Franklin 150 - on one cyl., plug won't start back in...what

[alfybill]

#3, top...easy to reach/see. I tried for over an hour to get the same plug back in after cleaning (it had come out without difficulty). Tried many other plugs as well. All were happy to try to start at a bad angle...didn't go there. Looked at the hole carefully, doesn't look like the helicoil has backed out (still flush with top), but the thread area on one side looks a little buggered. I ordered a thread chaser to try to gently clean the area, but it won't arrive for a while. Thought I would probe the pundits in the meantime.

 

[Rhunter]

Franklins with 14mm spark plugs are notorious for stripped holes. They all have 14mm spark plugs except for the 6A-350 Series that use 18mm plugs like most other aircraft engines. Most NAPA car parts stores sell a good chaser for this since most all cars use 14mm spark plugs.
Are you sure it has a helicoil in it? Originally the hole was bored and the spark plug was threaded directly into the aluminum cylinder head. If there is a helicoil that means the hole has been repaired at one time which is very likely. Just be careful threading it in because the helicoil is made of some very hard steel and sometimes the chaser can't cut it's way through the pinched spot. If this happens the chaser will jam in the insert and the helicoil will back out along with the chaser when the chaser is withdrawn from the hole. Should this happen, don't panic. Just go get a 14mm HeliCoil Spark Plug Kit and follow the directions and put a new insert in it. You won't need to drill and tap it unless there is no insert to begin with and you're repairing a stripped hole. It MUST be a HeliCoil Brand kit to be a legal! I don't think any other kits are recognized by the FAA.
Remember, ALL spark plugs have METRIC threads! This had to do with patents and such back in the old days when spark plug ignition was first invented in Germany. If an American manufacturer wanted to use spark plugs in their engines they had to order them from Germany and machine their heads to the metric specs because the plugs were not offered in SAE sizes. It's been that way ever since. Just a bit of trivia there.