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I'm adding to what has already been posted on this question.
You certainly can re-ring an aluminum cylinder B&S. A ball hone can be used to break the glaze on the cylinder wall. Install new B&S rings gapped at their recommendation or .0035 " per inch of cylinder bore.
Small engines are very resilient and will run in spite of numerous things being not up to spec such as bore taper (which is what you correct when you bore, in addition to removing scarring.)
If you are doing your own work, the cost will be fairly low providing you already have all the normal tools and are good with your hands. Rings, borrow a ball hone, gasket set.
As you get into the rebuild you will see why it is easier to buy a new engine.
It could be a fun project and a good learning experience.
IF YOU REPLACED THE HEADS WITH REBUILT/NEW HEADS: A side effect of increasing compression when you bolt on fresh heads, is broken piston rings. It's beneficial to replace the rings & bearings and at least remove the ring ridge at the top of the cylinder wall if not re-bore when replacing heads due to the higher compression that will cause.
If you just put on a set of used heads.
There's also the chance that the new headgaskets didn't seal completely, (usually due to a problem with surface prep or a warped head/block), but if you've got high mileage, it's usually the rings.
A set of rings can be replaced, but you'll want to make sure they haven't cut into the bore so much as to cause blow-by once new rings are installed. Eliminating the ridge by carefully using a ridge reamer at the top of the cylinder to prevent breakage of the top ring from impact. Hone the cylinder walls gently & reinstall pistons & new rings! Good luck.
If you can turn your engine over by hand easily by pushing down the kick lever, and your not Arnold Schwarzenegger, You have a compression problem. Do a compression check. If your bike has under 100 PSI you need to disassemble the top end , bore or hone the cylinder (depending on condition) and install a new piston, rings, and gaskets. This should fix low compression. If the lever is going down but the motor is not turning over then you need to remove the side cover and check for broken kick gears. Reed valves have nothing to do with this problem.
Low compression, and lots of blow-by forcing oil into the air filter via the the crankcase breather. Sounds like you need a top end rebuild. If you can get your hands on a compression tester, then check it. If it comes in under 120, then certainly time for a tear down and top end rebuild.
If you put a quick squirt of oil down the spark plug hole and if the compression pressure goes up on a second compression test, then rings/piston/bore are worn. If it does not come back up, then it may be leaky valves, or a blown head gasket.
do the compression test first. That will tell you where to start. Or, if you have the tools, tear the top end down and inspect. A motor re conditioner/machine shop will help to measure up and tell you if piston, and bore are ok. May just need new rings to freshen up.
Happy to answer any questions that you may have.
My first thought is low compression... which leads to a few questions...
What fuel to oil ratio have you used?
What do you mean by 'fresh top end'? To me a fresh top end includes piston and rings and cylinder bore, gaskets and crank seals. Anything less on a 2-stroke is an incomplete job and usually leads to premature failure.
Have you followed break-in procedure on your fresh top end? Double check the head bolt torque.
hi some had same size rings fit them with gaps 90 deg apart some had different width rings but they will only fit 1 way if you have stripped it put new rings in and run a cyl hone down the bores hope this helps merlin v12