the 48cc got A hole in the case in A accident. the 66cc was ****.
I torqued the head to 10 pounds of torque. the first ride it
blowed A heat casket.Then head stud striped I replaced it
with 1/4 in threaded rod .Then the spark plug and the thread
came out about a mile from home.I got A new head then another
stud striped this time I replaced all of them. I put 6mm heli coils
in the mounting holes to the frame at that point I thought it would last
A long time.I got about five miles from home it quit running
I pulled the spark plug to check it for spark. I stoped right there
the spark plug was smashed flat from the piston hitting the spark plug.
At thet point I thought f%&* it . I will not get my hands dirty
on it again
Lets take this one step at a time.....
First of all 10# of torque is not enough for the head, but more on that later. (had that happen on my first build)
Second ALL the studs on these engines are junk, and need to be replaced. Chinese studs are about equal to a grade 3, very soft. They stretch very easily. All the head, exhaust, intake and mounting studs need to be replaced with a metric grade 8.8 which is equal to our grade 5. Grade 8 US is overkill. This cast aluminum won't take the torque that you can put on grade 5 hardware to begin with.
Thirdly I recommend when installing the studs to use lock tite...many use BLUE, I use RED. Double lock nut the stud, with a drop or two on the threads to be inserted screw the stud till it bottoms out then back off about 1/4-1/2 turn. Let sit and it's there. When installing the head I use some copper kote (bought at Auto Zone/Advance, spray can) It is a gasket sealer for metal gaskets, been using it for the past 40 years off and on.
Fourth Before installing the head, mill it true. I do that by using DA 220 grit paper, stuck to my garage window (true flat surface). Move head back and forth, round and round till the surface is milled. Don't take any more off then needed.
When installing nuts on the head studs I use nylon locking nuts. I have never had to re torque during inspection. I either use locking nuts or double nuts on the reaming studs.
Spark plug holes that strip out can be repaired. They make plug inserts for them. As far as the piston hitting the head, there are a number of reasons why the piston hit the plug..you changed heads, might not be the correct one. Did you replace the head gasket/s, incorrect plug. Beats me at this point.
6mm studs in cast aluminum have a max torque of 60-70 inch pounds and 8mm studs 204 inch pounds. For foot pounds divide by 12. I torque 6mm grade 5 to 50 inch pounds, and 8mm grade 5 to 150 inch pounds. 120 inch pounds/10 foot pounds and Chinese junk hardware don't mix. Besides, if you used a pound torque wrench you don't really know is it was 10#'s or even 8#'s.
Proper engine mounting in the "V" frame of the bike is very important.
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