into the substrate and hold the washer in place, while external-tooth SEMs perform the same task with teeth extending from the outside diameter of the washer.

Metric Internal ScrewsMetric External Screws

Double SEMs contain two washers instead of just one. In addition to a lockwasher, a double SEM also has a flat washer that sits on top of the locking washer. The lockwasher may have internal or external teeth, but more commonly it comes in the form of a split lockwasher. Split lockwasher see lots of use in automotive and aerospace manufacturing due to their ability to fit into narrow holes. Double SEMS come in brass, steel and stainless steel in sizes ranging from narrow to wide.

Metric Double Screws

Of course, sometimes all these options can get a little confusing. Which SEM is right for your needs? Do you need more than one kind? If you want to make sure you’re ordering the right products, don’t just order blindly — contact us and we’ll be glad to discuss it with you and help you with your purchase.

About the author

Mr. Metric is the spokesperson for, an online store for metric fasteners. Get $7.50 flat shipping with no minimum orders, plus advice and best practices for using metric fasteners in any application.

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