teefly-RCU

trying to repair a stripped muffler bolt in a US41. the newer pitts style muffler came on the engine and before running I used red locktite. have about 30 flights and the last 10 ended with muffler falling off! finally drilled out the threads in the muffler and got 2 flights before it came loose. now.......lost threads in one engine muffler mounting holes.
bought heli coils (5/16-18x1.5) and the label on it says to drill 8.3mm or 21/64. the auto parts guy says just drill it and the coil will expand and hold. i thought all heli coils HAD to be tapped and for high vibration applications only the "screw lock" type would hold. am i right or is the auto parts guy just guessing at it? help!

IowaBoy

Yes, to install a heli coil insert you first drill out the hole to the recommended size, then tap the hole with the supplied tap. This will create the proper size threads for the coil. Next screw in the coil until it goes in about 1/2 turn below the surfase and break off the tang at the bottom of the hole with a small pair of pliers or punch. If done properly, this will create a new thread that is actually stronger than the original.
I've installed dozens of these in threads ranging from 4-40 to 3/4 in, and they work great. This is also a terrific way to make extremely strong threads in glass filled nylon engine mounts.
Hope this helps, Wes

teefly-RCU

thanks Wes. I plan on going to Nappa tomorrow and get the right stuff. my previous visit was to Auto Zone and all they apparently carried were the heli coils without the installation kit (drill bit, tap, and inserting tool).

S D FlyR

My Feedback: (1)

It sounds like Iowa Boy has got it right. I would Question the 5/16 x18? Are you sure about the size I think 1/4 x28 would already be oversize. If you need to use 5/16 at least get the fine thread x24 size. It will be much easier to keep the bolt tight. Hope this helps Steve

teefly-RCU

thanks guys! for sure on the fine threads.

Flypaper 2

Muffler bolts on a Quadra are 3/16 24 or 10-24.

jjmiller1

I was gonna' say "I've never heard of 1/4-24, there's 1/4-28 and there's 1/4-20". As a machinist, I've installed thousands of helicoils, and I still think to myself "What a great idea these were". They're often tougher and longer lasting than the thread that they replaced. We use them in brand new aluminum parts for longer lasting threads than aluminum can provide. I wish I'd invented them, or anything for that matter.

JJ

raverys12

Make sure your tap says helicoil on it. Remember the pitch of the thread is always the same as the one you are replacing. Actually you are just enlarging the original hole and tap size so that you can retrofit a secondary thread in the hole. Actually if you look at the specs. The new one should outlast the old ones. Less wear and tear on the stainless replacement thread. The kit should include the recommended drill size, Proper tap, and a helicoil and insert tool. One other important thing. Some times the tang that is used to drive the helicoil is still down inside the hole. That is designed to break off when you are done. Make sure it's not still in the bottom of the hole, could cause problems. Good luck, Bob.