Why hot-dip galvanized nuts are tapped oversize
Galvanized nuts tapped oversize are not a supplier's shortcut — they are what ISO 10684 requires once a bolt thread has been hot-dip galvanized. The zinc goes on every surface of the thread, including the flanks, and it is thicker than the clearance the thread was made with. Press play to watch what happens to a standard nut, and what the oversize allowance does about it.
What to ask for
- The letter Z or X stamped after the property class on the nut — e.g. 8Z. ISO 10684 requires it on every nut tapped oversize.
- Confirmation the nuts were tapped after galvanizing. The standard forbids retapping a coated nut.
- The coating certificate: ISO 1461 / ISO 10684, minimum local thickness 40 µm on a spun fastener.
- Bolt and nut supplied together as a matched set — and never a Z or X nut on a bolt marked U.
What ISO 10684 actually says
ISO 10684 is the fastener standard for hot-dip spun galvanizing, covering coarse-thread fasteners from M8 to M64, property classes up to 10.9 for bolts and 12 for nuts. It does not recommend galvanizing anything smaller than M8 or with a pitch under 1,25 mm. On thread fit it is direct: the coating is always in excess of 40 µm, so the thread has to be made to special limits to accommodate it, and it gives two ways to do that.
Two more rules from the same standard that matter on site. Nut threads are tapped after galvanizing and retapping is not permitted (clause 5.7). And the coating on the bolt thread should not exceed one quarter of the minimum clearance of the thread combination (clause 6.2.2) — which is where the table below comes from.
The oversize allowance, size by size
From ISO 10684:2004 Table 1, for the combination MF supplies — a 6AZ nut on a 6g bolt. The fundamental deviation is how far the nut thread is cut beyond the plain 6H position; the clearance is that deviation plus the bolt's own 6g deviation; the coating limit is a quarter of the clearance.
| Pitch P | Thread | Nut 6AZ deviation | Bolt 6g deviation | Min. clearance AZ/g | Max. coating on bolt |
|---|
What that means for an M20
So on an M20 the galvanizer is working to a window of roughly 40 to 98 µm on the thread. Below it the coating fails the certificate; above it the nut fits badly even though it was tapped oversize. That window is why a fastener galvanizer spins the parts straight out of the bath and why a general galvanizer's thicker, non-spun coating can leave a Z nut tight.
Questions we are asked
Can I use a galvanized nut on a plain, uncoated bolt?
Not one that is tapped oversize. A nut marked Z or X has extra room cut into its thread to make space for zinc. On a bare bolt that room is empty: the fit is loose and less of the thread is engaged. Use the nut the bolt was supplied with, and if the bolt is uncoated, use a plain 6H nut.
Why won’t my galvanized nut go onto the galvanized bolt?
Almost always because the nut is a standard 6H nut, not one tapped oversize. The zinc on the bolt has used the clearance the thread was made with, and ISO 10684 names the result thread interference. Look for a Z or X after the property class on the nut face. If it is not there, it is the wrong nut — forcing it strips the coating off the flanks and leaves bare steel in the joint.
What does “tapped oversize” actually mean?
The nut’s internal thread is cut after galvanizing to tolerance class 6AZ or 6AX (ISO 965-5) instead of the plain 6H. For 6AZ the thread is cut beyond the 6H position by a fundamental deviation of +0,325 mm to +0,42 mm depending on the pitch — +0,35 mm on an M20 — so it runs freely over a bolt that was threaded 6g or 6h before coating. The nut is stamped Z (or X) after its class mark so it can be told apart on site.
Does oversize tapping weaken the nut?
ISO 10684 gives reduced proof loads for oversize-tapped nuts only in the two smallest sizes it covers, M8 and M10 (Annex A). For M12 and above it relies on the ISO 898-2 values for the nut’s property class. The strength question that does arise is the one above: an oversize nut on an uncoated bolt, where the extra clearance is not filled with zinc.
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