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Bucking Bars by Weight and Profile: Which One for Which Job
How to choose a bucking bar without buying eleven of them, why tungsten costs what it costs, and what a pitted face does to a shop head.
A bucking bar is a lump of steel, which makes the range of them look like an upsell. It is not. The bar does half the work of setting a rivet, and the wrong bar is the most common reason a shop head comes out clinched, smiling or offset.
Weight first
Weight is the primary decision, because it determines how many blows a rivet takes.
The bar has to have enough inertia to resist the gun rather than bounce. Too light and the blow pushes the bar away instead of forming the head, so you shoot for longer, hammer the material and work harden it. Too heavy and you cannot hold it accurately overhead for a whole rivet line, which introduces its own errors and its own shoulder injury.
| Rivet | Bar weight, roughly |
|---|---|
| 3/32 in | 0.5 to 1.0 kg |
| 1/8 in | 0.9 to 1.6 kg |
| 5/32 in and above | 1.6 kg and up |
Those are starting points, not specifications. Access changes them more than rivet size does: a bar you can only get two fingers behind is effectively lighter than the same bar held properly.
Then profile, which is really access
Profile is not about the rivet, it is about what is in the way.
Flat. Open skin, both hands available. The default, and the one that produces the best heads because the face sits square with no effort.
Offset or stepped. For a rivet next to a flange or a rib, where a straight bar can only reach at an angle. An angled face is how you get a clinched head, so this is the bar that fixes the most common problem in the empennage of a kit build. Our offset bar is 47 EUR ex VAT.
Heel or toe. A narrow projection for a corner or a channel, where nothing else fits at all.
Tapered. A confined bay, where the bar has to go in at an angle and then sit square.
For most people a four bar set covering flat, offset, heel and taper at 165 EUR ex VAT answers more jobs than four separately chosen bars would, and it is where to start. Add specific shapes when a specific job defeats you.
When tungsten is worth the money
Tungsten is roughly two and a half times as dense as steel. So a tungsten bar with the same effective mass as a steel one is dramatically smaller.
That is the entire proposition, and it is a real one: it is the bar that fits where a steel bar of the correct weight will not. Inside a wing bay, behind a spar, in the tail cone. Our 1.1 kg tungsten bar is 196 EUR ex VAT against 45 to 50 EUR for a comparable steel bar.
Honest advice: do not buy tungsten first. Buy it when you have met the specific rivet that a steel bar cannot reach, because until then it is an expensive version of something you already own. The exception is production work in confined structure, where it pays for itself in access alone.
Face condition, which nobody mentions
The face of a bucking bar is a forming surface. Every mark on it transfers to a shop head, and every shop head is an inspection item.
Keep faces ground and polished. Keep bars off concrete. Do not use one as a hammer, and do not throw them in a drawer together, because a bar dropped on a bar is how faces get pitted. A bar with a pitted face marks every rivet it touches, and it is the sort of gradual fault nobody notices until somebody queries a whole panel.
Which is a genuinely practical argument for cutting foam pockets for a bar set rather than keeping them loose. It is not tidiness, it is protecting a forming surface, and the same insert stops a two kilo steel bar becoming a projectile if the drawer is upended in a van.
Technique, briefly
Hold the bar against the rivet before the gun starts, keep steady pressure, and let the gun and the bar work against each other rather than chasing the gun with the bar. Never lift the bar mid rivet, because the shank will start to bend and there is no recovery from that.
If a shop head is coming out oval, the bar is not square. If it is coming out too flat and too wide, there is too much pressure or too many blows. If the skin around it is marked, the bar is slipping or the face is not clean.
Once the head is formed, check it with a gauge rather than by eye. A shop head should be at least 1.5 times the shank diameter across and at least 0.5 times high. A rivet gauge checks both in one movement and costs 19 EUR, which is less than the first panel you would otherwise redo.