Update: Finally able to get back to this, took the top cap of the engine off and here are a couple of pictures. I didn't see any scratches/gouges on the cylinder walls, the piston has one clean spot on it, not sure if that points to anything specific. Will get that cylinder section removed next and see what we find.

 

Lot of carbon build up on one side of the head.Since you're this far,pull the cylinder and check the piston and rings out for obvious scoring. Pull the piston,remove the rings and push the piston through the bottom of the cylinder and hold one side of the piston skirt flush with the cylinder sleeve. Just take a simple feeler gauge set and see what size slides in with a light snug feel to it. Same principle as adjusting valve clearances. If a .005 or larger feeler gauge can fit,the piston is worn out. The cylinder needs to be measure with a good set of calipers to see what over size piston and rings are needed to clean up the bore.Any dealer or machine shop can do that.Look at the 5:52 mark on this video and it explains this better.

 

Finally able to get back to this and got the cylinder off, didn't feel any play up and down on the rod but will check again when we get that piston off. Wanted to post a couple of pictures, the piston has stains of both sides and the cylinder has discolored areas on both sides as well. Is that where the compression is being lost or spots where oil wasn't getting to? I'll get the piston off and do that gap check next.

 

Pull the rings off and check the clearance with feeler gauges like in the video shown at the 7:25 mark.Lower part of the piston is starting to seize.This can be from heat caused by too much clearance or lack of oil.My guess is the piston to cylinder clearance is too much and ready for a new piston and bore job.

 

Got a mechanic friend who has a hone tool thats a bunch of silver ***** so we can clean up the cylinder and he also has a telescoping T gauge measure device. Will also do that feeler gauge and report back. During the ATVs use, I have had to add a little oil several times so I know its using some, not sure if that makes me in the clear on that part. Thank you OPT for the continued guidance.

 

If he has a set of t gauges and calipers he can get a better reading than the feeler gauges. If the piston to cylinder clearance is .005 or over time for a new top end.Oil pumps were reliable on the 300 except that you might have him pull the pump and make sure it doesn't have a plastic drive gear that some of the early ones had. If it has,it needs to be changed out to the metal one used by the 250 and later 300 models. Item #17.

 

Quick update, the T gauge was too large for the cylinder and without the rings on the piston a .012 fit in there. Also you could pretty easily compress the piston rings by hand and my mechanic friend said they shouldn't be that way. Cylinder bore cleaned up very nicely and quickly with the honer. Will have to watch that video again and see what that .012 will lead me to, the next step up Wiseco over standard piston is 75.00 mm bore and 0.50 oversize.

 

If you had .012 on the piston to cylinder then that's way too much. If it's the ring end gap in the cylinder you're talking about,then that's not bad at all. The piston to cylinder clearance is what matters.

 

He used the feeler gauge between piston and wall but that was with piston at the top of the cylinder, looking at the video the guy measured at the bottom of the sleeve and piston area. I tried that and the tip of the .006 just goes in but its tight. So I guess I was going in from the wrong side based on that video? Also, I didn't see a .005 on the gauge set but have one at home I can bring to where the cylinder is and see how tight that is.

 

Clearance is measured with the bottom of the piston flush with the bottom of the cylinder sleeve just like in the video. If a .006 gauge will barely fit,you might as well say there's at least .005 clearance which is too much and have the cylinder bored and honed for the next oversize piston.