Fix for cracked mounting posts

Nicely done, rickoshay.

I have had success with a somewhat different method which I will describe if anyone is interested.

bf.....absolutely.  There's never too many approaches.  Skill and comfort level vary greatly among us.  Various approaches to a fix is a plus.  Your input would certainly be appreciated.

Jack

My method seems to work best where the threads inside the mounting posts are merely stripped out,* but will also work if the post is cracked, as in "rickoshay's" example.

Essentially, I drill out the hole and clean the debris out as best I can. If possible, I try to enlarge the bottom of the hole just a bit so that is is wider at the bottom than at the top. Then I select a screw to replace the original. The original is often a coarse-thread screw and I prefer a machine screw with finer threads, but you can reuse the original if you want. Then mix up a small amount of virtually any two-part epoxy putty or adhesive. I have used several types successfully, from the 5-minutes types (read half an hour for a full cure) to J.B Weld to Marine-Tex even. Fill the hole almost to the top with the near-liquid epoxy. Use a tooth-pick to get the air out and to achieve good coating of the inner walls of the post.

Now here's the trick. Some of these epoxies cure as hard as rocks -- much harder than the plastic post itself. Consequently they are very difficult to drill and tap without doing more harm than good. So I coat the screw threads, including the undersides of the heads, with ordinary household Johnson's paste floor wax, which serves as a parting compound. You just need to coat it well. One way is simply to drive the screw into the wax, remove it and wipe gently. You only want a thin film of wax, without any globs or chunks.

While the epoxy is still quite liquid, drive the now-waxed screw into the epoxy almost all the way to the bottom of the screw-head. It is best if you can keep the screw head a 32nd of an inch or s0 above the epoxy, but if you can't, the wax should do the job. Take care to keep the screw  centered as closely as possible. Brace it if necessary to keep it upright. When the epoxy has cured, gently twist out the screw and wipe any remaining wax off.

With any luck, you now have a post with a perfectly "machined" thread cast into it. Some of the better epoxies cure so hard you can pretty much cast a "nut" that behaves just like a steel one. You can gently screw into and out of this "nut" many times, which is a useful feature where it is necessary frequently to remove the floor of the car -- to change a bulb or add little people to passenger cars, for example. You can even secure the screw from backing out due to vibration by using a little Blue (temporary, rather than permanent) LocTite thread locker product.

If the epoxy won't adhere to the plastic, you may be out of luck, although by making the bottom of the hole wider than the top, and roughing things up a bit inside the hole, you may get an acceptable mechanical bond if not a chemical one.

Always test this procedure beforehand on a piece of scrap wood, by drilling a hole in it and then doing the epoxy thread-casting trick. In my experience, the screw will back out of the casting with virtually no effort -- which is good because one doesn't want to place much stress on the plastic when it comes time to try it for real -- but you need to know that it will work. A test also allows you to get a feel for the timing and general behavior of the materials that you intend to use. You might also make a test run by placing a small blob of the epoxy on the actual plastic in an inconspicuous place to see whether adhesion is likely.

If the plastic post is badly cracked, you can always use rickoshay's method, or a combination of his and mine. Mine generally avoids any filing or sawing (read weakening) of the web of plastic that sometimes connects the post to the body of the car. I often use small regular clamps, or wedge a block of wood in to act like a clamp, but my favorite "clamps" are the tiny rubber bands that young girls use to make little pony tails in their hair. I think they are called "tangle-free rubber bands" and can be found in most places that sell hair styling supplies. Dentists, especially orthodontists, use similar small rubber bands. Thin O-rings might work too. They might also be used to keep the bottom of the screw head up out of the mix -- you can always cut away an O-ring if necessary.

In summary, filling a hole in plastic is scarcely new, but using the wax to enable casting new and better threads in epoxy just might be. Good luck -- and always do a bit of testing first. This method has worked for me many times with no failures so far. No guarantees though.

* I have run into several passenger cars where it seems as if the original screws were driven so hard that they have already weakened the plastic threads to the extent that there is no hope of reusing them. One can always try substituting a larger screw, but there is relatively little plastic material to work with and often that method does more harm than good.

Thanks for the ideas guys. I too have a tender, a 6466, that I bought a while back that had the mounting posts cracked and broken. I tried regular glue and had little luck. Epoxy it is.

Mike