How can my cylinder to piston clearance be this?

[Jefferson]

Hi there,
This is my first post and was hoping for some help. I was gifted two blasters. One rips and the engine sounds great. The other one rips and the engine sounds horrible with an overly loud knock. Compression was fine. I pulled the cylinder off and there is no rod play on either end. I found a vertex piston in it with +25 inscribed on the top and I measured the bore with telescoping gauges all over and came up with an average of 66.248. The piston is scraped and scratched all over and is obviously toast. It measures 66.09 n the skirt 10mm up from the bottom. That is a 0.158 piston to cylinder clearance!!! WTH? Is that just from normal wear? How can it be so far out of spec? Already sending to KOR racing for bore and matching piston. Thanks!

 

[Whatsup2day]

Hi there,
This is my first post and was hoping for some help. I was gifted two blasters. One rips and the engine sounds great. The other one rips and the engine sounds horrible with an overly loud knock. Compression was fine. I pulled the cylinder off and there is no rod play on either end. I found a vertex piston in it with +25 inscribed on the top and I measured the bore with telescoping gauges all over and came up with an average of 66.248. The piston is scraped and scratched all over and is obviously toast. It measures 66.09 n the skirt 10mm up from the bottom. That is a 0.158 piston to cylinder clearance!!! WTH? Is that just from normal wear? How can it be so far out of spec? Already sending to KOR racing for bore and matching piston. Thanks!

That would probably explain your "knocking" or piston slap..is it a Chinese cylinder/jug?..could have been bored before,worn out then overly "bench/drill honed" ...that's alot for home hone tho..I did a Chinese top end before and specs were terrible so had to send back..what where your highest to lowest measurements on cylinder? Not average. Is cylinder egg shaped ?...any pics?..scaping/scratches from being lean, dirt/debris, or parts not champhered?..while your top end is out and getting done clean the inside of pipe out..(stock pipe or a/m)..if you can't or don't want to burn the pipe out you can take a length of braided cable and untwist the last 6" and wrap tape or zip tie at point of unbraid and use a drill to knock stuff loose inside or a hand full of nuts and bolts or a small chain and shake around real good..(a stock pipe has baffles in it and these could get stuck inside)..could also soak and flush with heavy duty degreaser or oven cleaner..if there is rust or carbon built up in exhaust it could get sucked back in the cylinder..make sure it's clean when reassembling..what carb, filter, exhaust does it have?...

 

[Whatsup2day]

I just noticed you said .158 and not .0158..maybe someone put to small a piston in with new rings .I have no idea how it would wear that much and still run ..guess it doesn't matter if you got Ken taking care of it..just do proper reassemble and testing(leak down and jetting)..

 

[Jefferson]

I just noticed you said .158 and not .0158..maybe someone put to small a piston in with new rings .I have no idea how it would wear that much and still run ..guess it doesn't matter if you got Ken taking care of it..just do proper reassemble and testing(leak down and jetting)..

OEM cylinder and carb. Stock exhaust. Still has stock balloon tires lol. Vertex piston with +25 inscribed on it which should measure very close to 66.25. Bore was 66.25 in all but 2 measurement which was 66.24 I took x any y axis measurements, 8 measurements total in cylinder. I measured the piston a dozen times with the same calipers used to measure telescoping gauges in disbelief!! I took the rings off and slid the piston in to the cylinder and there was a visible gap! Piston images too large for the forum to process but they are totally scored all over. I just never heard of a clearance like that and engine not just run but still rip with high compression. Previous owner must have run regular gas in it or ran way too lean possibly. I will clean that pipe though. It is a nice clean 2006 blacked out special edition

 

[Whatsup2day]

OEM cylinder and carb. Stock exhaust. Still has stock balloon tires lol. Vertex piston with +25 inscribed on it which should measure very close to 66.25. Bore was 66.25 in all but 2 measurement which was 66.24 I took x any y axis measurements, 8 measurements total in cylinder. I measured the piston a dozen times with the same calipers used to measure telescoping gauges in disbelief!! I took the rings off and slid the piston in to the cylinder and there was a visible gap! Piston images too large for the forum to process but they are totally scored all over. I just never heard of a clearance like that and engine not just run but still rip with high compression. Previous owner must have run regular gas in it or ran way too lean possibly. I will clean that pipe though. It is a nice clean 2006 blacked out special edition

Nice..I found I could not post pics from my phone..said same thing but if I screenshot the pic then it will let me post the screen shot for some reason.