ISMS scope mount (Ideal Scope Mounting System) screw size
M4 / M5 · length not stated in the sources
Our records put ISMS scope mount (Ideal Scope Mounting System) on M4 and M5 screws. None of the sources we found gives an exact length, so measure the original before you order.
There are 2 separate fitment records for it, including ring and interface clamping screws (M4) and main clamping screws for scope mount base (M5).
2 of the 2 records are marked confirmed, drawn from 1 source (gunsmoke.eu).
M4 means a 4 mm nominal diameter with a 0.7 mm thread pitch. Imperial look-alike: #8-32 (4.17 mm) — the standard US PC/rack screw, close but not the same.
ISMS scope mount (Ideal Scope Mounting System) screw specifications
One row per fitment record. Head, drive are shown only where the source states them; pitch is the ISO coarse pitch for the thread.
| Thread | Length | Pitch | Head | Drive | Where it goes | Status |
|---|---|---|---|---|---|---|
| M4 | not stated | 0.7 mm | not stated | not stated | Ring and interface clamping screws | Confirmed |
| M5 | not stated | 0.8 mm | not stated | not stated | Main clamping screws for scope mount base | Confirmed |
Fitment records and sources
Ring and interface clamping screws
pitch 0.7 mm · tap 3.3 mm · clearance 4.5 mm · PH2 / Torx T20
Main clamping screws for scope mount base
pitch 0.8 mm · tap 4.2 mm · clearance 5.5 mm · PH2 / Torx T25
How to check the screw before you buy
- Outdoor hardware should be stainless or coated to stop corrosion seizing it in place.
- Diameter: across the threads an M4 screw measures about 4 mm with calipers (usually a hair under). This device also uses M5, so measure each screw rather than assuming they match.
- Pitch: 0.7 mm from one thread crest to the next (14.29 crests per centimetre). A thread pitch gauge or the pitch-gauge tool settles it in seconds.
- Length: measure from under the head to the tip for pan, button, wafer and cap heads; countersunk (flat) heads are measured overall.
- Driver: the source does not name the drive. Standard M4 heads take PH2 for Phillips and T20 for Torx, and a 3 mm hex key for socket heads; a bit that fits loosely will cam out and strip the head.
- Tightening: no device-specific torque is published in these records. The site's general figure for M4 is about 2.0-3.0 N·m into metal threads and 0.8-1.4 N·m into plastic bosses; snug it up and stop.
Common mix-ups with M4 / M5
- Ideal Scope Mounting System manuals disagree between revisions (M4 vs M5 clamping screws) - check your own manual.
- Display mounts mix systems: the screen side is metric M4/M6, but the wall side of many mounts uses imperial wood screws such as #14 x 2.5in.
- Ski and snowboard binding screws vary by brand and insert system (4x4, Channel, Tyrolia, Marker). Aftermarket suppliers standardise on M5/M6 but OEM screws can be a different pitch.
- The 'GoPro thread' is M5-0.8, a de facto standard on third-party mounts, and is not the imperial 1/4-20 tripod thread.
More in Sports, Outdoors & Optics Mounts
The same screw also fits
Devices whose records share a thread with ISMS scope mount (Ideal Scope Mounting System), closest length match first.
- Wakeboard / water-ski binding plates also M5
- Ski and snowboard binding insert screws also M5
- Archery/bow accessory set screws (generic component, not confirmed as sight-specific) also M4
- Sea-Doo personal watercraft socket head screw also M5
- Ski binding mounting screws (Binding Freedom) also M5
- Elliptical pedal arm bolts also M5
- Ski/snowboard binding mounting screws (Binding Freedom insert screws) also M5
- Powersports battery terminal bolts (alt size) also M5
Questions about ISMS scope mount (Ideal Scope Mounting System) screws
- What size screw does ISMS scope mount (Ideal Scope Mounting System) use?
- M4 for ring and interface clamping screws; M5 for main clamping screws for scope mount base.
- How long are the screws in ISMS scope mount (Ideal Scope Mounting System)?
- None of the sources states a length. Remove one screw and measure it from under the head to the tip (overall for a countersunk head).
- What is the thread pitch of an M4 screw?
- 0.7 mm (coarse), which is the standard pitch for M4 machine screws; a 0.5 mm fine pitch exists but is uncommon. The tap drill is 3.3 mm and the normal clearance hole is 4.5 mm.
- What screwdriver do I need for ISMS scope mount (Ideal Scope Mounting System)?
- The sources do not name the drive. For M4, Phillips heads normally take PH2 and Torx heads T20; socket heads take a 3 mm hex key.
- Can I use an imperial screw instead of M4?
- No. Imperial look-alike: #8-32 (4.17 mm) — the standard US PC/rack screw, close but not the same. Use a true M4 screw so it does not cross-thread or strip the hole.