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Post by dstrappazon on May 17, 2018 8:48:23 GMT -5
I recently bought a non running 50cc gy6 chinese scooter. I did not try to get it running first but just ordered a 50mm big bore performance kit. it came with cylinder, piston/rings, head, cam, myffler, carb, variator, ect. after assembly it fired right up and sounds great, lots of power. but it had a knocking noise that gets louder once the engine is warm. I pulled it all back apart and there is some scuffing on the piston skirt. I set ring gaps to spec, but I noticed the piston to wall clearance at the skirt is pretty much non existent. so I had the company send me new piston and cylinder, only to find out the new one is exactly the same. anyone else have this problem or have some info for me, that would be great. I'm guessing I need to hone the cylinder out a Lil bit. thanks
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Post by scooterted on May 17, 2018 9:21:58 GMT -5
Shouldnt be much clearance. .0015-.002 inch is typical for cyl-piston clearance.
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Post by dstrappazon on May 17, 2018 12:16:07 GMT -5
the top of the piston had a Lil gap, I can move it back and forth a Lil bit. but the skirt has literally like none. I can't move the bottom back and forth at all. I'll try to measure the gap when I get home.
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Post by dstrappazon on May 17, 2018 17:09:24 GMT -5
Just got home and have those measurements, top of the piston is 1.950" bottom is 1.968" and the cylinder is 1.969". like literally almost no clearance at the bottom. is that right or should I hone the cylinder out some. like I said, there's barely even 10 miles on this new Big Bore Kit but didn't want to drive it cuz it's making this noise
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Post by scooterted on May 17, 2018 17:37:40 GMT -5
Unfortunatley i dont know what the exact diameter of a 50mm bore should be. I would expect 1.970" but im not specifically sure. Be careful honeing. If you take too much youll get piston slap and that will make noise too.
Youre sure no other bearings are bad on the crank? You said it was non running.
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Post by AtariGuy on May 17, 2018 19:40:48 GMT -5
Possibility that the piston skirt is just kissing the crank wheels? At warmer temps that may get louder as the metals heat and expand. Idk. Just throwing that thought out there
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Post by dstrappazon on May 18, 2018 10:33:21 GMT -5
Thanks, well I put it all back together last night after honing a little bit and I even sanded the skirt down some to get very minimal clearance just enough to see some daylight through there put it all back together and it still makes the ticking/ knocking noise. I don't know , im about to just drive it see what happens
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Post by scootnewb on May 23, 2018 12:40:49 GMT -5
Did you figure out how to fix it?
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Post by GrumpyUnk on May 24, 2018 10:27:01 GMT -5
convert 1.950" to mm == 49.53mm
convert 1.968" == 49.9872mm
A tight cyliner:piston will end up scoring the cylinder and scuffing the piston. Not enough room for heat expansion and minimal lube clearance. I dunno how much room you need for erl, but figure .001" should be ok as plain bearings are less than that in many cases.
I would be checking the connecting rod big end bearing for wobblies, or motion in the up/down direction. There should be no appreciable movement of the 'big end' on the crankshaft journal. Side-to-side is ok.
Check also the fit of the piston pin in the small end of the rod. It should have no movement, but slide readily side-to-side. If you feel movement in the reciprocating motion(what the piston does...), you have a bigger problem. Maybe want to shop for a new crankshaft ... lots of labor time, but not a lot of money.
I *think*, MY opinion, that the piston should fit well enough that it will sink, w/o rings installed, from top to bottom and from bottom to top, when installed in the cylinder, and one end of the cylinder is covered/sealed by a hand palm. If the piston is too tight for that, or falls readily in either direction, I think too tight or too loose. Ballpark. YMMV
An easy check for other sources(there are not many) is to pull the cam cover, and try to rotate the cam sprocket. If the chain tensioner is loose, or was not released or plopped onto the outer side of the chain, there could be slack, allowing the cam to chatter in its movement. Most times it causes a 'grindy' noise, rather than clack or click, but you never know. You should not be able to wobble the sprocket. You should barely be able to hear/feel the rocker arm movement with the piston @tdc, both valves closed. There should be a tiny bit of movement.
tom
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Post by dstrappazon on May 26, 2018 10:09:23 GMT -5
Ya I'm almost 100% it's in the bottom end. I'm ordering new crank with rod arm, bearings, oil seals, and I have new oil pump as well. is there anything else I should replace while I'm in there.
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Post by GrumpyUnk on May 28, 2018 11:05:54 GMT -5
For a few dollars more....
I think I would inspect & likely replace the timing chain, the oil pump chain, and take a good look at the timing chain guide.
The crankshaft sprockets for both chains will be replaced, and the oil pump sprocket on the new pump. Might as well replace both chains, and inspect the cam sprocket while you have it all apart. The teeth should not be worn, and the chain should sit nicely between the teeth. Wear would occur normally on the 'load' side of the sprocket teeth, the load in question being that of rotating the camshaft, so on the 'left' side of the teeth as viewed from the sprocket end of the camshaft assembly.
tom
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