Thread Repair Kits
At some point, you're going to experience stripping a thread (damaging a threaded hole so that you can now longer successfully screw a bolt into it). There are several methods to fix this, the one you choose will depend entirely on what is the most suitable for the job, or what tools you have to hand. The easiest method is probably to drill the hole out and re-tap it to take a bigger thread size, if possible. Next, you could get the hole welded up, then re-drill and tap as required. Thirdly, you can try repairing the thread using a thread repair kit.
There are two main repair kit types; those that have inserts that look like small springs and those that have solid threaded inserts. They are usually referred to as Helicoils or Timeserts respectively. These are trademark names, and genuine Helicoil and Timesert kits are good, but very expensive. Fortunately there are some cheaper (look alike) kits available, but are they any good? I tried a couple, and here's how I got on.
If you look on Amazon and Ebay, you’ll see many thread repair sets like this.The kit pictured is a Bergen branded item, bought for around £30. It covers the most popular
metric sizes:- M5 x 0.8 M6 x 1.0 M8 x 1.25 M10 x 1.50 M12 x 1.75 and contains a drill bit, tap, insert driver tool, tang removal tool and a few inserts.
The Kennedy kit (the red boxed kit below) cost around the same price as the Bergen set. This kit only covers one size, M7 x 1.0. A fairly unusual thread size, but one that Suzuki chose for the GS500 cam cover bolts.
An essential item missing from both kits is a tap wrench. So, that’s an extra item you’ll need to buy if you don’t already have one. It is also worth noting that only the Kennedy kit comes with any instructions.
The kits in use.
I can’t claim that I’ve exhaustively tested either of these kits. I’ve needed to repair one M7 thread with the Kennedy kit - which worked perfectly. And one M6 thread with the Bergen - which didn’t go to plan.
The thread repair with the Bergen set started off Ok. Hole drilled as required, tapped with appropriate tap and then insert screwed into place. Unfortunately I then found that I couldn’t remove the driver tool. It should just pull out easily, but this definitely wasn’t going to happen. The only way the driver tool was going to come out was by unscrewing it, which meant that the insert came back out as well.
After some head scratching as to what was going on, I measured the diameter of the driving tool and found that it was 5mm - the same as the nominal inner diameter of an M6 thread. So, as the insert was screwed in, it was also jamming on the shaft of the driver. A check of the Kennedy driver revealed that it only measured 4.5mm in diameter, so clearly, the Bergen driver didn’t have enough clearance. A quick touch on the bench grinder reduced the shaft diameter of the driver to 4.5mm, and the next insert went in without trouble. The slightly modified driver is shown below. The silver area is where the shaft diameter has been ground down.
So, in summary both sets worked, but with the cheaper sets, check that the diameter of the insert driver shafts are smaller than the inner diameter of the thread you’re trying to repair!
One more thing:-
Well two or three more things that the instructions (if provided) don’t mention:-
It is important to make sure that you drill and tap in line with the existing hole, otherwise things won’t want to bolt back together squarely.
When tapping, after every half a turn forward, back the tap up to break up the metal chips that are forming. If you don’t, you run more risk of the tap jamming and possibly breaking. You don’t want to break a tap - they can be a nightmare to remove.
Use tapping or cutting fluid to ease the way. Toolstation do an aerosol drilling and cutting oil for around £4 a can which does the job nicely.