splitting the B-Motor crankshaft on YZF-R125

B-motor crankshaft rebuild progress in pictures…

what we’re starting with..

lower timing gear and bearing removed

trying to figure out the balance factor

Y-plate from 8mm steel to fit the webs and spread the load evenly

main plates either side..

Y-plate to support the back-side of the web

The Y-plate fits these webs quite snugly and support the thin edge while pressing

I guess that worked then


comparison of the stock rod vs the forged one from motogaga going in

noticably wider

the forged rod is a joy to behold.

[LOTS of measuring and calculating follows]

but then….success!

there’s no bearing or lower timing gear on it yet so it looks a bit naked

This is now the crank pressed back together but now with the forged conrod and piston pin (big end) kit from Motogaga. It still needs aligning/trueing but that’s the next step.

I did a LOT of measuring and weighing and spreadsheets of different combinations and comparing of balance factors and where I ended up is with this setup, which should almost completely restore the balance factor to stock, even with the bigger 180cc piston.

Whilst the rod is heavier than the stock rod, and the crank pin is lighter than stock (so the overall pin + rod weight doesn’t change vs stock), the weight is moved from the pin to the rod and somehow magically that seems to fix the balance factor.

As far as I could tell (measure), stock Balance Factor (BF) is about 0.48.

Apparently “ideal” BF is about 0.5, but anywhere from 0.4 to 0.6, 0,7 even are acceptable and it seems to mostly depend on the engine as to what works best in practice. (allegedly) race bikes which have shorter life expectancy for bearings and components are sometimes run with super low BF but seems like 0.4-0.6 is probably sorta where we want to be, and ideally as close to stock as possible, since it revved beautifully before.

So if we started off at a BF of 0.48, adding the heavier 180cc piston seems to have taken us to 0.4 (so actually looks like it could have affected which rpms the engine is happy at), but hopefully now with a 180cc piston, and the forged rod and lighter pin, that looks to bring us back to about 0.49, so very close to stock.

That’s the theory anyway! 😉

I’m editing a video going through all this atm but though ti’d get some pics or progress up in the mean time..

2 thoughts on “splitting the B-Motor crankshaft on YZF-R125”

  1. Hi there, we’re did you get the replacement crank shaft sprocket, mine broke and can’t get one for a 2016, bought a 2020 one and it’s different

    1. hiya, crank shaft sproket as in the lower timing gear? I got mine from sxparts.com – they seem to be able to get most/all of the engine bits. Postage isn’t cheap from indonesia but they have all the toys. Presumably you realise that you need a hydraulic press to fit it, right? Let them know I sent you! I hope that helps 🙂

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