Identifying Your Bus Bar Bolt: M10 vs M12 vs M16 Quick Reference

Why This Matters

The lug at the bus bar end of every dangler has a single bolt hole. That hole has to match the bolt that secures the dangler to the bus bar. Wrong bolt size means the dangler can't be installed — the lug either won't fit over the bolt or won't make solid contact.

This article gets you to the answer in under a minute, with no calipers required. If you have a wrench in your toolbox, you have everything you need to identify the right size.

The Three Sizes You'll See in Plating

In North American plating, three metric bolt sizes account for the overwhelming majority of bus bar configurations: M10, M12, and M16. The number is the bolt's nominal diameter in millimeters — M10 is a 10mm bolt, M12 is a 12mm bolt, M16 is a 16mm bolt.

Approximate distribution of what we see across customer orders: M12 is the most common, appearing on roughly 70% of bus bars across zinc, copper, and brass plating lines. M10 appears on about 20% of installations, mostly smaller barrel lines running under 50A per dangler. M16 appears on the remaining 10%, almost exclusively on heavy-duty zinc-nickel and chrome lines running 200A+ per dangler.

The Wrench Test (Most Reliable)

The simplest and most reliable identification method: pull a bolt from the bus bar and check the head with a wrench. The wrench size that fits perfectly tells you the bolt size.

M10 bolt → 17mm wrench (or 11/16" imperial). Hex head measures 17mm flat-to-flat. M12 bolt → 19mm wrench (or 3/4" imperial). Hex head measures 19mm flat-to-flat. M16 bolt → 24mm wrench (or 15/16" imperial). Hex head measures 24mm flat-to-flat.

If you don't have metric wrenches, the imperial sizes above are close enough for identification — they're not perfect fits but they confirm which metric size you're looking at. A 3/4" wrench will fit an M12 bolt with very minor play; a 17mm wrench will fit an 11/16" bolt with very minor play.

The Bolt Diameter Chart

If you've already removed the bolt and have a ruler, you can also identify size by measuring the threaded shaft diameter directly:

M10 bolt → threaded shaft is 10mm (0.394") in diameter. M12 bolt → threaded shaft is 12mm (0.472") in diameter. M16 bolt → threaded shaft is 16mm (0.630") in diameter.

Note: bolt length is irrelevant for dangler spec. Only the diameter matters. Whether the bolt is 25mm long or 75mm long, the dangler lug only sees the diameter at the contact point.

The Visual Comparison

If you don't have a wrench and can't pull the bolt, you can usually distinguish the three sizes visually if you've seen them before. Most plating shops use one consistent size across the line, so once you know what your shop uses, every bus bar in the building will match.

M10 bolts look proportionally similar to common automotive bolts. M12 bolts look slightly larger, similar to what you'd see on a structural steel beam connection. M16 bolts are noticeably larger — about the diameter of a US dime stacked on edge. If you're holding the bolt in your hand and it feels like a "small bolt," it's probably M10. If it feels like a "regular bolt," it's M12. If it feels heavy and substantial, it's M16.

What to Do When You Have a Mixed Line

Some plating shops have inherited multiple production lines built in different decades, and the bus bar bolt sizes vary across lines. This is normal. The dangler order spec is per-line, not per-shop. Identify the bolt size for the specific line you're ordering for, and note any other lines separately if you'll be ordering for them in the future.

If a single line has mixed bolt sizes (which occasionally happens after partial bus bar replacements), the simplest path is to standardize on one size during the next maintenance cycle. We can quote danglers with the existing mixed sizes if needed, but the unit cost is slightly higher because of the small batch breakdown.

Special Cases: Imperial Bolts on Older Lines

A small percentage of older North American plating lines — typically pre-1985 — use imperial bolt sizes (3/8", 1/2", 5/8") rather than metric. These are decreasingly common but they still exist. If a metric wrench doesn't fit any of your bus bar bolts, you may have imperial bolts.

Eagle can produce dangler lugs to match imperial bolt sizes, but it requires custom tooling and the lead time extends accordingly. The standard recommendation in this case is to replace the imperial bolts with metric equivalents during the next maintenance cycle: 3/8" → M10, 1/2" → M12, 5/8" → M16. This converts the line to standard sizing and eliminates future custom-spec orders.