BOB’S CUSTOM KNOBS
Threads are specified by diameter, pitch, and length. Screw diameter is measured across the threads. Thread pitch for standard (Imperial) screws is given in number of turns per inch, and metric pitch is given in millimeters per turn. Thread length does not include the screw head or knob, but does include the unthreaded shaft in partially-threaded screws.
Smaller standard screws are designated by numerical thread diameter, such as #6 or #10, followed by pitch in turns per inch. Thus a 6-32 screw has a #6 diameter and 32 turns per inch. (We provide thread diameter in inches for these screws on our order pages to help you select the correct size.) Large standard screws are designated by thread diameter in inches, followed by pitch in turns per inch. For example, a 1/4-20 screw has a thread diameter of 1/4 inch and a thread pitch of 20 turns per inch.
A metric screw is designated by an "M" followed by its thread diameter in millimeters. An M4 screw, for example, is metric with 4 mm thread diameter. Thread pitch on a metric screw is usually a function of its diameter. For example, an M4 screw has a common pitch of 0.7 mm per turn.
Sometimes it's difficult to determine whether a screw is standard or metric. Most telescopes and tripods made in the US use standard screws for the main tube and mount, and (if made since the year 2000) metric screws on the visual back and finder. Telescopes and mounts manufactured outside the US are usually all metric. There are exceptions, though, and you can strip threads by forcing a metric screw into a standard hole or vice versa. So if there's any doubt about which screw you have, it's best to try a nut of known thread size on your screw before ordering knobs. Also, some earlier telescopes have screws with non-standard pitch and no nut can be found that will fit the screw.
If you don't see the knob you need in our charts, contact us using the link below with details and we'll give you availability and price. All custom knobs can be returned for a full refund if they don't meet your needs.