Using helicoil, you can fix stripped threads in aluminum rails. In plastic materials, you should not use it. For plastic, a new insert is usually the best solution.
After pull of old insert, you measure its length for choose the right replacement. Around insert in open hole you can add epoxy or superglue, so that it stay snug in its place.
Fix Stripped Threads with Helicoil and Pick the Right Drill Size
For helicoil-drill size present the right amount are important. You must have cylinder hole of precise amount, so sometimes need to drill it a bit more widely. One way estimate the current hole is by using the size of drill.
Find the biggest bit, that fits, and later search the nearest helicoil-tap drill help to start. Different kits require different drills. For instance, 3/8-16 helicoil go with 25/64” drill for STI tap.
M10 helicoil require drill of 10.3015 mm. For M14 you uses 14.3 mm drill. Some boxes point even 21/64” drill.
Sizes range according to kinds. 1/4-20 helicoil-tap requires precise drill, usually 1/64-inch bigger than the normal thread size. For 5/16 BSW hole a kit can require 8.3 mm drill.
At 7/16-20 STI the tap is too big to accept the helicoil. Sometimes suggests you 8.4 mm drill, with range of 8.27 until 8.48 mm. The outer diameter of such insert is 9.27 mm.
14.3 mm drill for M14 have shank that is the same size. Notably, that between 10 mm and 10.25 mm drill the difference is only 0.1 mm for every side of the hole.
For repairs in steel or iron parts you usually follow the recommended drill size. You can use oil, for instance cutting or motor oil, when you tap. At aluminum parts as intake manifold, drill 1/64-inch less big than suggested works.
For thin materials, whose thickness is less than twice the thread size, standard drill depths can break through the back side. Tape around drill bit help to avoid too deep drilling. Like this the repair stays strong.
