Could it be like a retaining ring like a spirolox to keep the piston on the pin?
How about one of those rare spring loaded link bars used on roller lifters in some aftermarket kits. These bars lift the lifters clear of the cam when the pushrods are removed, so cam changes can be done without removing the intake manifold?
How about a rev kit spring?
Instead of a helicoil, how about a key locking threaded insert?
maybe those 4 will jog some additional ideas
Andy