Interesting failure

Skyliner70cc

Member
Mar 8, 2008
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One of the small pins that prevents the piston ring from rotating fell out or broke off. It scored the piston and the cylinder was spit out (didn't lodge and seize the engine). It also left a hold in the piston where it was press fitted into the piston. The piston rotated and the ends snagged a port and the rest is obvious.

I'm not sure if the piston and pin were defective or if the pin was not inserted properly. I already pull apart each engine before installation and checking these piston pins will be another check I will perform.

If the pin can be removed with pliers, I will reinsert with a touch of JB weld to keep them seated.

The engine was a bushing Dax and has a little over 1 year use of use on a bike I built for someone. The owner doesn't abuse his engine and rarely exceeds 12mph on his 55 tooth sprocket geared bike. Its an easy fix, I have a spare cylinder and a new piston and rings are 30 bucks from Dax.

later!
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
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up north now
Ouchy....how many rpm is the engine running at that speed with a 55T? How many "miles" do you think the engine has seen with that gearing? Maybe the ring was worn enough the PIN snagged the port?
 

Skyliner70cc

Member
Mar 8, 2008
138
0
16
The 55 tooth sprocket tops out at 23mph. The person who owns the bike rides slower than a grannie. When I built him a bike, he desired a bike that can puts around slow up steep Colorado hills. I can run faster than the speeds he rides at.