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What are single-start threads and twin threads?

Single-Start Threads

A single-start thread screw has one continuous thread running along the body of the screw. 

 

Screws with single-start threads are the most commonly found and are usually less expensive than screws with twin-start threads.

Twin or Double-Start Threads

A screw with a twin or double start thread has two threads running along the body of the screw instead of just one. 

 

Screws with twin-start threads often have a larger pitch, which means they can be inserted or removed twice as fast as a screw with a single-start thread. 

 

They will also hold material more securely. However, screws with twin/double start threads are usually more expensive.

High-Low Threads

On some types of screws with twin-start threads, the two threads will be different sizes. One thread protrudes high above the screw’s body, and the other lies low against it. The higher thread is sharper than the lower one, and so will bite further into the material, holding it more securely. 

 

Manufacturers of high-low threads claim that their sharp threads decrease the likelihood of the material splitting. However, screws with high-low threads are generally more expensive than ones with single-start threads.