Testing rivet strength after a repair

Why every riveted repair deserves a second look

A rivet that went in with a satisfying snap is not automatically a rivet that will hold. Pop rivets and solid rivets both rely on the hole being the right size, the sheets being tight together, and the rivet being the right length for the thickness of material it is clamping. Get any of those wrong and the joint can feel solid in your hand while quietly working loose the moment it sees real load.

Testing is cheap. It takes a few minutes and a handful of simple tools, and it saves you from discovering the failure later, usually at the worst possible moment, on a trailer bracket, a heat shield, a wing skin or a boot floor patch. Here is how to check a riveted repair properly, in the order you should do it.

Start with a careful visual inspection

Before you touch anything, look closely. Good light and, if you have one, a magnifier will show you more than you expect.

• Broken mandrels: every blind rivet should have its mandrel snapped off cleanly inside or just below the head. A mandrel sitting proud, or one that pulls out when you tug it with pliers, means the rivet did not set properly.
• Head seating: the rivet head should sit flat and tight against the sheet. Any visible gap, or a head you can slide a feeler gauge or a piece of paper under, is a fail.
• Cracked or stretched material: look for hairline cracks radiating from the hole, a dark ring of fretting dust, or paint that has cracked in a circle around the head. All of these suggest the sheet has been moving.
• Distortion: a wavy or dimpled panel around a row of rivets usually means the sheets were not clamped together before setting, so the rivet pulled them into each other rather than joining them.

The wobble test and the tap test

Once it looks right, see whether it feels right. Press firmly on one side of the joint and lever gently against the rivet head with your thumb or a plastic wedge. You are not trying to break it, just to detect movement. Any click, any rock, any sense that the sheets are sliding against each other is a rejection. There should be no perceptible give at all.

For a more objective check, mark the edge of the joint across the seam with a fine marker or a scribe line. Load the joint a few times by hand, then look again. If the two halves of the mark have shifted relative to each other, the repair is creeping and will only get worse.

The tap test is quick and surprisingly effective. Tap each rivet head with the handle of a small screwdriver or a light hammer. A tight rivet gives a short, dead, solid sound. A loose one rattles or sounds hollow, and you will hear the difference immediately once you have tapped a few good ones for comparison. Work along the whole row, not just the rivets you fitted.

Check the mandrels and the grip range

On blind rivets, the retained mandrel head does not add strength once the stem has snapped, but a loose piece of mandrel rattling around inside the joint is a clear sign that something is not right. Pull on any exposed stem with pliers. It should either be gone or immovable.

Grip range is the usual culprit behind weak repairs. Every rivet is rated for a specific thickness range, typically something like 1.5 to 3.0 mm or 3.0 to 5.0 mm on common 3.2 mm and 4.8 mm rivets. If the stack of material you are joining is thinner than the minimum grip, the rivet will not clamp and will sit loose. If it is thicker than the maximum, the mandrel will break before the tail has formed properly. Check this by measuring the stack with a vernier or a rule before you choose the rivet, and recheck it if you added sealant or a backing washer into the joint.

Load testing in stages

For anything structural, a static proof load is worth doing. Do it progressively, not in one big pull, so you can spot the first sign of trouble rather than a sudden failure.

• Light load first: apply roughly a quarter of the expected working load by hand, using a lever or hanging weights, and hold it for a few seconds. Watch the joint, listen for creaks, and recheck for movement.
• Step up: increase in stages, checking after each one. Stop immediately if you see or hear anything move.
• Cycle it: load and unload the joint five or ten times. Fatigue and creep show up under repetition, not a single pull.
• Recheck after use: on a vehicle or a machine, inspect again after the first journey or the first few hours of work, and again after a week.

If it fails, drill it out and start again

Do not be tempted to run a second rivet alongside a loose one and hope. Drill the failed rivet out with a bit matched to the rivet shank, commonly 3.3 mm for a 3.2 mm rivet or 4.9 mm for a 4.8 mm, and use a drill stop or a sleeve so you do not punch through the rear sheet. If the hole has elongated or cracked, step up one rivet size, use a backing washer, and rivet the joint again with the sheets firmly clamped. A riveted repair is only as good as its weakest point, and a few minutes of testing now will tell you exactly where that is.