What a Fine-Thread Bushing Does in a Mount
A fine-thread bushing gives an adjustment screw a brass thread to run in when the mount itself is aluminium. The mount gets a plain reamed bore, the bushing goes in up to its flange, and the screw — an ultra-fine adjustment screw with a ball tip — turns in the bushing. EKINSUN LTD is a custom parts manufacturer in Guangdong, China, that machines these bushings in the common sizes below and to drawing from 1 piece, alone or as a pair with the screw. The same part is asked for under several names, and three neighbouring parts are often mistaken for it:
| Name on the request | What it is | Where it goes |
|---|---|---|
| Fine-thread bushing, threaded bushing | flanged brass body, internal thread of 0.25 mm pitch, plain mounting diameter | this part: pressed or bonded into a metal mount |
| Adjuster nut, adjustment screw bushing | the same part, named after the screw that runs in it | mirror and lens mounts, stages, gauge heads, fixtures |
| Threaded insert for metal, embedded nut | the same part when the thread is ultra-fine; with a coarse thread it is a catalogue insert | an aluminium plate whose own thread would wear |
| Heat-set or press-in insert for plastic | knurled brass insert, coarse thread, melted or pressed into a plastic boss | threaded inserts for plastics |
| Plain bushing, sleeve bearing | no thread; a shaft turns or slides in the bore | bushing size chart |
| Self-clinching nut | pressed into sheet metal, coarse thread | self-clinching nuts |
Threaded Bushing With Flange — Common Sizes From M3 to M6 × 0.25
The common flanged bushings are M3 × 0.25 on a 4.5 mm mounting diameter, M4 × 0.25 on 5.5 or 6 mm, and M6 × 0.25 on 8.6 or 9.5 mm, in lengths from 5 to 15.6 mm; M5 × 0.25 has an 8 mm mounting diameter and is 8 or 10 mm long, and M2 × 0.2 and M2 × 0.25 have a 3.5 mm mounting diameter and are 6 mm long. M3, M4 and M6 each come on a thinner and a thicker wall, so the same screw fits a small bore or a bore with more material around it. Turns of thread is the length divided by the pitch, before the chamfers.
| Thread A | Mounting diameter ΦB | Length L1 | Wall, (ΦB − A) / 2 | Turns of thread over L1 |
|---|---|---|---|---|
| M2 × 0.2 | 3.5 mm | 6 mm | 0.75 mm | 30 |
| M2 × 0.25 | 3.5 mm | 6 mm | 0.75 mm | 24 |
| M3 × 0.25 | 4.5 mm | 5 mm | 0.75 mm | 20 |
| M3 × 0.25 | 4.5 mm | 6.5 mm | 0.75 mm | 26 |
| M4 × 0.25 | 6 mm | 6.5 mm | 1 mm | 26 |
| M4 × 0.25 | 6 mm | 8.5 mm | 1 mm | 34 |
| M4 × 0.25 | 6 mm | 10.5 mm | 1 mm | 42 |
| M4 × 0.25 | 5.5 mm | 6.5 mm | 0.75 mm | 26 |
| M4 × 0.25 | 5.5 mm | 8.5 mm | 0.75 mm | 34 |
| M4 × 0.25 | 5.5 mm | 10.5 mm | 0.75 mm | 42 |
| M5 × 0.25 | 8 mm | 8 mm | 1.5 mm | 32 |
| M5 × 0.25 | 8 mm | 10 mm | 1.5 mm | 40 |
| M6 × 0.25 | 9.5 mm | 8 mm | 1.75 mm | 32 |
| M6 × 0.25 | 9.5 mm | 9.6 mm | 1.75 mm | 38 |
| M6 × 0.25 | 9.5 mm | 10.6 mm | 1.75 mm | 42 |
| M6 × 0.25 | 9.5 mm | 12.6 mm | 1.75 mm | 50 |
| M6 × 0.25 | 9.5 mm | 15.6 mm | 1.75 mm | 62 |
| M6 × 0.25 | 8.6 mm | 8.5 mm | 1.3 mm | 34 |
| M6 × 0.25 | 8.6 mm | 10.5 mm | 1.3 mm | 42 |
| M6 × 0.25 | 8.6 mm | 15.6 mm | 1.3 mm | 62 |
Buying one of these sizes? Click a row and the button asks for that size. Standard sizes can also be supplied from stock; every other thread, length or material is made to order from 1 piece. 1, 10, 100 and 1,000 pieces are quoted together: the more pieces, the lower the unit price. Quote in 12 hours, DDP to your door.
