Heli-coil

pilotlaw · Post by **pilotlaw** » Wed Oct 18, 2006 12:43 am

Anyone out there have any experience in this?

During annual the other day, the bottom plug on the No. 5 jug of my O-300D wouldn't budge. After oiling it down, I applied a fair amount of torque and felt the plug "break loose" (last install was not mine and no anti-seize was used, I would guess). In fact, the heli-coil, or adapter, or sleeve in which the spark plug resides was backing out of the cylinder.

Mech helping me with the annual had never seen it before and after some calls, decided the jug needed to be removed and replaced.

After some later research on my own, I found a number of people that have experience with reinstalling these heli-coils on the cylinder without removing the cylinder. While I do not want to be penny foolish, I also do not want to replace a cylinder that is in otherwise good condition if I do not need to.

Anyone have any suggestions?

Bruce Hain
'54 "B"
26263

Post by **Bruce Fenstermacher** » Wed Oct 18, 2006 2:36 am

Yes this happen to me but with my A-65 in my Cub which has similar cylinders.

The spark plug helicoil is spun into the cylinder and locked there with a round rod similar to a round bronze welding rod which is driven into a hole drilled between the cylinder and the helicoil which locks the helicoil in place. You more or less stripped out the helicoil destroying the rod. Look close at the other spark plug helicoil in the cylinder and you will probably see the round rod I'm talking about.

First use some heat to remove the spark plug from the helicoil. Then looking close at the helicoil find the half round hole and clean out whats left of the rod. Do the same to the threads of the helicoil. Then do the same to the cylinder.

Test fit the helicoil back into the cylinder, looking at how well it spins back in. If all seams OK it probably is.

You and your mechanic would of course want to do a real good inspection around the plug hole to make sure the cylinder is not cracked which lead to the helicoil becoming loose in the first place but you probably just trashed the jam rod.

In my case the helicoil spun right back in with no trouble. As I recall we reinstalled the spark plug and torqued it and the helicoil into the cylinder lining up the two halves of the hole for the rod. We used a slightly over sized piece of bronze welding rod as a new wedge driving in the rod with a hammer and expanding it in the hole. We then removed the spark plug and inspected the cylinder.

This cylinder ran this way for another 250 hours till the engine was overhauled. At overhaul the cylinder was inspected and replaced back in service and remains that way till this day. I'd actually forgotten about it.

BTW my repair was done in the field without removing the cylinder with crude borrowed tools. I'd stopped at a local airport about 30 miles from home to visit and upon closing the throttle for landing the exhaust valve in this cylinder seized open in the cylinder. To much carbon buildup.

No problem I'll just do the rope trick and get it loose. That's when the helicoil came out. I was fortunate that an old time WWII mechanic who had served with the Flying Tigers came over to enjoy the mechanical wonder that was unfolding on his door step. When I showed it to him he said no big deal I've got a torch and rod in the hanger. And so it goes. I was back in the air with a clean exhaust valve and repaired plug helicoil in less that 2 hours. I did have to borrow a serviceable C-26 spark plug as we destroyed the old one removing it despite having heat available.

Good luck with your cylinder.

pilotlaw · Post by **pilotlaw** » Wed Oct 18, 2006 11:56 am

Bruce,

Thanks for the reply...your story matched exactly what I have heard from other mechanics (other than the one helping me out on the annual). My Dad was a mechanic in the Air Force in the 50s and he had experience with these things on old P & W radials and described the procedure exactly as you did. I hope to get it done today and will let you know...thanks again.

Bruce Hain

Wed Oct 18, 2006 12:02 pm

Writing that last night after along day I said we used a bronze rod. I'm not actually sure of the metal in the rod but it was a brazing rod that we used to replace the helicoil which is what I meant.

Bill Hart · Post by **Bill Hart** » Wed Oct 18, 2006 1:04 pm

Hey Bruce,

What is this rope trick. I have heard of it and I think I know what your talking about but just thought I'd ask.

Post by **Bruce Fenstermacher** » Wed Oct 18, 2006 5:05 pm

I've described the rope trick procedure in detail here before. Basically it's using a rope (I like nylon) pushed into the cylinder through a spark plug hole till the cylinder is filled. Then very carefully using the piston to compress the rope against the valves to either dislodge them from a stuck open state or hold the valve closed while compressing the valve spring for its removal.