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Discussion Starter #1
Cb360 is getting a new top end. Starts and runs but compression is way down and can’t get over 55mph.
Opened the box one of the Pistons arrived in to find this...break out the file? leave it alone? Or absolutely don’t run it?
 

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I personally wouldn't use it, even if you filed the edge smooth. Would you want to rebuild the top end with that piston only to find that its no good and have to pull the top end back apart?
 

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it's not as bad as you think. The crown of the piston shouldn't be touching the cylinder wall anyway. It's the rings that are in contact with the cylinder and provide the compression needed.

That said, since it's a newly ordered piston, you should be able to get an exchange for it no?
 

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Discussion Starter #4
That’s kinda what I was hoping to hear...and no exchange as it is a NOS Suzuki gs850 piston
 

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I would still do some closer inspection with a magnifying glass and see if there are any cracks around the area. If so do not use it.
 

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Agree with 76TWIN as to the best choice of your three options......

Send it back for exchange, ....But since that's not feasible.....
Lightly dress and run it, ( live with it as-is )......
Fill the ding with Aluminum welding/brazing rod and then dress it......(lots of work for minuscule gain)
 

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Discussion Starter #7
Ok. I will get it under a magnifying glass and check it out, if it seems solid I will lightly dress it and proceed. This is not a race bike nor a spare no expenses type project so I’m not overly concerned with making big performance gains outside of knowing its running as well as it can.
 

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Wipe or dip it in gasoline, acetone or a similar THIN solvent. Wipe it dry then check closely. If there is a crack, the solvent will be creeping out of it. Old gunsmith trick for checking receivers. ;)
 

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Discussion Starter #9
Pro tip! Awesome, I will be sure to do that. Imagine, if I had a perfectly good piston I never would have learned this.
 

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As NOS Suzuki it's been out of production for years and you know you won't be able to get an exchange. I would try and get a few bucks back from seller. It won't be a problem though so just file it flat with a very fine file and use it. The alloy isn't so brittle it will crack easily and, real kicker for guy I worked with, the pistons for Suzuki GS850 were made by the company that Honda has controlling interest in so it's really a Honda piston anyway (Gerry was 'corporate Suzuki' at one time :D)

Oh, it will go over 55 if you rev it to red-line through the gears. Stick it in top at 35, no way ever unless you fall off a cliff
. 360 need to be running at more than 6K to get anywhere
 

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Discussion Starter #11
More good news. Awesome. Strangely I’ve always pushed this bike to redline (owned it for 5 years, taken care of it but also ride it pretty hard) but lately it Won’t rev over 6k...wondering if maybe carb slide is stuck acting as “limiter”? (Just regurgitating something I read) Hence why I’m pulling the carbs for an inspection/ deep clean.
 

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Slides only stick if bike hasn't been used in a while, usually a few months. More likely a damaged diaphragm? Could also be low battery voltage, have you fitted a new regulator/rectifier?
 

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Discussion Starter #13
Yeah new reg/rec this season and high output rotor. Electrics have never been more reliable on this bike...I struggled with weak charge for a long time. Maybe you’re right about the diaphragm though...
 

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Easy to check, just remove carb tops and pull slides out then run finger underneath diaphragm while you looking at top. Slight stretching shows even tiny splits but you have to look close. Usually damage is on inner part nearest the nylon hold down ring but I've seen damage all over the place over the years. Even poked screwdriver through one good diaphragm trying to get things apart. Whatever you do DO NOT FOLLOW CMC video instructions and 'glue' diaphragms to carbs. The float height needs to be 21mm, done correctly not 16mm done incorrectly as Brendan shows in his video

 

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I’m learning more and more there should be lots of disclaimers on their videos. Like the differences in air fuel mix assemblies on the cb350. The jet sizes. The floats. The wiring. They never really point out how specific the differences are say between a 71 and 72 but imply they’re the same. Which isn’t true. In my case I had an o ring and washer on my air fuel mix screw that aren’t supposed to be there
 
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