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Old 01-24-2012, 02:56 AM
recon chris's Avatar
recon chris recon chris is offline
Motorized Bicycle Senior Member
Join Date: Apr 2011
Location: pacific grove
Posts: 87
Default Re: Getting all 3 Horse Power out of your GT-5 Sky? (tear down)

After measuring both original and new piston there is no difference between the two including the wrist pin hole position on the piston. There is a slight difference in skirt length. The new skirt is about 2mm longer than original. And there is a difference in weight. The old piston weight is 87g and the new one is 83g (that’s a pretty big weight difference for both pistons measuring up almost exactly the same). So (and I cant believe I didn’t think to do this earlier) I installed the old piston and it is the exact same. None of the ports are ever fully open and the piston has the exact same TDC height as the new one. So I guess I was going 30 miles and hour before with half blocked transfer ports and semi blocked intake and exhaust ports.


If they would just mach the jug ports with the piston height (NOT THAT HARD) they would probably get twice the power out of these engines. This isn’t even about Chinese quality control this is just poor engineering. Casting proper ported jugs would cost just as much as casing incorrect ones.

How am I going to fix this?
Since all of the bottom ports (exhaust and transfer ports) can benefit from having the piston travel lower so they can be more exposed ill add a thicker seal to the bottom end of the jug. This will raise the jug and better expose the lower ports during BDC.
However doing this will also lower deck high and therefore lower compression. So to counter that ill sand away some of the top part of the jug. This will make the head bolt on lower on the jug (as opposed to the original) but the thicker jug gaskets on the bottom will keep the piston at the original deck height. The only port that will suffer in all of this will be the intake port but ill shave the piston skirt to make up for that.
What do you guys think will that work. And if so will it kill bottom end or top end performance? I would think it would give the engine overall more power by greatly improving the efficiency.

Last edited by recon chris; 01-24-2012 at 03:39 AM.
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