What the convention fixes
It fixes the approximate inside diameter of a plate’s hole, and the matching shaft on which the plate sits: nominally 30 mm. Bars and dumbbell handles built to it are usually threaded at the ends and take screw-on collars, which is where the common name spinlock comes from.
Unlike the 50 mm fitting, the loadable part of the bar here is often the same diameter as the part you grip — the bar is one shaft end to end, not a shaft with wider sleeves. That single difference is what makes the format cheap, and what limits it.
What it does not promise
It does not promise a load rating, and on this shelf the gap between the two is widest. A 30 mm bar’s published maximum is set by the shaft’s bending resistance, not by its diameter, and two bars of identical bore can be rated 60 kg apart.
It does not promise a consistent figure either. Listings in this format cite 30 mm, 31 mm, 1 inch and 25 mm across the same shelf, sometimes for stock from the same factory. Where a maker publishes one of the near neighbours, the page reports that figure rather than rounding it to the familiar one.
And it promises nothing about the thread. Screw collars are cut to a pitch the maker chooses; a collar from one set is not reliably a spare for another.
What it works with
It works with the usable length between the collars, which on a threaded bar is shorter than the bar and is the figure that actually caps a load.
It works with cement, vinyl and cast-iron plates in very different ways: for the same mass, cement is thickest and cast iron thinnest, and the bar fills up accordingly.
And it sits opposite the 50 mm fitting, which it never meets. A collection built on one bore is a collection that stays on it.
How we read it on this site
Bore comes first on every plate and bar page in this format, in millimetres as published, with the usable length beside it wherever the maker gives one. Where a listing offers only a photograph of a threaded end, the page records that no bore is published — because a thread in a picture is not a measurement.
Questions readers send in about this
Is 30 mm the same as 1 inch?
They are close and they are not identical: an inch is 25.4 mm. Listings translated from several markets use the two interchangeably, which is exactly the kind of substitution this site does not make. A page reports the figure its maker published, in the unit the maker printed, and says so when a listing gives both.
Can a 30 mm collection be moved onto a 50 mm bar later?
Not directly — the hole is too small, and no adapter makes a hole bigger. Moving up means replacing the plates, which is why the bore is worth deciding before the collection grows rather than after.
Why are spinlock sets so often sold with cement plates?
Because the format exists to be cheap and shippable, and cement in a plastic shell costs less than cast iron. The consequence is thickness: a cement plate takes far more sleeve length per kilogram, so a spinlock bar runs out of room at a much lower total than its rating suggests.
Last reviewed 20 September 2026