Possibly one of the most worrisome situations with a motorized bicycle is the prospect of a seized engine. The first thing to do is squeeze the clutch handle to disengage the clutch drive. If the bike now rolls freely, then it is quite possible the engine has seized. This is typically caused by either foreign material entering the combustion chamber, or excessive heat buildup due to a lack of lubrication (2-stroke oil).
If the clutch handle is squeezed, and the bike still does not move freely, then the chain has most likely jammed in the engine. This is usually due to a chain which has too much "slack". The best way to free the chain is to move the bike in reverse. Usually tools are required for this procedure - once the chain has jammed, it will require a fair bit of force in the opposite direction in order to free it.
I have attached a photo below. The clutch cover has been removed to show what happens when the chain jams. This should help!
Mine locks too. It is not the chain and it is not the piston. After it locks, I pull in the clutch and release it again and the drive is no longer locked, but the piston will not move in the cylinder. yet the cylinder will slide off the piston easily when I remove the head. I hear a click noise at exactly the same time the drive locks up.
The case you experienced sounded like over torquing of the head bolts caused the bottom cylinder gasket get too compressed that resulted the piston over extended to the top of the cylinder head that caused the seized. The remedy for this is to add either top or bottom gaskets to add additional spaces for the piston and the cylinder head . Hope this help!
My bike was doing that be cause my drive chain (from the motor) was to loose and when the bike would start as pop the clutch it would jam up like that and I couldn't you couldn't move the bike at all because both chains are connected to the wheel shortened the chain by taking out 3 links and the bike functioned properly
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