DKS Steel Door and Frame Systems, Inc. constantly strives to meet the demands of the hollow steel door and steel door frame industry by providing premium products. Superior quality, a strong service commitment, and value are DKS watch words. Standard features built into each steel door and frame reflect constant attention to detail and performance.
DKS Steel Door and Frame Systems is committed to providing the best in hollow steel doors and door frames to their valued customers.
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Any lock prep that deviates from those dimensional properties whatsoever is no longer defined as a #161 prep.
According to Sargent:
The Federal Specifications for Builders Hardware and Door Trim defined the dimensions of door cutouts for various locks. This Federal Government Specification Number FF-H-00106 listed a lock as “Series 86 Cylinder Lock and Latchsets” is now obsolete. The door cutout requirements for this mortise lock became known as an 86 cutout. In the same specification it also defined along with others a 160 and 161 Series Bored Lock and Latch Series. The door preparation for these lock series became known as a 160 and 161 cutout. Although the Federal Standard is obsolete the cutouts are still used today. ANSI standard cutout numbers have replaced these federal cutout numbers. The Federal and ANSI cutouts will be discussed in detail later in this book.
And the passage previously quoted can be reviewed in their technical manual as seen here :