Stripped Mag mounting hole threads on engine casing

[Steve Brown]

I had to send end my E/P-Mags for some service and was just reinstalling them yesterday.

My installation has studs for one mag, bolts on the other. Got the mag with the studs back on with no problem.

When starting to torque down the other mag, the bottom bolt started turning freely. I was torquing them progressively, so this was at less than 100"/#. Metal from the casing side threads was evident on the bolt threads.

There seemed to be no damage to the bold and the hole apparently goes all the way through the casing. I tried threading the bolt in past the stripped section (no mag) to assess the condition of threads deeper in the hole.

The bolt really won't go in with the force I was willing to apply. I'm thinking I need to clean up the threads with a tap and use a longer bolt, but I'm not sure if this is right.

Also, I'm wondering why this mag has bolts, and whether or not I should just replace the bolts with studs in the process of doing this repair.

Advice on this greatly appreciated

[avee8tor13]

I don't think a longer bolt is the answer. However, there is a work around using heli-coils. This technique is used on cylinder heads for spark plug threads.

I had this happen where the alternator bracket mounted on the bottom of the case. I purchased a heli-coil kit. I drilled out the old threaded material, tapped the new thread size (larger than the original bolt size) then screwed in the new heli-coil (brought down to the original thread size) with a little locktite.

Do you have access or enough room to get an angle drill to the area where the holes are located?

Did you say the thread area goes all the way through the case?

[RV8Squaz]

Steve,

First be very careful cleaning out the threads with a tap. It will be necessary but be careful. Ask me how I know...
Recently I was replacing my spark plugs. They have been getting more difficult to get off and on so I used one of the popular thread chasing tools (a tap basically). One of the thread chaser flutes caught the edge of the helicoil and sprung part of it out (I learned that even new cylinders come with helicoils installed). Ughh. I realized the task was now beyond my pay grade so I called for help. My engine overhaul shop (JB Aircraft Engines), a local enigine shop (lucky for me less than 30 minutes from the hangar), the local CAF squadron, and ECI were all extremely helpful. I saw that replacing a helicoil or installing a new one is not real difficult. The only snag we had was when the old helicoil sprung out, it damaged the first thread and thus made it diffucult to install the new helicoil. We had to clean the spark plug hole with a special tap and it was wanting to cross thread. We took off just a little of that first thread and it allowed the new heicoil to go in. Just realize the helicoil for a sparkplug hole and tools required are special and are specifically designed for threading spark plug holes.

For your situation it will be a lot easier, since it is a smaller hole. A thread repair kit (tap and helicoil) may be available at your local auto parts store. The hole may already have a helicoil in it. However, I would start by calling the engine shop you got your engine from.

Good luck. Let us know how it turns out.

Jerry RV-8 N84JE

Originally Posted by Steve Brown

I had to send end my E/P-Mags for some service and was just reinstalling them yesterday.

My installation has studs for one mag, bolts on the other. Got the mag with the studs back on with no problem.

When starting to torque down the other mag, the bottom bolt started turning freely. I was torquing them progressively, so this was at less than 100"/#. Metal from the casing side threads was evident on the bolt threads.

There seemed to be no damage to the bold and the hole apparently goes all the way through the casing. I tried threading the bolt in past the stripped section (no mag) to assess the condition of threads deeper in the hole.

The bolt really won't go in with the force I was willing to apply. I'm thinking I need to clean up the threads with a tap and use a longer bolt, but I'm not sure if this is right.

Also, I'm wondering why this mag has bolts, and whether or not I should just replace the bolts with studs in the process of doing this repair.

Advice on this greatly appreciated

[Steve Brown]

On the advice of several, including the guy who built the airplane, I'm pulling the accessory case.

Once I made that decision I had nothing to loose, so I used a little more force and found that the hole was threaded all the way through. I went and bought a longer bolt to try. With the longer bolt I got it to 100 inch/pounds before it stripped again.

