Winter Projects 2019/20 - pt 5 - Repairing the AFM mounting threads.

 Feb 16, 2020 - Gretchen's engine had many threaded holes that were stripped. Repairing the mounting holes for the Air Flow Meter with Heli-Coil screw thread inserts and replacing the mismatched hardware with the correct parts was next on the list of things to do. 

Drilling out the stripped holes to take a Heli-Coil STI

The die cast aluminum AFM on a Porsche 944S is fastened to its bracket by three shock-mounted bolts threaded directly into it. In Gretchen's case all three holes were stripped, and they all had the wrong bolts installed as well. The AFM was removed from its mounting bracket, cleaned, and then each of the three threaded holes were drilled out, tapped, and a stainless steel STI (screw thread insert) installed.

I had made up my mind early on (in fact even before buying the car), that only the best parts and hardware that I could afford would be used on my Porsche. I did all kinds of research prior to buying Gretchen, and my thinking, based on some early material I had read, was that only a Time-sert would be good enough for any stripped threads I came across. But after further reading and research, particularly after watching this video, it was obvious that the Heli-Coil was actually the far superior STI and the right way to go.

Once each of the holes was drilled to the correct size for the M6 screw thread insert, The specially-sized STI plug tap was mounted in the drill press chuck, and a few threads were cut by hand-turning the chuck and applying gentle downward pressure on the quill. These initial few threads would act as a guide for the hand tapping operation to follow. The AFM was then removed from the drill press, and the tapping operation continued by hand. A bottoming tap was used for the two blind holes so that the threads would reach to the bottom of those holes, allowing the STI to be threaded in properly. I used WD-40 as a drilling and thread cutting lubricant; it's excellent on aluminum.

The initial threads made on the drill press were continued by hand. A bottoming tap was used on the blind holes.

This photograph shows the difference between a bottoming tap (upper) and a plug tap (lower). Notice the differences in the profiles of these tools. The bottoming tap makes full threads much closer to the bottom of the hole. This is necessary when an STI is installed into a relatively shallow hole, and there's not a lot of free space below the bolt.

The end result of the tapping operation on one of the blind holes. You can see the light chamfer at the top of the hole. this makes it easier for the bolt to engage the threads.

After drilling and tapping the holes, the inserts could be installed. In a small diameter Heli-Coil like an M6 or M8, it is necessary to pre-tension the insert, or wind it up slightly, so it can be threaded in. The inserts are actually slightly larger than the hole they are installed in. Once threaded in, the STI will expand outward against the threaded walls, locking it in place. The Heli-Coil kit usually comes with a prewinder, the mandrel, and several STI's.

The prewinder compresses the insert slightly as the mandrel is wound into it, prior to its entry into the tapped hole. 

I've found it's easiest to install the STI if you make it project out of the prewinder by a thread or two, then thread the whole prewinder/insert/tap wrench assembly into the threaded hole, just until the prewinder is snug against the casting.

Hold the prewinder steady and continue to thread the insert in until it's free of the body of the prewinder; you'll feel it spring free.

At this point you can remove the prewinder and thread the STI in the rest of the way using only the installation mandrel. Continue until the top coil of the STI is two threads deep in the hole. This prevents the top coil from being pulled out of the hole when the bolt is inserted and tightened down.

Finally we have the finished hole. You can see the V-shaped tang on the bench below the hole. The tang is removed after installation by placing a round flat punch against the tang and rapping with a small hammer to break it off. You may need to get it out of blind holes with a blast of compressed air.

Test fitting with a new M6 bolt; a perfect fit. A properly installed Heli-Coil will withstand the full rated torque of the base material.

Close-up of the broken-off tang.

If you're careful, use the right tools and the right insert, installing a Heli-Coil is easy. As long as the base metal is sound, you can apply the same torque to a repaired hole that the original thread/fastener combination was rated for. In some circumstances they are even used in brand new castings to provide the original thread. It's also possible to remove and replace them, but it takes some skill and practice; I tried it once and was not successful!

Next time, we will be repairing a few stripped holes in cast magnesium. The cam cover, cam timing pulley/distributor covers, and intake manifold are all made out of magnesium on the 944S. Specially coated inserts have to be used to avoid galvanic corrosion. This interesting metal has strengths and weaknesses which we will explore in detail!