Both of these posts are absolutely correct to a degree but have some technical inaccuracies that I would like to point out:
If the threads stripped due to soft aluminum using a regular bolt, why would anyone think that the HeliCoils will hold any better in the same soft aluminum?
First off most of the time Rohmell I don't think we have thread failure on the China girls because the base being metal is aluminum. It is usually due to improper mounting techniques, over torqueing and aluminum metal fatigue.
- Improper Mounting techniques:
The engine must be mounted securely to avoid thread failure from vibration.
- Over Torqueing:
Some people think the tighter the better use a torque wrench. Acorn nuts on cylinder studs are the worst offender if they are bottomed out you will transfer the torque to the crankcase holes and strip them out.
- Metal Fatigue:
Even when properly torqued over a period of time threads will fail because the torque is being carried on the pitch of the threads only and friction of torqueing and the elasticity of cast aluminum they will fail.
Realistically due to the material that Helicoils are made of and it being more elastic and the fact of them being spreading out the load of the torque more efficiently simply due to their size they will last longer and be far stronger if properly installed.
Helicoil is basically spring steel. You install it in a slightly larger hole. The slightly larger hole you then cut treads into for the helicoil. You put red lock tight into the threads on this hole only. There is a tool that hooks onto the helicoil that gently grabs it to thread it into the prethreaded hole. Now viola you have a hole threaded with spring steel that is your original fastener size. Note this section is not at all where red lock tight goes. Although blue lock tight which is the serviceable grade would be fine here for our application.
I actually put red lock tight in holes at the engine block where the cylinder head studs go. As I am competent not to strip mine out doing something wrong. and I will have no need to take them out of the block ever any way.
helicoil‏ - YouTube
When I had my shop I was a Sthil, Echo, Target, Honda, Tecumseh and Briggs dealer and repair station. And had almost a daily need for HeliCoils. One of the things I kept when I sold the business was my HeliCoil rack that has virtually all sizes of coils in 3 different lengths, drill and tap, installation tool and tang breaker for each size in both metric and SAE all in individual trays. Out of 6 techs I had only 2 I though competent that I would allow access to the racks because they understood when a coil would make a good repair and when it wouldn't, how to do the repair right and make sure all of the tools got back in each tray.
A couple thing to consider when using a coil to repair threads (especially on these engines) is if you have enough wall surface (or meat) to drill and tap into as well as Depth to insure a good repair. For motor mounts both front and rear you defiantly have enough depth but you will might be pushing it in sidewall surface on one axis after dri9lling and tapping, For cylinder studs I am not sure on side wall surface area but you might get in trouble on depth.
As I said I have not considered HeliCoil repairs for China Girls in favor of either filling it with a TIG or Alumi-Welding and then locating the hole with a transfer punch and drilling and tapping a new hole.
Most places that sell HeliCoil brand coils made by Emhart Teknologies sell coils with the proper drill and tap for that coil I suggest not using cheap taps from kits as the pitch can be slightly different and that you use that specific drill recommended for that tap or you will get a sloppy repair. Also using proper tapping techniques such as not trying to hog through the casting because it is soft aluminum and instead turning the tap only 1/4 turn clockwise then an 1/8th back to break the chip will give a good thread. Knowing the difference between a through tap and a bottom tap helps and use TapMatic thread cutting fluid and not oil.
Below is a PDF that has everything you need to know about HeliCoils and how to properly install them:
HeliCoil Specifications
Some of us have road more than you might think?
