Power at Work: Quick and handy repairs for damaged threads
Thursday, April 2, 2009
This is not a simple, cheap do-it-yourself repair. But it can be done with the proper kit, or by your local repair shop
by KEITH BERGLIND
Missing parts are always frustrating and there's nothing more annoying than missing threads. Yes, you try to tighten the bolt into the engine block, but there are no threadsthere – and no way to put a nut on the other side.
Well, back when I was a boy and my dad had this problem with some old tractor, he would just drill the hole out, rethread the hole to the next size larger bolt and move on.
But modern castings do not leave room for this kind of repair. And there are a lot of special bolts, such as head bolts, which simply can't be duplicated locally. I've worked in small towns where the local hardware store, which has the only bolt supply around, doesn't even know what a Grade 8 bolt is. The main hardware stores in my town today do not sell fine thread bolts and nuts.
The use of aluminum has aggravated this problem. Corrosion and over-tightening create many damaged threads. Spark plug holes can be an expensive repair if the head has to be removed.
So, the common thread repair is installation of a threaded insert. There are many brands and designs, most of which are easy to install and work well. Most auto parts stores and engine parts suppliers will sell some brand of threaded insert.
In the metal repair trades, the common name is HeliCoil. (Just like Kleenex, this is both a trade name and a common word.) This name is such a habit to use that mechanics will ask for a HeliCoil and then install a different brand, such as Perma-Thread. To many mechanics, this is just the name of a repair method, not a brand.
In my collection of repair coils, I have two types of inserts: Flexible and rigid. The flexible repair devices look like springs made of square wire. The rigid repair devices are slightly thicker, basically a thin tube with threads inside and out.
Cost. Just for one stripped hole, this is not a simple, cheap do-it-yourself repair. You cannot just buy one insert and install it yourself. There are special tools involved. One basic kit, with a special threading tap, and installation tool and drill bit are required, just to install the first coil.
The first kit includes a few inserts, and will probably last your shop forever. A separate kit is required for every size of threaded hole being repaired. Once you have the tools, you can buy more inserts alone.
Because of the cost of this kit, it is much more practical to just take the damaged part to a local shop, which can install just the needed insert without charging you the cost of the whole kit.
Installation Method. There are three steps for a flexible coil insert – drill, tap and install. In all cases, follow the instructions with the kit.
• Drill: Use the drill size specified in the kit.
• Tap: The thread size will be marked on the tap shank. This is a special tap.
• Install: Screw the proper insert onto the installation mandrel. Then turn the insert all the way into the newly threaded hole.
• Remove drive tang: Use a thin punch to strike and snap off the short drive tang.
If there ever is a need to remove this insert, it can be "un-screwed" from the top end.
The rigid insert, which is commonly used for repairing spark plug holes, is usually a similar step process: drill, tap and then install.
The main caution with installing spark plug inserts is keeping the drill and tapping chips from falling into the engine. Use lots of grease on the drill and tap to collect the chips. When the flexible inserts are used in spark plug holes, the drive tang must be collected. The best way is to use thin needle nose pliers to twist off the tang. BF
Keith Berglind is a licensed heavy-duty mechanic.