Riveting using pop rivets is straightforward. But if you need to put a rivet at the bottom of a channel, such as you might find on a yacht mast, things get a bit more complex. Most rivet tools have a fairly wide nose and won’t get into the ~20mm wide channel you find on smaller masts. And if the rivet tool can’t press down on the rivet, it can’t do it’s job. Fortunately, there’s a few different ways to solve this tricky problem.

Stack Of Washers

The first trick to know about is using a stack of washers or nuts placed onto the rivet mandrel to act as a spacer. This allows you to have the rivet tool a little bit away from the rivet itself, but still have it apply the necessary pressure. However, this technique is limited to shallow channels. Most rivets have 25mm of mandrel, and rivet tool often need at least 20mm of mandrel inserted inside before they can even start to pull – and if they’re only grabbing onto the very end of the mandrel, it’s likely that it’ll just shear right off.

Rivet Tool, meet Angle Grinder

Another approach is to use an angle-grinder to narrow the nose of your rivet tool so that it will fit in the channel. The tool is in compression during use, so it’s possible to remove some material without compromising the tool too much. However, if you try to remove too much material, you’ll start to expose the inner workings.

Rivet Transplant

The final approach is possibly the most elegant. The main difficulty with riveting in a narrow deep channel is that standard rivets (eg. the size you want to use) typically only have 25mm mandrels. It’s not easy to buy normal sizes with extra-long mandrels. But it is possible to buy long rivets (like you’d use to rivet two 20mm plates together) and transplant their mandrels into your rivet bodies.

I bought a large pack of 4.8 x 50mm rivets from part-on.co.uk for £5. Using a vice to lightly hold the rivet body, it was easy to tap the long mandrel out with a hammer. The same method was used to remove the mandrel from the 4.8 x 8mm rivet I wanted for the job. Finally, the long mandrel was tapped into the short rivet, thereby producing a super-long mandrel variant of the desired rivet size.

The one drawback with this approach is that the “weak spot” on the mandrel where it breaks off is not in the right place for a smaller rivet size and will leave a lot of mandrel protruding. This isn’t a problem, so long as you have enough space behind the rivet to allow you to tap the mandrel all the way through and out the back. It’s worth double-checking this after you’ve drilled out the holes for the rivet to go in.