Wrong piston?

Daniel sheffield

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Oct 11, 2019
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Hello everyone just join the forum can’t wait to get information from people who know what they’re doing. I currently have a 66cc bike kit that I am trying to do performance upgrades on to get better speed, so when I took the head off I noticed that the deck height was around 2 mm. I checked the crank and I have a 40 mm crank which from what I could find online should put my deck height at zero.
could I have the wrong piston in the stock engine from factory.
 

Greg58

Well-Known Member
May 1, 2011
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Probably not, there are only two piston types I know of, there’s a big difference in the two. How many base gaskets does it have? Sometimes two are used to get the port timing correct.
 

Daniel sheffield

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Oct 11, 2019
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Probably not, there are only two piston types I know of, there’s a big difference in the two. How many base gaskets does it have? Sometimes two are used to get the port timing correct.
It has only one base gasket. I did some lite porting with a file to round some of the sharp edges but want to get more top end out of it. I have gutted the exhaust but left the cap on and trued the mating surfaces between the head and cylinder. Tops out at about 33mph but gets hot. Running about 20:1 oil. Need it to do 30 mph consistently for a 12 mile one way trip to work.
 
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Greg58

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May 1, 2011
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Is it new or fully broken in? It will get better with time, also what rear sprocket are you using? Smaller sprocket increases top speed but hurt low end torque.
 

Daniel sheffield

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Oct 11, 2019
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Is it new or fully broken in? It will get better with time, also what rear sprocket are you using? Smaller sprocket increases top speed but hurt low end torque.
About 5 tanks in. 44 tooth. Going to get a 36 with hub soon and designing a expansion chamber to make using stock exhaust
 

Greg58

Well-Known Member
May 1, 2011
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20/1 is more oil than most of us run, I've been running 32/1 in my bikes since 2011. I didn't like oil dripping on the shop floor where I parked.
 

Daniel sheffield

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Oct 11, 2019
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20/1 is more oil than most of us run, I've been running 32/1 in my bikes since 2011. I didn't like oil dripping on the shop floor where I parked.
Yeah I just figured it would aid in break in kind a like four strokes, and because it’s running so hot and I have to travel such a long distance I like the idea of the engine having plenty of oil. not too worried about the mess
 

Greg58

Well-Known Member
May 1, 2011
4,112
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Newnan,Georgia U.S.A.
I went from a 41 to a 38 on my 66cc bike and it made my ride a lot better, if you drop down on the sprocket and add a good expansion chamber you can see several mph.
 

Daniel sheffield

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Oct 11, 2019
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I went from a 41 to a 38 on my 66cc bike and it made my ride a lot better, if you drop down on the sprocket and add a good expansion chamber you can see several mph.
Would you recommend going to a high compression head to help with the heat and case matching the ports for better flow
 

Greg58

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May 1, 2011
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Newnan,Georgia U.S.A.
A lot of the performance heads on the market do little to increase performance, they can help cooling. Proper squish band clearance will help more than higher compression.
 

Daniel sheffield

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Oct 11, 2019
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A lot of the performance heads on the market do little to increase performance, they can help cooling. Proper squish band clearance will help more than higher compression.
My problem is the piston sits so low in the cylinder that I feel like my squish clearance is too high
 

Greg58

Well-Known Member
May 1, 2011
4,112
1,020
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Newnan,Georgia U.S.A.
I would check the compression first to have a baseline, a stock engine should be 90-100 psi minimum for good performance. To get top rpm
you need a degree wheel and use base gasket thickness to adjust port timing, then decking or gasket thickness to set squish.
 
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