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2008 YZF-R125 (180cc)
552 Posts
Discussion Starter · #1 ·
I'm starting a separate thread for this so I can just dump pics and progress without confusing my main bike thread.

This is a spare motor i bought as broken which I'm restoring to use in my main bike. The basic idea is to build the exact same engine as I have in my bike, but sort out a couple of issues.

Because of a scored little-end bearing I've had to get the tools to split the crank and replace the con-rod, for which there is a nice forged item ready to go in (it's a thing of beauty) .

scoring inside the little end from having to press the pin out.
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There's one thing though, namely since adding the 180 kit on my main bike I've always felt the vibrations were a bit off. Like it was happy to scream all day long at 8-10k before as a 125 but now it's kinda "reluctant" in that range. Some of that was fuelling and no doubt some of it will be due to the stretched timing chain on my main bike and probably quite worn bearings and basically everything from havign been beaten on relelntlessly for 28k miles.

So I'm hoping just freshening everything up will help a lot in general, but while I've got the crank in bits i'm measuring/weighing some stuff and seeing if i can do somethign about the vibration thing. It's looking liek the crank has a slightly weird balance factor whcih doesn't really make sense to me (hardly any counterbalance) but the bigbore piston is about 30g heavier than the stock and you can't tell me that doesn't affect the balance of things flying back and forth 80+ times a second, and must increase load on bearings, and may even mess with the counterbalance weight/shaft.

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So whilst I don't really understand the choice of balance factor, when it was 125 it worked great so my aim is to see if we can get it back to whatever the 125 balance factor was, and then hopefully it'll rev a bit more freely, even with the bigbore kit.

It looks like there's two choices - either drill some holes in the crank webs at the light end, effectively ging the heavy end more weight (i could prob do holes). Or adding weight to the heavy end, either by drilling holes and filling with lead/tungsten plugs (ha, nope), or be potentially welding on a small steel patch in just the right place.. seems preferable, but only if there's clearance to do so.

drilling the light end would be something like this:
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pics and measurements to follow, just wanted to get the ball rolling here.

The rough plan for this engine is all new internal bearings, new close ratio racing gearset, forged conrod, 180cc cylinder, stage 2 cam and for now stock head but freshly lapped valves etc, R15 super duty clutch like my current bike etc, brand new timing gears, chain and guides..basically all new internals...but all of that only works if i can get the crank apart and back together again, and straight, to within tolerances.

If it works out as planned that should make this a beautifully smooth big bore conversion :).

will follow this up with more pics once I've got them off the phone
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