the changfa has the tapered seat as well, and the body is the same size as the VW glowplug
but it has courser threads. the reach of the changfa unit is longer so even if one were to rethread the VW
plug i think you would have to turn down the hex down to the body size so it would reach in about a 1/4 inch deeper
to get to the taper seat.
i have considered getting a metric thread die and just chasing the threads on the VW unit,, likely the threads would not
be perfect but i don't think it would take much to hold it in. lots of pressure on it, but very little surface area so even
rather mangled threads would likly hold just fine. i know this will be my first option i am going to try.
it would appear the difference is about 1 less thread per inch, maybe 1.5 threads less than the VW per inch.
i am thinking it might work, for the following reason
several years ago i ordered a box of 5/8 grade 8 course threaded nuts,
i opened the box and found them to be double threaded, they had been run twice
with both course and fine threads!
for kicks i torque tested a couple of them and found them to hold 170ft/lbs on the fine thread and 180lbs on the course thread
and that is spec and they held just fine, cool
the bowman guy wanted to get them back and i told him no way,, they are mine now
he wanted to know why i wanted them, i told him i now have one nut to fit both threads for that emergency job
along the road where i am constantly digging for one or the other and always have the one i don't need
the plug was installed at about 25 ft/lbs torque, no more
even at 25ft/lbs the force is far greater than the less than 1000psi at combustion put against the ~3/8" seat area.
what does that work out to be, about 110lbs of force exerted on it if the combustion pressure reached 1000 psi
i am relatively certain that 25 ft/lbs of screw force is far greater than 110 lbs.
so i am thinking it will probably work , i will let you know
bob g