Winter Projects 2019/20 - Pt 7 - Intake Manifold Thread Repair

Last time we discussed the use of magnesium in the engine bay of the Porsche 944S, and also looked at some unexpected corrosion damage inside the cam timing gear housing. Now we'll get into the repairs that were done to the magnesium intake manifold.

March 2, 2020. This was a challenging repair, from the standpoint of work holding. To do a good thread repair you have to ensure that both the drill and the tap are perfectly in line with each other and in line with the original hole. The most effective way to do that is to hold the workpiece in a vice and use a drill press for both operations, but the hole is often in a location where you can't do that;  then you have to hand drill & tap instead.

Two of the stripped fuel rail mounting holes were located in the flat intake port flange, so I was able to do the drilling and tapping for those holes using my mini drill press by clamping the flange down to the little 6 x 6 inch table. The manifold was huge compared to the tiny table, and I had to come up with a creative way to clamp the flange down in two different positions. One thing I had going for me was that the magnesium intake is super light; I think it only weighs two or three pounds.

Drilling the front fuel rail mounting hole, beside intake runner #1. At the back you can see the C-clamp used to hold one side of the intake mounting flange. In the foreground, the aluminum bar  clamps the other side of the flange. Note the stack of washers under the bar, to the right of the bolt. Even though the intake is big and bulky on this tiny drill press, it only weighs a couple of pounds, so this arrangement held it pretty firmly.

I found that I could use a C-clamp right at the edge of the table, and then  position a 3/16 piece of aluminum bar using a single bolt through a slot in the table and a stack of washers under one end to clamp down against the flange at the other end, kind of like a machinist's toe clamp and step block arrangement. This worked really well, because the intake is so light it didn't want to move around a lot. I could clamp it lightly, and easily move the intake around to center the drill perfectly over the hole, then tighten it a bit more to lock it in place. After drilling, I put the tap into the drill chuck and tapped the hole by manually turning the chuck, so the threads were accurately aligned with the hole. The screw thread inserts were installed at the end, after all of the holes were drilled and tapped.

Threads were tapped immediately after drilling . The tap was placed in the chuck and turned manually to take advantage of the chuck's perfect positioning relative to the hole. The STI's were installed later, after all the drill press operations were done.

The manifold was re-positioned and the second threaded hole along the intake port flange was drilled and tapped the same way. Here, a bolt has been passed through an injection port along with the aluminum bar clamp arrangement to clamp the flange down to the table.

Drilling and tapping the hole for the other three holes had to be done by hand; since there was just no way to secure the manifold onto the drill press table in a way that would present the holes to the chuck.  The drill press is so small there wouldn't  have been enough vertical clearance between the table and chuck to get it in there anyway.

 I carefully drilled out the hole a size or two at a time until I got to the 6.3mm tap drill size. By drilling in small steps the hole stayed straight. But I needed something more foolproof to tap the hole straight. Luckily, there is a flat registration face at every hole location. I made a tap guide by taking a 5/16" coupling nut, drilled out enough to just clear the threads of the tap. I ground one end flat and square to its length. It worked really well. By firmly holding the flat face of the guide against the flat surface at the hole, I was able to tap the hole at 90 degrees.

I was able to tap a straight hole by hand using this improvised tap guide made from a 5/16" coupling nut. It is held firmly against the flat surface while the tap is started. After the tap is in a few turns it will stay on course and you can remove the guide.

 

The new threads came out great. It's quite true that magnesium machines really nicely. The next step is to install the coated STI.

In all, I installed five STI's (screw thread insert) in the intake manifold: three in the fuel rail mounting bolt holes, one in a fuel damper mounting bolt hole, and one in a throttle cable mounting bracket hole. I was tempted to proactively install STI's into every threaded hole , but the coated inserts are quite expensive at around $6.00 each, and the threads in the other holes were all in perfect condition, so I decided to leave them alone for now. Except for the throttle body mounting holes, I can easily drill and tap them in place in the engine bay, now that I have my tap guide.

A bottoming tap was used for the blind holes. The tap guide can be seen in the upper right corner.

All of the STI's were installed after drilling and tapping operations were completed. A prewinder tool was used, but is not shown here. The STI's were installed one to two threads below the surface, shown below.

As with most STI's, breaking the installation tang off with a punch is the final step.

Even though there were a couple of obstacles to overcome, it was really a lot of fun to think up creative ways to hold the manifold and position the tap for hand operations. The intake turned out great. When I installed it later in March and bolted up the accessories I used new yellow zinc fasteners and zinc anti-seize. The bolts were tightened to the factory specs (10 Nm) without any problem at all.

Thanks for dropping by, I hope you enjoyed what you read. 

Next time, I install the new belts and rollers, and repair some cracked and exposed thermo fan switch wiring.