Thread Data for M2 to M6 Ultra-Fine Pitches
A 0.25 mm pitch thread is 0.27 mm smaller at its minor diameter than at its nominal diameter, so an M6 × 0.25 bushing thread measures 5.729 mm across the crests and is tapped from a 5.75 mm hole. M2 is made with 0.2 mm and with 0.25 mm pitch; M3 to M6 with 0.25 mm. These pitches are outside the ISO 261 fine series, so the taps and plug gauges are made for the job and kept for repeat orders. The standard coarse and fine pitches are on the metric thread pitch chart.
| Thread | Pitch | Pitch diameter D2 | Minor diameter D1 | Tap drill | Travel per turn | Travel per degree | Ball-tip screw lengths in our range |
|---|---|---|---|---|---|---|---|
| M2 × 0.2 | 0.2 mm | 1.870 mm | 1.783 mm | 1.8 mm | 200 µm | 0.56 µm | 3–20 mm |
| M2 × 0.25 | 0.25 mm | 1.838 mm | 1.729 mm | 1.75 mm | 250 µm | 0.69 µm | made to order |
| M3 × 0.25 | 0.25 mm | 2.838 mm | 2.729 mm | 2.75 mm | 250 µm | 0.69 µm | 8–25 mm |
| M4 × 0.25 | 0.25 mm | 3.838 mm | 3.729 mm | 3.75 mm | 250 µm | 0.69 µm | 8–50 mm |
| M5 × 0.25 | 0.25 mm | 4.838 mm | 4.729 mm | 4.75 mm | 250 µm | 0.69 µm | 8–50 mm |
| M6 × 0.25 | 0.25 mm | 5.838 mm | 5.729 mm | 5.75 mm | 250 µm | 0.69 µm | 8–75 mm |
Pitch diameter D2 = D − 0.6495 × P and minor diameter D1 = D − 1.0825 × P, from the ISO 68-1 basic profile; tap drill = D − P. Travel per turn and per degree are geometric values from the pitch; the repeatability of a mount also depends on thread fit, preload and the tip.
How to Tell a 0.25 mm Pitch From 0.35 and 0.5
Ten threads of 0.25 mm pitch measure 2.5 mm along the screw; ten threads of M3 × 0.35 measure 3.5 mm and ten of M3 × 0.5 measure 5 mm. Lay the screw that runs in the bushing on a rule or under a caliper, count ten crests and read the length, or try the 0.25 leaf of a pitch gauge. A coarse M3, M4 or M6 nut is a shelf part; a 0.25 mm pitch bushing is the table above or made to order. For the screw side of the same thread, see the M3 × 0.25 adjustment screw.
Press-In or Bonded — the Bore, the Wall and the Flange
A press-in threaded bushing is held by interference between its mounting diameter and a reamed bore; a bonded one sits in a slip-fit bore with adhesive. The flange stops the bushing at the face and takes the push of the screw. A press fit closes a thin wall by part of the interference, and at 0.25 mm pitch that is enough to tighten the thread, so the bore and the fit belong on the drawing; the fit classes are on the limits and fits chart. Where the screw has to be locked after adjustment, the locking threaded bushing screws into a tapped hole by a hex and holds the screw with a nylon-tip screw in its side.
| Feature | What to give | If not given |
|---|---|---|
| Thread A | diameter and pitch, M6 × 0.25; tolerance class if the fit matters | 6H, checked with a go / no-go plug gauge |
| Mounting diameter ΦB | the bore in the mount and whether the bushing is pressed or bonded | ΦB as in the size table |
| Length L1 | overall length; it sets the turns of thread that carry the screw | a length from the size table |
| Flange ΦC and flange end L2 | flange diameter and thickness, or no flange for a bushing that sits below the face | flanged with a relief groove behind the flange, as in the photo; dimensions on the approval drawing |
| Material | brass, phosphor bronze, 303 or 304 stainless | brass |
| Quantity | 1 piece as a sample, and the yearly number if it repeats | quoted at 1, 10, 100 and 1,000 pieces |
A typical request: an M5 × 0.25 bushing, 8 mm long, in brass, for the adjuster of an optical mount, with the screw to match.
