If two parts won't screw together — different diameter, different pitch, or one's coarse and one's fine — you can't force them. The reliable fix is a thread adapter cut to each thread. Here is how to pin down what you have and get the exact adapter made — from a sketch, no CAD, one piece up. Skip ahead and send your two threads →
First, why it won't fit
Two threads only join when both the diameter and the pitch agree. When they don't, one of these is usually the reason:
Different diameter
An M24 won't enter an M30 — the simplest mismatch. You need a reducer between the two sizes.
Different pitch
Same diameter, but one is coarse and one is fine (e.g. M24×3 vs M24×2). They cross and jam — they don't engage.
Different standard
Metric vs imperial (UNF/UNC), or BSP vs NPT. The forms are incompatible even at a similar size.
Same gender
Two males, or two females, that need to meet — you need a piece that presents the opposite gender to each.
Don't force it. Cross-threading shaves metal off both parts and never holds load or pressure safely. A jammed coarse-into-fine joint feels "almost right" for a turn or two, then strips. The correct answer is always an adapter cut to the real threads.
The fix: an adapter cut to each thread
A thread adapter is a single piece with one thread on each end and a hex in the middle to drive it. Because it's machined to order, each end can be whatever you actually have — any diameter, any pitch, coarse or fine, metric or imperial — even a combination no catalog stocks. That's the difference between a custom adapter and an off-the-shelf one: the stock part has to match a standard pair; a custom one matches your pair.
How to get the right one — 3 steps
Identify both threads
Get the diameter and pitch of each side — e.g. M30×3.5 and M24×2 — and note whether each part is male or female. Not sure? Send photos or the parts; we identify them with thread gauges and calipers.
Choose the form
Pick male-female (to extend or convert a single line) or double-male / female-female (to join two same-gender parts). If you're unsure, tell us what each end screws into and we'll advise. Compare adapter types →
Send it to be made
A sketch or the two thread callouts is enough. We draw and confirm the adapter, then turn it — each end cut to its own thread and gauged. One piece or a small batch, no tooling cost. How ordering with no CAD works →
Real examples
Both of these started as a hand sketch of a thread combination that isn't sold anywhere — M30×3.5 paired with M24×2 — and were machined to order:
Electrical installations throw up the same problem in a different vocabulary: a connector on one side, a cable gland or protective conduit on the other, and two threads from different standards. That case is covered on the cable gland & conduit adapter page, with the PG, metric and NPT charts.
Two Threads That Will Not Meet — What the Catalogue Covers
Some pairings are stocked and you should just buy them. These are the ones people send us, and each one is a single part with your thread on each end.
| What you need | In the catalogue? | How we machine it |
|---|---|---|
| Two of the same standard, common sizes | Stocked | A plain reducer exists for this. Buy it off the shelf — you do not need us. |
| BSP one end, NPT the other | Stocked in common sizes | Widely available up to about 1 inch. Beyond that, or in mixed forms, it comes to us. |
| Metric M one end, NPT or UNF the other | Thin | The two systems share no thread form. Both ends single-point cut on one bar. |
| JIC 37° or ORFS against a metric port | Rare | Hydraulic hose end meeting a machine port from another system. |
| Same diameter, different pitch | No | The pairing that looks identical until it jams. Each end cut to its own pitch. |
| Left-hand one end, right-hand the other | No | Not stocked in any size or standard. |
| A thread you cannot identify | No | Send the part or the two mating pieces. We measure pitch, major diameter, flank angle and taper, then cut both ends to match. |
| An obsolete port from a machine out of production | No | Copied from the part, not from a chart. |
State both ends and the form you need. If we think a stock part will do the job, we will tell you that instead.
When the two threads are a lathe headstock and a chuck, the joint has an extra rule — the register shoulder does the locating, not the thread. That case is covered on wood lathe spindle thread adapters.
Frequently asked
Can I force two threads of different sizes together?
No. Cross-threading damages both parts and never holds load or pressure safely. Threads only join when diameter and pitch match. If they don't, you need an adapter that presents the correct thread to each side.
The diameters match but the pitch is different — is that the problem?
Often, yes. Same diameter with a different pitch (coarse vs fine) won't engage and will jam or strip — and it's a common reason a stock adapter won't fit. A custom adapter is cut with the exact pitch on each end. Full guide to connecting different thread pitches →
Do I need to know the exact thread sizes?
It helps but isn't required. Send a photo or the two parts; we identify the diameter and pitch, then draw and confirm the adapter before machining. How we identify threads without a drawing →
Got two threads that won't meet? Send a photo, a sketch, or just the two sizes (e.g. "M30×3.5 to M24×2"). We'll tell you the right adapter, draw it, and quote — usually within 12 hours. Get a quote → or explore custom metric thread adapters.