Thread renewal is a way of replacing a stripped thread with a coil of (usually) stainless steel carefully manufactured to mimic the dimensions of the original thread once installed.
It is a relatively simple procedure once you know how ( as are most things) but could be daunting to someone with little knowledge and some stripped threads!
There are various kits on the market, Helicoil being the best known (and usually the most expensive) thread insert. Your local tool store should be able to help or there is always the ubiquitous eBay.
Usually each kit is for a dedicated size of thread.
The threads being renewed here are the 6mm x1mm for the rocker box screws.
OK, HERE WE GO!
YOU WILL NEED THESE TOOLS. Thread insert kit, drill, hammer, tap wrench, small screwdriver, thread lock solution, grease.
Make sure to cover everything with cloths or paper towels pushed into all the crevices to stop the swarf getting into places it shouldn't.
It still will, so be anal when cleaning up after you have finished the job!
Check out how deep the drill has to go with the small screwdriver.
Mark the depth on the drill with some masking tape or a drill collar if you have one.
This is the bit that usually goes wrong so check, check, check again. Borrow a friend's eyes so that you check from the top and they check from the side.
You need to be 90 degrees both ways!
Hold the drill very firmly and use a slow speed because it will likely try to grab!
A dab of grease on the flutes will help lubricate it and also trap some of the swarf.
A little grease on the end of the tap will help lubricate it and trap some of the swarf.
Loads of swarf trapped but there will still be more down there. Use the small screwdriver with some grease to remove the rest.
You'll need your friend again when you go to tap out the hole. Very easy to run off with a tap so make doubly sure it's at 90 degrees.
When you have finished tapping out the hole, clean it free of grease before proceeding any further.
I use an aerosol carb cleaner to blast the threads clean. The reason for this is so that the insert is held firm by the thread lock solution.
The new insert coil is slipped over the installation tool with the coil tang towards the bottom of the tool.
In this instance we want the top of the coil to be slightly below the surface so the installation tool collar is set as above.
Put a little thread lock solution in the outside of the coil before inserting it.
Screw the coil into the hole slowly down to the stop collar.
REMEMBER. YOU CAN SCREW IT IN FURTHER IF NEED BE; YOU CANNOT SCREW IT BACK OUT IF IT'S TOO DEEP!
Once the coil is in place use the thin metal rod to knock the installation tang off the insert.
Finally, check that the intended bolt fits the new thread.
Only another seven to do! To give you an idea (admittedly I've done this kind of thing before) all eight were done in about half an hour.
Time for clean-up and the job is done!