Knocking Tomos A-55

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Knocking Tomos A-55

Postby mountainmoped » Sat Nov 05, 2016 10:42 am

A Tomos ST with 675 miles on it was brought into my shop with a loud knock that seemed to come from the top of the cylinder. I pulled the head and cylinder off and found a carbon buildup on the piston. The piston cleaned up and there was no sign that it had seized. Assembled the engine with new pin and upper rod bearing and started it up. The knocking was still present. There is no sign that the piston had hit the head. I disassembled it again and noticed there was a small amount of end play between the crankshaft and the rod, I suspect something wrong with the roller bearings between the rod and the crank. Has anyone ever seen anything like this? Why would the bearing fail at only 675 miles?
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Re: Knocking Tomos A-55

Postby tortoise » Sat Nov 05, 2016 11:39 am

Potential bearing issue.
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Re: Knocking Tomos A-55

Postby John » Sun Nov 06, 2016 7:22 am

Think that reply nailed it Bryan. Chinese creep .....
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Re: Knocking Tomos A-55

Postby mountainmoped » Sun Nov 20, 2016 10:13 am

I tore the A55 down and found no damage to the piston/cylinder and the ball bearings on the crankshaft looked good. I could see no indication that the engine had been run too lean and there was no scuffing on the piston. There was side play and end play in the rod bearing and the crankshaft showed signs of intense heating in that area. Upon close examination I could see that at least two of the needles on the rod bearing were missing. There was no sign of those needles. I am installing a new crankshaft and new ball bearings as I'm afraid the old ones have metal in them. The moped is adult owned and not run hard or abused. Any ideas as why the bearing failed. If it was a lubrication problem I would expect the piston to fail before the needle bearings.
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