Brass Threaded Bushing, Bronze or Stainless
Brass is the default material for a threaded bushing because a stainless or hardened screw runs in it without galling and the wear goes into the bushing. Phosphor bronze lasts longer under daily adjustment, and 303 stainless replaces brass where zinc and lead are not accepted. Machining data for brass is on the brass machining page.
| Material | Used when | Runs against |
|---|---|---|
| Brass | the default; free-cutting, so a 0.25 mm pitch thread comes out clean | 304 stainless and hardened steel screws |
| Phosphor bronze | adjusters that are turned daily; harder-wearing than brass | stainless and hardened steel screws |
| Stainless 303 / 1.4305 | vacuum, clean-room and marine parts where the zinc and lead in brass are not accepted | a screw of a different hardness: hardened steel or bronze |
| Stainless 304 / 1.4301 | the drawing names it; slower to thread than 303 | hardened steel screws |
| Aluminium 6061 or 7075 | the mount body, not the bushing: a fine thread cut straight into aluminium wears and picks up | the reason the bushing exists |
From Stock, or Made to Your Bore
One or two catalogue adjusters with their bushings are bought from an optics supplier's stock. The bushings that are made to order are the ones outside that shelf: a thread such as M8 × 0.25, a length or a flange of your own, another material, a bushing for a screw you already have, and quantities from 10 pieces up where a price ladder applies.
| What you need | On the shelf? | At EKINSUN |
|---|---|---|
| One or two catalogue adjusters with their bushings, for a mount you are building this week | Yes — optics suppliers | Buy it from stock from the optics supplier you already use; one catalogue pair costs less than a one-off. Standard parts: ask us too — we may have them in stock. |
| M2 to M6 bushings in the sizes of the table, 10 to 1,000 pieces | Some sizes | Standard sizes can also be supplied from stock; the more pieces, the lower the unit price. |
| M8, M10 or M12 × 0.25, or M2.5 × 0.25 | Rarely | Made to order from 1 piece, with the mounting diameter cut to your bore. |
| A length between the listed ones, a longer body, no flange or a larger flange | No | Same thread and gauge, the body to your dimensions. |
| Phosphor bronze, 303 (1.4305) or 304 (1.4301) stainless in place of brass | No | Made to order in the material on the drawing. |
| A bushing for an 80 or 100 TPI inch adjuster, without buying the screw again | Rarely | Threaded and gauged to your screw; send one screw or its thread. |
| Bushing and ball-tip screw as a pair, fit checked together | No | Made as a pair from our screw range, M2 × 0.2 to M12 × 0.25. |
| The mount plate in 6061 or 7075 aluminium, bores reamed, bushings fitted | No | Machined to drawing and delivered with the bushings in place. |
Bushing and screw from one source? The bushing is cut to the same thread gauge as the ball-tip screw, so the pair fits on arrival. Send the thread, both lengths, the bore and the quantity; the quote covers the pair at 1, 10, 100 and 1,000 sets, within 12 hours.
Answers by Size
What size is an M3 × 0.25 threaded bushing?
An M3 × 0.25 fine-thread bushing is made with a 4.5 mm mounting diameter in lengths of 5 and 6.5 mm. The thread has a minor diameter of 2.729 mm and is tapped from a 2.75 mm hole; the wall between thread and mounting diameter is 0.75 mm.
What size is an M4 × 0.25 threaded bushing?
An M4 × 0.25 fine-thread bushing is made with a 6 mm mounting diameter in lengths of 6.5, 8.5 and 10.5 mm or with a 5.5 mm mounting diameter in lengths of 6.5, 8.5 and 10.5 mm. The thread has a minor diameter of 3.729 mm and is tapped from a 3.75 mm hole; the wall between thread and mounting diameter is 1 mm or 0.75 mm.
What size is an M5 × 0.25 threaded bushing?
An M5 × 0.25 fine-thread bushing is made with an 8 mm mounting diameter in lengths of 8 and 10 mm. The thread has a minor diameter of 4.729 mm and is tapped from a 4.75 mm hole; the wall between thread and mounting diameter is 1.5 mm.
What size is an M6 × 0.25 threaded bushing?
An M6 × 0.25 fine-thread bushing is made with a 9.5 mm mounting diameter in lengths of 8, 9.6, 10.6, 12.6 and 15.6 mm or with an 8.6 mm mounting diameter in lengths of 8.5, 10.5 and 15.6 mm. The thread has a minor diameter of 5.729 mm and is tapped from a 5.75 mm hole; the wall between thread and mounting diameter is 1.75 mm or 1.3 mm.
What tap drill for an M6 × 0.25 thread?
An M6 × 0.25 internal thread is tapped from a 5.75 mm hole, which is the diameter minus the pitch; the minor diameter of the finished thread is 5.729 mm. The same rule gives 3.75 mm for M4 × 0.25, 2.75 mm for M3 × 0.25 and 4.75 mm for M5 × 0.25.
How far does a 0.25 mm pitch screw move per turn?
