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Discussion Starter #1
New to the forum and I just picked up this sweet looking all original 1971 cb175 k4. I heard it run and bought it... I thought it needed some carb work, Unfortunately there is no compression on the right cylinder. It will run at idle. Pulled plug and found what I believe to be melted aluminum on the plug and it's wet with looks like water or condensate, no gas smell. I feel no compression with my finger over the plug hole while cranking. Left side plug is dark and the exhaust pipe has blued.
Are there any replacement engines available? I really like this bike but dont really want to get involved with an engine rebuild at this time...
Anyone on the forum offer engine rebuilding?
Thanks for any info
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Welcome to the forum. I moved your post to the introduction area since it essentially is one. It's a very nice bike you've found, but sadly it sounds like there's a hole burned in the right piston based on your mention of aluminum on the plug. These engines are not all that difficult to rebuild if you have any mechanical experience. If you decide not to do the job yourself, perhaps another member would be interested in doing the job but in that case it would obviously be helpful to know your location.
 

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Discussion Starter #3
Hi Tom

Thank you for the response. I think you're correct with the piston being damaged. What would cause that? I know the guy before me installed a different coil maybe the timing was way off? Not sure.
I live in New York, Dutchess county area. I'm not sure if anyone is local to help me out, please reach out to me if anyone can guide me in the right direction to get this little bike running.
I bought it as a small project thinking maybe a carb rebuild would fix it. Never thought it would have been a top end engine replacement.
I appreciate any info anyone can give me.
Thank you
Joe
 

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Joe, there could be many reason for the holed piston if in fact that's what happened. Running wide open (70 to 75 mph on that bike, possibly 80 at best) for a long period could do it, running too lean could also cause it, possibly too advanced timing... but the cylinders will have to be bored, new pistons and rings and all the other top end parts inspected for wear limits and replaced as needed. The engine has to come out of the frame to do the job but the bottom end - crankshaft, transmission, clutch - would not have to be disturbed unless something more serious was seen on inspection of the top end (cylinders, pistons, cylinder head, valves camshaft, rocker arms) like an issue with either the small end of the rod due to the holed piston, or the big end of the rod if the cause had to do with oil loss or starvation. At the least, the right crankcase cover should come off to clean the centrifugal oil filter and check the oil pump and screen
 

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Discussion Starter #5
Thanks for that info ... I've been searching around and cannot seem to find any piston/cylinder kits that I can just bolt on. Unless I am overlooking or searching for the wrong items. Anyone here know where I can find some pistons and where can I bring my cylinder to be honed out? I appreciate all the info.
 

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Member Tools1 does machine work. He's in KC, MO so you'd ship him your cylinders and pistons after you get them. No one sells kits for these bikes, you buy a set of pistons, rings, pins and clips and the machinist measures your cylinders and bores them to fit the new oversize pistons. Generally we tell people to take their cylinders to a machinist first and have them measured so you'll know how big the oversize pistons need to be, but if you bought a set of 0.50 mm oversized pistons or larger, up to 1.0 mm over, that generally covers minor cylinder damage caused by a holed piston. Send Tools1 a private message (Conversation) and he'll advise you - Tools1
 

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Discussion Starter #8
So I poked around a little this afternoon and was actually able to see a hole in the top of the piston by shining a light in the spark plug hole. I was able to get a picture too...
I began the disassembly and will be looking for some parts to repair it. Anybody know a good source in the USA for pistons? I'll keep updating as I go along...
Thanks for any info...
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