Helicoils How To... (And How Not To!)

I got home today to find my Helicoil kit waiting on the doorstep. I threaded the alloy rear oil seal housing at the weekend and decided it'd be a better idea to repair it rather than buy a new one and risk threading that too! It's cost efficient too, a new housing would've been £20 whereas the Helicoil kit was £15! I got my kit from Chronos Engineering because they sell kits that contain everything you need so you don't have to spend ages searching for obscure size drill bits.

The kits are specific to a certain bolt size and contain around 25 Helicoils, a drill bit, a tap, an allen key, an insertion tool and a small drift all in a nice blue case!

So here's the problem. Basically a classic case of Triumph using the wrong fastener in the wrong place. The alloy is really very soft and it would have been better if Triumph had used a UNC thread like they did on some other alloy parts such as the intake manifold. Instead they used a UNF thread on both this and on the engine front sealing block.

First thing to do was to drill the threads out. I checked the depth of the holes and wrapped a bit of tape round the drill bit from the kit so I didn't go too deep! The alloy was very soft and drilled nice and easily.

The next stage of the process is to thread the hole for the Helicoil insert. The kit comes with the tap but you'll need a tap wrench if you haven't already got one. I borrowed a tap holder from my Clarke Imperial tap and die kit and that worked a treat. With tapping it's always best to take it slow and keep the tap well oiled but when the metal is as soft as this, you can go pretty rapidly!

Once the hole is tapped to the bottom you can remove the tap and run it through a few times. I cleaned the swarf out with light oil from an aerosol to clear it.

Inserting the Helicoil is very straightforward. It's basically a spiral of metal that has a small 'tang' sticking into the middle at the bottom. To fit it you use an insertion tool that is basically a drift with a slot in the end which engages with the tang on the Helicoil. The one in my kit has a depth stop so you can sink the coil down to a set depth. This is secured using a grub screw and the allen key from the kit.

With the Helicoil in place, all you need to do is knock the tang off. The tang is weakened and should knock off pretty easily using the drift from the kit.

This worked well for me on one of the holes but not on the other! On the one hole the drift didn't seem to knock the tang of very cleanly. The last thread ended up sticking out slightly and when I screwed in a bolt it seemed to push the thread down underneath the bolt. This meant that the bolt wouldn't screw fully home and left the Helicoil in a right mess! I had to get it out!

I thought this would be near impossible but it was actually fairly straightforward. You've just got to bend out the top thread until you can get a pair of pliers on it and then the whole thing will unscrew just as easily as it went in. Once the mangled Helicoil was out, I cleaned up the threads again and fitted a new Helicoil. This time I snapped the tang off by gently wiggling the insertion tool forwards and backwards. This worked a treat and after running the Helicoil through with a tap I was happy that the job was done!