A screw with 0.25 mm pitch moves 0.25 mm (250 µm) per turn and 0.69 µm per degree in its bushing, whatever the diameter: M3, M4, M5 and M6 × 0.25 all travel the same. An M2 × 0.2 screw moves 200 µm per turn and 0.56 µm per degree.
Is the M2 bushing 0.2 or 0.25 mm pitch?
Both exist: M2 × 0.2 moves the screw 200 µm per turn and is tapped from a 1.8 mm hole, M2 × 0.25 moves it 250 µm per turn and is tapped from a 1.75 mm hole. Ten threads measure 2 mm on the first and 2.5 mm on the second, which is how the screw you have tells you which bushing to order. Both have a 3.5 mm mounting diameter, a 0.75 mm wall and a length of 6 mm, which holds 30 turns at 0.2 mm pitch and 24 turns at 0.25 mm.
Price, Lead Time and Delivery
Fine-thread bushings are quoted within 12 hours from the thread, the mounting diameter, the length, the material and the quantity. One piece is made as a sample; if it fits the bore and the screw, the drawing and the thread gauge stay on file and a repeat order is one email. Quantities of 10, 100 and 1,000 are quoted together, and the more pieces of one size, the lower the unit price. Production is about 5 working days, then 5–7 working days by courier; every quote is DDP to your door with duty and VAT paid. If your current supplier has dropped a size, send the old bushing or its dimensions and we quote it as a second source.
What to Send for a Quote
Four numbers quote a bushing: the thread, the bore it goes into, the length and the quantity. A sketch with those numbers, a photo of the old part next to a caliper, or a sample in an envelope all work; no CAD file is needed. A dimensioned drawing comes back for approval before the parts are cut.
Related: precision adjustment screws · M3 × 0.25 adjustment screws · set screws · knurled nuts · brass fasteners
Fine-Thread Bushing FAQ
A fine-thread bushing is a small flanged nut, usually brass, with an internal thread of 0.25 mm pitch or finer. Its plain outside diameter is pressed or bonded into a bore in a mount, and the adjustment screw runs in the bushing, not in the aluminium of the mount. One turn of an M3, M4 or M6 × 0.25 screw in the bushing moves the tip 0.25 mm; one degree moves it 0.69 µm.
The common sizes are M3 × 0.25 with a 4.5 mm mounting diameter, 5 or 6.5 mm long; M4 × 0.25 with a 5.5 or 6 mm mounting diameter, 6.5, 8.5 or 10.5 mm long; M5 × 0.25 with an 8 mm mounting diameter, 8 or 10 mm long; and M6 × 0.25 with an 8.6 or 9.5 mm mounting diameter, 8 to 15.6 mm long; M2 × 0.2 and M2 × 0.25 have a 3.5 mm mounting diameter and are 6 mm long. M8, M10 and M12 × 0.25, inch threads of 80 and 100 TPI, and any length or flange are made to order from 1 piece.
Two different metals run on each other without galling. A 304 stainless or hardened steel screw in a brass thread turns smoothly and the wear goes into the bushing, which is the cheaper part to replace; a stainless screw in a stainless or aluminium thread picks up and seizes. Phosphor bronze is the harder-wearing alternative, and 303 stainless (1.4305) is used where brass is not accepted, with a screw of a different hardness.
The mounting diameter goes into a reamed bore until the flange sits on the face, and it is held by a light press fit or by adhesive in a slip-fit bore. A press fit closes a thin wall slightly, so on the sizes with a 0.75 mm wall (M2 in 3.5 mm, M3 × 0.25 in 4.5 mm, M4 × 0.25 in 5.5 mm) state the bore and the fit on the drawing, or bond the bushing; the thread is then gauged for the fit you chose.
Yes. The ball-tip screws in our range run from M2 × 0.2 to M12 × 0.25, and the bushing is cut and gauged to the same thread, so the pair arrives with the fit checked screw-in-bushing. For a pair, send the thread, the screw length, the bushing length and the bore it goes into.
Fine-thread bushings are quoted within 12 hours from the thread, the mounting diameter, the length and the quantity. One piece is made as a sample; 10, 100 and 1,000 pieces are quoted together as a price ladder, and the more pieces of one size, the lower the unit price. Production is about 5 working days, then 5–7 working days by courier, DDP to your door with duty paid.
EKINSUN, a custom parts manufacturer in Guangdong, China, machines flanged fine-thread bushings in brass, phosphor bronze and stainless steel, M2 to M12 with 0.2 and 0.25 mm pitch and 80 or 100 TPI, to a drawing, a sketch or a sample, from 1 piece, alone or as a pair with the adjustment screw.