Steel bolt, alloy thread: sooner or later that will be a mix that ruins your day, especially if you're heavy handed. Care must be taken when tightening bolts and screws into alloy threads, as over tightening can pull the thread right out of the component. If it is something that is tightened regularly, such as an oil drain bolt, the likelihood of a failure increases. The good news is that the thread can usually be repaired back to its original size for a lot less than a new crank case or engine cover.
Although fatigue does play a part in a thread failure, an overzealous wannabe mechanic is usually the reason behind something like this. Not so long ago somebody cam to us with his motorcycle the alloy thread was pulled right out of the clutch cover when the inspection bolt was over tightened. It can be repaired, but cutting the next size thread is not a good option. You would go from 6mm to 8mm in this case, which is a 33 percent increase! A thread repair maintaining the original 6mm diameter is a much better option.
For this case we have chosen a Recoil stainless insert to repair the thread. You can also use Helicoils, Timeserts, Locinserts or any other to repair threads in different applications. The correct size drill is critical to a successful repair and is chosen depending on the type of material the insert is going into. We are using a 10mm drill to repair this 6mm metric thread. Obviously, the drill needs to be straight and and, since we are sealing oil, it needs to be perfectly perpendicular to the sealing surface of the alloy. The next step is to cut a new thread using the Recoil tap provided in the kit. This tap is over 6mm in diameter and just the right size to allow the stainless steel insert to fit in the tapped hole and regain that 6mm by 1.0mm pitch thread. Some CRC or kerosene can be used to lube the tap as you cut the thread. A little on the drill bit in the previous step will also help a clean neat cut. Remember to keep the tap straight, as you only get one chance at this!
The best thing to do if you are repairing a thread on an engine cover is to remove the cover from the engine. That way you won't get any swarf (metal fillings) inside the engine during the repair. If you must complete the repair with the cover in position, wipe a large amount of grease over the drill biit and also grease up the tap. The majority of swarf will stick to the grease and be extracted from the engine. It's a good idea to drain the oil and flush out as much as you can since there will always be some metal left behind. Not ideal inside an engine!.
Screwing in the stainless insert might seem simple enough, but it can turn into a disaster if you are not careful. As you turn the insert tool, the insert coils up and the diameter is reduced. When the tool is released, the insert springs back out and grips the alloy thread, locking the insert in position. If you apply too much downward pressure as you turn the insertion tool, you run the risk of the leading part of the insert to jump a thread. As you turn the tool, pull up slightly and this will keep the insert in the correct thread.
The insert needs to be wound in far enough to sit flush or just below the surface of the alloy. In this instance, we are repairing an oil level check bolt hole, so the bolt that screws into the repaired thread needs to seal oil. To do this, the insert definitely needs to be below the sealing surface of the alloy so it does not interfere with the sealing washer when the bolt is inserted. This is vital if you are repairing an oil drain bolt thread in the crankcase, as well as drilling and tapping a perfectly straight hole to begin the repair.
The small tang on the bottom of the insert which is gripped by the insert tool needs to be removed once the insert is in position. There is a small nick near the end of the tang to aid removal. If you turn the insertion tool 90 degrees, or use a small punch, you can easily snap off the tang with a quick tap. With the cover removed, this is no problem, It can't be done if the cover is in place, however, as it will leave a very hard piece of stainless steel floating around in your engine, just ready to do huge amounts of damage.
If it is not possible to safely punch out the tang after the insert is in position, you can use pointy nose pliers. Using very thin (you may need to grind the pointy nose pliers down for a 6mm thread) pointy – nose pliers, reach through the insert and grab the tang. Hold firmly and wiggle the pliers in and out of the thread. Do not try and force it to break, just keep wiggling. After 20 or so wiggles, the tang will neatly snap off the end of the insert and can be removed.
Once the insert is fitted, it is quite difficult to remove. You can not use the insertion tool to unwind the insert, as this just expands the insert and it jams in the thread. However, you can use a removal tool to take out the insert, even if the tang is broken off. This is basically a T-piece with a hardened V on the bottom. The leading edges of the V are quite sharp and grip the stainless insert. As you turn the insert backwards from the top, it contacts in size and screws out. Perfect if you have jumped a thread and need to re-fit the insert.
Inserts are made for many different thread diameters and pitches. You have spark plug taps, designed to screw into what is left of the damaged thread and then cut away the alloy with out the need for a drill. This keeps the tap at the same angle as the original and reduces the likelihood of a crooked spark plug. Unfortunately, they are not available in other sizes. They also have inserts available in different lengths. Always use one of the correct size and if you are unsure of what to use, seek advice.Tag: Thread-RepairThread-InsertThread-DamagedRecoilElicoilsTimesertsLocinsertsMetric-ThreadScrewsBoltRepairMechanical