The new (to me) engine is now ready to fire up. Electronics are all done, except for the kill switch. I ordered some reed-style switches to make that happen, but it could still be run as-is if the weather would cooperate.
There was one other repair I needed to do to make the engine ready, and that was to repair the threads on one of the carb mounting holes. A previous owner got a little over zealous with the torque and one of the two bolts that hold the spacer to the cylinder was stripping out. Trying to run it like this would certainly lead to an air leak and poor running characteristics, so I picked up a Helicoil thread repair kit. Step 1 was removing the carb & spacer and covering the intake to keep metal shavings out.
An appropriate size drill bit is then used to over-size the hole a bit, followed by the thread cutting tap included in the Helicoil kit. There's a certain "pucker factor" doing this kind of work on your engine....!
Here's the meat and potatoes of the Helicoil kit - a stainless thread insert, the install tools, and the die. I had to grind the tip off on the die so it could go a little deeper into the blind hole I drilled.
Using the guide, the new threads are simply threaded onto the newly cut threads in the cylinder. The new stainless threads are far stronger than the metal of the cylinder. Note in the picture below the final thread is still about 1/32" above the surface. Normally you want just the opposite, with the end of the last thread just below the surface, but this is as far as I could tap and I didn't want to risk boogering up the threads by trying to remove the insert. To fix the situation I simply clipped about 1/8" off of the last thread, which resulted in all new threads being below the surface as required.
I used to do a lot of metal fabrication and auto repair, so I picked up this tap & die set from Harbor Freight about 15 years ago. While it's not the highest quality (shocker) it has been very useful. Today I used the tap handle to cut new threads, and its far easier to keep this type of tool steady than it is to use a ratchet wrench. While I had it out, I also chased the threads on all the original hardware with the M5x.8 die. The original bolts had a little gunk built up on them, but this cleaned the threads up very nicely.
Here's the new Helicoil installed on the Zenoah G38. Note that the other carb mounting bolt hole is also looking very clean, as I ran an M5x.8 tap through those holes as well. Once done, everything was reassembled and is ready to go. The Helicoil kits are fairly cheap - mine was around $20 for the brand-name Helicoil kit in M5x.8 size, including the tools shown and a dozen threaded inserts. For about $30 you can get a Chinese-made kit with a few sizes.
