I was cruising along and the engine started getting a tad bit louder (like an exhaust leak or weird vibration. Then it started losing power and eventually died. I peddled home, found the spark plug contacts pushed up making contact with each other. I took the engine top off and found some minor damage to upper piston edges. I took the top half off and found the piston had a lot of play. I looked under the piston and saw that the bushing was practically gone. It had broken apart and just the part inside the actual arm was still there. I punched that part out but can't find anything around the house to make another. Is it brass or copper? It looks kinda like brass.
I purchased the engine on EBay from a seller named Powerkingshop. I emailed them to see if they offered replacement or exchange. It is a Power King 80CC.
The wrist pin bushing is brass and the fit to the wrist pin has a clearence of about .0005" to .00075". Yep! It's that close.
The big end of the connecting rod is probably one piece. In other words, there are no rod bolts on the big end. This means it is a pressed-up crank with roller bearings in the big end or crankpin as some call it. If this is the case, the case must be split, the crankshaft removed, pressed apart, the connecting rod repaired or replaced, then the whole plot cleaned 'n reassembled.
When I worked on my Mercury outboards in the early '60s, they had bushed small ends on the rods, we ran 28~32 to 1 oil 'n gas. They smoked!
Some of these Chinese engines have roller small ends, it might be cheaper just to buy one of those. I have no idea what brand or type they are. Someone here is sure to know.
If you were running cheap oil, mixing it lean or ran with a fresh air leak into the crankcase for a time or ran over heated for long periods ( like long full throttle runs on 100+F days, any of those could have helped kill the small end. Could have been a defect. The seller might have felt it going bad and sold it before it went catastrophic. Who knows? Only you and the seller.
Thanks. I am running synthethic. 36:1 ratio. When I first took the piston off the arm, it looked like the bushing that goes through the hole at the end of the upper arm was in the hole unevenly. The hollow steel pin that goes through the piston, then through the bushing and the upper arm is good to go. I hope to hear something from the seller tomorrow or maybe something from Dax about parts. Daxs is supposed to ship my 36T sprocket this Friday too.
This guys info is dated, just like these Chinese engines. Current state of the art strokers can run 50 to 1 with ease but not these crude, inexpensive power-plants. Sure wish I had one to blue-print for review.
Thank you for the pic.
Just about all strokers and most of the old 4 stroke m/c engines had cranks like that, those are called built-up or pressed-up cranks. The rod big end has no rod cap or the bolts used to remove/install a rod cap. This is lighter, permits a smaller crankcase for compression on the down stroke and maintaining tolerance during manufacture is easy compared to a plain beaing crank/rods. The roller 'n ball bearing bottom ends are also tolerant of more dirt/grit or misalignment, than plain bearing cranks.
The wrist pin or small end is a bushing and that IS the weakest link in this motor. The better way to install a new piston would be to lube the bore areas where the pin rides, chill the pin in the freezer, warm the piston 'n rod small end with a hair dryer on high, then slide the pin home with the piston facing the right way and one circlip inplace to stop the pin from going too far. Open portion of the circlip always faces down.
Ya know OP,
If the small rod end and pin were properly hardened a caged roller bearing might fit. Please do a good follow up in this thread.