streaks in fresh cylinder bore

[125rmcrazy]

hey guys, i got my new head ready and put in on my bike tonight. 175 psi. perfect. when i had the head off i inspected the cylinder wall and except for almost all fresh hone marks, (only 1 tank of fuel was burned so far) on the exhaust side above and below the port were a few streaks. not anything you could feel with your fingers and no scratches or anything, but just shiner spots where you couldnt see hone marks anymore. any one know what causes this? could it be the piston to bore clearance being inssuficent and i need to let it warm up longer before riding (it is a wiseco piston, forged, expands quicker. the endgap is set to 0.010 factory spec is 0.008 to 0.014. any thoughts?

 

[whenfoxforks-ruled]

What is the skirt clearance? Did you drill the exhaust bridge holes? You can probably clean the cylinder with a skotch brite pad and wd40, then wash with hot soapy water, rinse and coat the bore with your favorite 2 stroke oil. How long does the manual say a set of rings should last?

 

[125rmcrazy]

didnt measure the skirt clearance. just endgap 0.010 the whole length of the bore. the exhaust bridge holes are already in the piston. the manual is in the garage and i cant recall reading on how many hours the rings can be used. but theres no use on the new rebuild at all. not even a full tank of fuel burned.

 

[ohmthis]

125rmcrazy,
Did you measure the bore to ensure that it was in spec. Typically The bore wears irregular. What I mean is it will be tapered with the bottom slightly larger than the top. If the cylinder is out of spec it can let the piston skirt slide on the wall. Another idea I have is that you didn't clean the cylinder, reeds, carb, or there was some dirt in the crankcase that could have been drawn into the cylinder and scratched the wall. I would have the piston and cylinder wall measured and double checked to make sure there isn't a condition of too tight or too loose of a piston to wall clearance. If everything checks out fine. Clean everything extremely well and put it back together. Good luck!

 

[griffbones]

Sounds like the normal burnishing that occurs after re-plating a cylinder.

 

[whenfoxforks-ruled]

Did you do anything to the cylinder besides checking the ring end gap? Did you have it honed? Ring gap is necessary for the rings. It tells you pretty much nothing about the bore. Checking the skirt clearance when it is in its normal position, and normal length of stroke, you can get some decent idea about the bore. Since it is a used piston, you will have to measure the od at the skirt, to see how much it has worn. Check clearance with a feeler gauge at the top middle and bottom of its normal stroke. For a baseline measurement, check further down its normal stroke and flip the piston over and measure the top. If it is tighter at the top or bottom your cylinder should be replated, it has moved, probably from going to long on the maintenance. Check the bridge clearance also, it should be .0015 to .002 more than the bore. Sometimes it does not get relieved, also clutch material will build up when the right seal is bad. Flashing builds up also. The piston is aluminum after all. If you have more than .0015 difference in your initial measurements of the bore, then it needs to be replated. Extended service and sometimes they come out of spec. Or you can get a bore gauge and make life a lot easier! But I believe it takes some idea of what you are doing to determine if it is out of round, the piston is your best bet. But the skirt clearance needs to be in spec like the rings.

 

[125rmcrazy]

the bore has been resleeved with a cast liner. and was bored to 2mm over. the piston is a brand new wiseco pro-lite. everything is brand new from top to bottom.. so things being worn is out of the question. like i said its just shiny streaks no gouges or scratches and the exhaust side of the piston there are no marks whatsoever. just those shiny spots on the cylinder wall.

 

[whenfoxforks-ruled]

Allrighty then. A liner runs a little looser than the plated bores, more tolerance. Check the bridge. A guess wold be around .003 to .004. If you ran 1 tank of gas, and have a definitive scoring, it could be to tight and cause trouble down the road. The bike was designed to run a plated cylinder, stock tolerances mean nothing. Whoever did it should shine some better light on it than me, hopefully!

 

[SBC383STROKER]

Streak marks?

The only reason that your cylinder would have "marks" on the exhaust side of the cylinder is because there isn't enough piston to wall clearance. There might be enough clearance on the intake but since the exhaust side creates more heat therefore causing it to expand faster and then you have streak marks! Now the shiny marks are where the tight piston to wall clearance has made the piston to rub with cylinder therefore BURNISHING the cylinder and thats why you cant see the honing marks anymore. The ideal piston to wall clearance is .002".