Im missing something

Cardboard box and scissors! Regular paper will work. Soda cans cut to shape. Use one or stack them to height.
My present cylinder is raised a bit with thick cardboard and a soda can on either side to protect it from tearing.
The aluminum cuts easily with scissors. You can use the soda can alone to drop the cylinder.
You can grease the cardboard, paper or cans, or use dry.
I leak test on assembly by pressurizing to 7 psi max. More will blow your seals out.

I hate using sealer but will use sealer alone on some builds if needed.
I file and sand the cases flat first. RTV sucks, I prefer ThreeBond for this.
This is a last resort due to sizing issues and it won't come apart easy.

4-6 sheets of paper are about the stock gasket height.
Soda can gaskets on either side keep the paper reusable many times.
 
Now we are cooking with butter... Im using some cardboard first thing in the morning. Ive got an entire day to work on my bike.... A small list of things as well... 1. Make gasket 2. Cast my new custom levers 3. Oil all my cables 4. New brakepads 5. Source a new freewheel for pedal side 6. Clean carb 7. Begin building new fuel tank rear mount.... Woohoo
 
Update... Pulled motor apart to fix the gasket and found a new problem. I dont even know which piston to order... SMH
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Just order one for a regular 66/80cc engine

Look for any other scratches or rub spots and make sure they aren't going to affect anything
(I'm guessing you don't have a 49cc?)
(And I don't think they ever made a black Grubee)

I can't tell what even happened but your piston is fried, if your like me then you would want to upgrade to something better since it broke.... I would look for lightweight pistons then polish it when it arrives

This should add a decent power increase:)
 
Ouch! You need both a piston and a cylinder. Looks like it overheated, noting the discolouration on the piston and perhaps slight signs of detonation beads. The lack of scuffing on the skirt says your oil mixture was fine. Piston got hot and swelled until it rubbed.

Why did it get hot? Fuel mixture/jetting wrong? I don't think so, no huge signs of detonation. I suspect hot weather and hard load. More cooling air speed and more pedaling might have helped this poor engine.

The Grubee cylinder and piston will work, even if it comes in the different height. Assemble it and check how high the piston comes in the bore. It should be level.

If the piston is too high, Use a thicker or stack basgaskets or stack headgaskets to get proper squish (@1mm) with the head.

If the piston is too low, thinner base gasket (cut paper) or file the top of the cylinder (finish by sanding on sheet on glass plate) to get proper squish with the head.

Steve
 
look at the mzmiami site on ebay and it will show the measurements - measure your piston to see which to order
 
Thanks everyone. Yes its a good time to upgrade. Ordering new parts soon. On the bright side, good time to break the bike down to the frame and give it that new Black cherry pearl paintjob Ive been putting off. Lesson learned, GA heat and my weight = more breaks to cool the motor... *Side note* What would happen if I took the 90cc off a small ATV and installed on a bike? Motor seems to fit the spot. Im sure I can whip up some brackets. Its chain driven.
 
Thanks everyone. Yes its a good time to upgrade. Ordering new parts soon. On the bright side, good time to break the bike down to the frame and give it that new Black cherry pearl paintjob Ive been putting off. Lesson learned, GA heat and my weight = more breaks to cool the motor... *Side note* What would happen if I took the 90cc off a small ATV and installed on a bike? Motor seems to fit the spot. Im sure I can whip up some brackets. Its chain driven.
I thought a seized piston was from heating and expanding the piston _before_ the cylinder has time to warm up and expand ("hard load"!). So I don't know if taking breaks is the answer, perhaps just a more gentle first ten mins would be better (and build to prevent detonation too) so the piston and cylinder warm up together. Not an expert but that's the impression I got from my reading.
 
I thought a seized piston was from heating and expanding the piston _before_ the cylinder has time to warm up and expand ("hard load"!). So I don't know if taking breaks is the answer, perhaps just a more gentle first ten mins would be better (and build to prevent detonation too) so the piston and cylinder warm up together. Not an expert but that's the impression I got from my reading.

Been there, done that too!
Mostly happens with watercooled engines with forged pistons. exactly as you describe.
Waterjacket doesn't warm up as fast as the piston.
Causes a "four-square" scuff on the piston. Also called a "cold seize".
Forged pistons swell much more than cast, much more prone to it.

Even without detonation, engines can overheat.
Richen the mixture will liquid cool the piston. That liquid being fuel.
More rpm helps, as does more speed for air flow if it can be done with less throttle.
Cooldown breaks and lower loads and slower on the hills or a lower gear help too.

Hey Built,
90cc bike engine weighs over 3 times what your China Girl does.
I'm sure it can be done, but getting kinda heavy for a weak frame and wheels.

Steve
 
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