Cal Royal 88-JO Offset Top Arm Assembly, Mortise Type

The Cal-Royal 88-JO is the "JO" type offset top arm assembly for the Cal-Royal 88 Series concealed overhead door closer. On a concealed overhead closer, the closer body is hidden inside the door's transom or header, and a top arm connects it to the door, with the arm type chosen to match how the door is hung. The 88-JO is an offset arm, used for offset-hung doors (butt-hinge or offset-pivot applications), and it's the mortise-type version sized for a 9/16" depth top rail. It's the genuine Cal-Royal part to specify or replace when an 88 Series closer is set up on an offset-hung door with that rail depth.

What the top arm assembly does and when you need the JO

This is the key thing to understand, because the arm is what makes a concealed closer fit a specific door. The 88 Series closer is concealed in the header, and the top arm transfers the closer's force to the door to swing it shut in a controlled way. Cal-Royal offers different arm types for different door-hanging methods: the S (side loading) and A, K, and PT (end loading) arms are for center-hung doors, while the JO and VO offset arms are for offset-hung doors on butt hinges or offset pivots.

So you choose the 88-JO specifically when the door is offset-hung and the application calls for the JO offset arm in the mortise type. It's single-acting (the door swings and closes in one direction), which is how offset-hung doors operate. Getting the arm type right is the whole point: an offset-hung door needs an offset arm, and ordering a center-hung arm by mistake means the closer won't mount or function correctly.

JO mortise type and 9/16" top rail

The 88-JO is the mortise-type offset arm, and it's sized for a 9/16" depth top rail. That rail-depth spec is how you tell it apart from the related 88-VO offset arm, which is sized for a 7/8" depth top rail. The top rail is the horizontal member at the top of the door, and its depth determines which arm assembly seats correctly, so matching the arm to your door's actual top rail depth is essential for a clean install and proper closer geometry. If your top rail is 7/8" rather than 9/16", the 88-VO is the arm to order instead.

Because it's the correct factory component, the 88-JO keeps the closer within its designed operation, which a mismatched or generic arm cannot guarantee on a concealed closer where the geometry has to be exact.

Cal-Royal 88-JO specifications

• Type: Offset top arm assembly ("JO" type), mortise type, for Cal-Royal 88 Series concealed overhead door closers.

• Application: Offset-hung doors (butt-hinge or offset-pivot), single-acting.

• Top rail depth: 9/16".

• Arm family: One of the 88 Series offset arms (JO and VO), as opposed to the S/A/K/PT center-hung arms.

• Part type: Genuine Cal-Royal replacement/specified arm assembly.

• Part number: 88-JO.

Why order the genuine Cal-Royal 88-JO

For a Cal-Royal 88 Series concealed closer on an offset-hung door with a 9/16" top rail, the 88-JO is the correct mortise-type offset arm to make the closer work as designed. A genuine Cal-Royal arm guarantees the fit and geometry the concealed closer requires, and ordering the right arm type by door-hanging method and top-rail depth is the most important detail on a concealed-closer job. Whether you're specifying a new 88 Series installation for an offset-hung opening or replacing an arm in the field, the 88-JO is the matched part for the 9/16" rail.

One thing to confirm before ordering: the 88-JO is the mortise-type offset arm for offset-hung doors with a 9/16" depth top rail. Verify your door is offset-hung (not center-hung, which uses the S/A/K/PT arms) and that your top rail depth is 9/16". If your top rail is 7/8", order the 88-VO offset arm instead, and note the closer body and bottom pivot are ordered separately.

Order the Cal-Royal 88-JO from American Locksets

We stock the Cal-Royal 88 Series concealed closer line and arms from authorized distribution, so every part ships with the manufacturer warranty. Find the closer bodies and related arms in the Cal-Royal 88 Series concealed closers category, see all overhead concealed closers, or browse all door closers.