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2008 YZF-R125 (180cc)
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if it just needs a new cylinder that's not a trivial task but totally doable if you're mechanically minded.

I would say the "danger" in this lies in any secondary damage that may have come from the failure. So if just the rings let go no worries, new cylinder and you're done. But if the rings went because he forgot to put oil in it (which happens) and then it overheated as it failed, then that could escalate into something more complicated. e.g. if you happen to damage the con-rod bearing surfaces removing the old fried piston then you may end up having to replace the con-rod which means bottom end apart. Ask me how I know this lol.

I did a few of videos walking thru this entire process of a cylinder swap, youtube link in my sig."blew the motor" and the next few from there.
 

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2008 YZF-R125 (180cc)
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632 Posts
oh right if you're a mechanic then you should be fine. I went the same direction and it's a little different. If you're coming from cars I'd say the main difference is that these have a crank which is pressed together so to do something which you'd think is simple on a car like swap out a con-rod, you actually have to disassermble the entire engine and gearbox (separate the crank casings) and then press the crank apart in order to fit a new rod. Plus the gearbox is part of the deal so added fun/complexity and there's no avoiding it ;).

So when you separate the knackered piston from the con-rod if the wrist pin is seized (quite likely).. be super careful not to score/damage the bearing surfaces on the little end of the con-rod or you're in for a bottom end rebuild. Rather a little heat & patience with the force than just force. My bottom end rebuild was only supposed to be a cylinder swap but exactly that happened and here we are.

There's a pdf service manual for the gen1 bikes which is fairly easy to find online and is supremely useful and the haynes is good too and has info the service manual doesn't. I'm just finishing up doing a full bottom end rebuild in my fairly makeshift workshop (so it's doable with pretty basic kit apart from needing a press) and that's really about as "bad" as it can get engine-wise. See the "B motor" thread on here and there's some bits doing the crank on youtube too.

Twostrokestuffing on youtube is really worth a watch and whilst his stuff is mostly 2-strokemotors, he covers a lot of engine rebuild stuff which also applies to what we're doing.

Hope that helps :)
 
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