Anyway, for me, pulling the accessory casing is a huge adventure into the unknown in terms of wrenching. However, I learned from a very young age that I can take things apart. Getting them back together can be a different matter entirely

Once I get it off, I may as well replace it or send it out for repair. When its time to put it back on, I will likely have my work checked by a local mechanic before taking flight.

The fact that I only learned that something called a heli-coil existed by reading these posts is evidence to me that I probably should not use one in this situation.

[Radomir]

Just one little suggestion -- when you go to put it all back together, try installing the mechanical fuel pump before putting the sump back on.. it's A LOT easier than putting it on last.. and you can easily ensure that the plunger is engaged properly..(and it's easier to hold the plunger in place... )

Good luck!

[Sam Buchanan]

Originally Posted by Steve Brown

On the advice of several, including the guy who built the airplane, I'm pulling the accessory case.

Once I made that decision I had nothing to loose, so I used a little more force and found that the hole was threaded all the way through. I went and bought a longer bolt to try. With the longer bolt I got it to 100 inch/pounds before it stripped again.

Anyway, for me, pulling the accessory casing is a huge adventure into the unknown in terms of wrenching. However, I learned from a very young age that I can take things apart. Getting them back together can be a different matter entirely

Once I get it off, I may as well replace it or send it out for repair. When its time to put it back on, I will likely have my work checked by a local mechanic before taking flight.

The fact that I only learned that something called a heli-coil existed by reading these posts is evidence to me that I probably should not use one in this situation.

Steve, while you have the accessory case off the engine, make sure the oil pump AD has been complied with. Your local A&P can assist you in determining compliance.

[Mike S]

Good info about thread repairs Here

Carb or acc case, the fix doesnt care.

[Steve Brown]

I went through all the steps of removing the accessory case to send it in for repair. Took out all the bolts and removed all the widgets. When it came time to remove the case, it wouldn't budge. Not the slightest sign of movement.

I contacted the builder to see if there was any reason for that and there wasn't. I talked to a local hanger neighbor and got a recommendation for a local mechanic who works on experimentals. We contacted Brain at Flying S and he agreed to take on the job. I was certain he would end up pulling the engine to get the case off and I was preparing myself for the inevitable wallet draining.

I'm not sure what effort he put into taking the case off, if any, but he ended up installing a heli-coil to fix the stripped threads for the mag mounting bolt without removing the case. The cost was about $130. Very painless.

I put the airplane back together, which meant replacing all the lock washers, the gaskets for the widgets, attaching the oil & fuel lines, checking & double checking the manual for torque, the safety wire, building a tool for the fuel pump bolts, set the E/P mag timing etc.

My original plan was to have Brian take a look at my work before taking flight, but I felt reasonably confident that I hadn't screwed anything up. I did a bit of a run-up in front of the hanger (wife standing by with fire extinguisher) with the cowling off to check for oil or fuel leaks. No leaks and the engine ran smooth.

After a solo trip around the pattern to make sure it would fly, I picked up my wife and we flew to Watsonville for dinner and back. No problems.

For you guys that built your airplane there are no lessons from this, but for me it was an adventure into the unknown.

Based on my skill level, I would have been better off calling the mechanic as soon as I stripped the threads. That doesn't mean advice to the contrary was bad advice, it means "a man has to know his limitations". Several good things came from this adventure:

1-I learned my "call the mechanic" threshold and I found one who is willing to divide up the work.

2-I found out my oil filter adapter has a quick drain on the bottom. No more spilled oil when taking the filter off

3-I learned how to take on a repair job that takes multiple weekends. This was a huge mental hurdle for me, but once admitted that I could finish it all at once I was able to pace myself and maintain some level of mental tranquility.

4-The original job, taking the E/Pmags off, putting them back on, and setting the timing ended up being simple and trivial. Add one thing to the short list of things I'm not afraid to do to the airplane.

[FrankK90989]

I like this story. Great ending!

[chunt0]

Steve,

These threads shouldn't be stripping at such low torque. Are you sure your wrench is accurate? One hundred inch-pounds/8.3 foot-pounds is really low torque.

Charlie