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Discussion Starter #1
Hi,

I am French I have had tw for years now, I love these bikes but like many europeans I think it lacks a bit of pop ;)

I started documenting about a possible upgrade to 200 and found thanks to this forum that it was actually doable.

However the parts needed to go from 125 to 200 are hard to find in Europe at reasonable price... this made me postpone my upgrade for years.

But it seems our Chinese friends on aliepress heard my prayers and it seems possible to now buy an XT225 cylinder kit AND and an YFB (or YBF I don't know) 250 cylinder kit too !

The only part missing now would be a 185/200 crankshaft or 225/230 etc. but I don't want to change the cam sprocket so...

After looking on Ebay and everywhere else the only crankshafts I find at an affordable price are pretty old and worn out.

This lead me to look for another solution, when looking at Yamaha parts and their serial number, it seems the only difference between a 125 crank shaft and 200 are the connecting rod and the first weight (starting form the teethed end) everything else being identical.

A 200 connecting rod is not hard to find thus the big question:

I don't imagine the difference between crankshaft weights being ridiculously high (but I am no expert) so the only last part to swap would be the connecting rod, do you guys think I can keep my 125 crankshaft changing only the connecting rod (for a 200) and run the bike with a 225 cylinder kit and 250 head cylinder?

I have read quite a lot on this subject on this forum and on the French forum too but couldn't find any real answer to this precise question, I apologize if it has been adressed already.
 

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I am guessing no based on the fact that a TW200 has a stroke of 55.7mm and a TW125 has a stroke of 48.8mm. The XT225 gas a stroke if 58mm.

I think if you use the 125 crank and the 225 piston and cylinder the piston would be 5mm short of reaching the top and 5mm short of reaching the bottom. The engine would probably run but not well. It would be very low compression.

Contact Sebastian and see what he says. He is, or at least was, very generous with his knowledge.

 

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Discussion Starter #3
Thank you for the quick reply.

I knew the distance traveled by the piston was different but was hoping it could still work one way or another...

Thanks a lot I'll try to find a cheap crankshaft ;)
 

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Discussion Starter #4
So juste to recap, the parts I think I can manage to find are :

  • SR185 crankshaft
  • xt 225 cylinder/piston etc.
  • YBF250 cylinder head BUT with 125 cam sprocket and chain

From what I understand I shroud be able to up my 125 to 200ish, could anyone confirm this setup would be ok please?
 

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I don't know what exact timing chain the 125 uses and what timing chain use 200 and 225 engines. But isn't the 125 chain weaker? It would make sense to me that bigger valves in bigher head would have stronger valve springs, thus putting more tension on the timing chain.
If someone knows the different timing chains specs, I'd be glad to hear them. I know a little about some newer model of xt that uses completely different chain?
I'm from europe too and I too would like some more performance. I've been dreaming about putting an xt/tt350 engine in my TW :D I know about the chain sprocket offset, but everything can be done.
 

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Hello,
The crankshaft of the 185 SR is identical and has the same stroke as that of TW200
The timing chain is the same on 125/185/200/225
On the XT225 (after 2006) the "chain and sprockets" timing is different, the chain is a "CL04-2 / 3"
If you place a 200/225 cylinder piston, you must also place a 200/225 cylinder head.
Ideally the balance shaft should be changed
The crankcase must be bored for the passage of the cylinder
 

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So juste to recap, the parts I think I can manage to find are :
  • SR185 crankshaft
  • xt 225 cylinder/piston etc.
  • YBF250 cylinder head BUT with 125 cam sprocket and chain
From what I understand I shroud be able to up my 125 to 200ish, could anyone confirm this setup would be ok please?
Hello,

Exactly this assembly in a 125 crankcase is correct, (after its reboring)
209695
 
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