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Hi all,

This is my first exercise in motorcycle mechanics and I'm pretty excited. I bought this bike last year and rode it for the summer. In August it puttered out of me at half-power and I new that one of the pistons was not functioning. On inspection as you can see there is a big ol' hole in the top of the piston. I now have the top end disassembled and am looking for advice.

Firstly please take a look at the cylinder walls--are they in decent condition? Or do I need to send them to be machined and get over-bored? I am having trouble researching whether I will need to re-bore them or I can just buy the same size piston and rings to replace them.

Secondly also take a look at the valves, too me they looked to be the only other part that might need replacement? ----but what do I know.

Thirdly I would like to know what people are thinking about whether there could be other damage in the lower crank case. After removing the oil there was a small amount of debris in the old (not sure if that's normal). Should I be worried that the debris from the piston hole could have damaged the lower engine?

Finally I would love to hear thoughts on how this could have happened in the first place! Was I running the bike wrong? I had previously had it in at a garage where they put in a new battery and replaced the spark plugs (this was before I was committed to taking care of it myself). I was wondering perhaps the garage put in a longer spark-plug--(ie hotter ignition closer to the piston?)

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Let me know what you think---anything is appreciated.

The only other thing is I'd rather not take apart the lower crank case. I'm not fully confident in that mechanical expedition/would rather avoid the downtime!
 

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Do you like the bike is the first question. Meaning to you want to fix the problem, build an awesome motor, or something in the middle.

With today's ethanol in the gas increasing the compression ratio would go along way. Sad thing is it cost about $350 for both pistons here's a link however they can be found cheaper. Product Search Engine For All Products - Wiseco Piston Inc.

Obviously your buying new pistons so you need to decide if you want to upgrade or go with a stock 1st overbore. I say 1st overbore for best results in the end.

The way those valves look I would suggest new valves and valve guides just because.

Then all you sould need is a gasket set and your done.

I can't trust anyone to work on my engines so I would assume they fudged it up.
 

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To rebore or not is not a question of 'does it look ok?' You'll need to take measurements of your cylinders and compare them to the specs in the factory service manual. The FSM is available online if you do a search.
 

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Same goes for the valves if theyre in spec and dont leak (acetone test) leave them, if they leak or are out of spec replace them
Youre going to need to hone the bore whatever you do even if the bores are wighin spec
Your holed piston could be timing, running rich and building up carbon or too lean or plug heat
 

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I would take the cylinders and head to a reputable machine shop (most automotive shops are more then capable of working on motorcycle engine parts) and have them measure the bores and inspect the head. They will be able to tell you if you need to bore the cylinders. If your cylinders are good, I have a set of standard pistons that look like they are brand new that I would sell cheap.
TOOLS
 

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Personally I would bore it to fit max oversize piston kit
There are cheaper sets than the Wiseco, take a look on the CB350 race sites
The scratches in cylinder will cause oil burning unless they are real shallow and can be hones out without going oversize?
Did you find out why it holed piston? (timing not set right one side?)
 
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