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Discussion Starter #1
Have some questions for you gear heads. I will be fixing the base gasket leak on my '07 this winter. I could just do a re-ring but how about boring it out as big as it will go with maybe one bore left for safety ? How thick is that cylinder liner and how many oversize pistons are available? Can I gain 5 or 10 cc with boring the stock liner? Also if I re-bore shouldn't I also do a valve job? That would be OK with me as I could do a minor port and polish job while the head was down. I don't have a lot of money but like working on motors. I also don't want to pump it up too much and loose any reliability but I'm thinking a few more cc's and a little better flow can't hurt. I also want to keep it simple, so what do you guys think? Your advice will be greatly appreciated.
 

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Have some questions for you gear heads. I will be fixing the base gasket leak on my '07 this winter. I could just do a re-ring but how about boring it out as big as it will go with maybe one bore left for safety ? How thick is that cylinder liner and how many oversize pistons are available? Can I gain 5 or 10 cc with boring the stock liner? Also if I re-bore shouldn't I also do a valve job? That would be OK with me as I could do a minor port and polish job while the head was down. I don't have a lot of money but like working on motors. I also don't want to pump it up too much but I'm thinking a few more cc's and a little better flow can't hurt. I also want to keep it simple, so what do you guys think? Your advice will be greatly appreciated.


Hello,

If this can be useful..!

I have these measures ...



 

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Discussion Starter #3
Thank you kindly christddel, it is very useful. I am trying to find a 70mm Wiseco piston kit but even their website says they aren't currently available? I thought they were common as corn dogs, hmmm. Maybe I'm doing the search improperly?
 

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IIRC Wiseco lists the pistons as YFM200 items.
 

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Discussion Starter #5
Ok I think I'm getting this sorted out a bit. I found Wiesco pistons for the YFM/YTM 200 &225. The biggest 200cc piston is 68mm and the 225cc piston is available in 70mm. I did my my searching and read that 70.5 would be max safe overbore although I would think 70mm would be safer. I also found posters saying we can use the 225 piston w/no problem and several more saying no way Jose'. So I' still confused on that issue. I don't want to go over Wiseco's advertised 10.25 to 1 compression ratio and I''m thinking I might with a 70mm bore and 225 piston? Can we use a stock Yamaha 70mm 225 piston of some sort to get compression of around 10 to 1ish? Then which head gasket to use? I also read Ben Parry posts concerned with proper head gasket sealing. This is starting to seem not so simple after all. But I'm hoping to get some expert advice from you all soon.



thank you kindly, mac
 

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Bear in mind compression ratio will increase a bit due to the overbore itself.
 

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Discussion Starter #7
Bear in mind compression ratio will increase a bit due to the overbore itself.


Thanks lizrdbrth. That is exactly what I was thinking. I don't want to loose any reliability. I'm trying to find out if the YTM/YTF 225 factory piston is a drop in? They look a bit dished. I think that would help keep the compression ratio more reasonable than the Wiseco.
 

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They are dished. Advertized c/r is 8.25. They offer them in 10.25 as well.



Standard bore for all the 225's and 230's is 70mm.If you want to keep compression in the nines, what you want is a standard bore 70mm TTR225/ttr230/XT225/YFM225 et al piston. More or less flat-topped, no dome.



214cc's with the TW stroke. Too thin on the walls for my taste. I'd want the 225 cylinder, but you'd have to expand the case mouth, and if you stsrted with a used cylinder you'd probably have to freshen it up to at least 70.5.
 

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My personal thoughts on this are, unless you are going to use a long stroke crank from a 225 engine with your over bore then it will not be worth the expense or trouble. It's the increased stroke that gives the extra performance more than anything.



Sorry if I sound like a killjoy but I don't think you will be happy with end result.



It's your choice though.
 

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Discussion Starter #10
My personal thoughts on this are, unless you are going to use a long stroke crank from a 225 engine with your over bore then it will not be worth the expense or trouble. It's the increased stroke that gives the extra performance more than anything.



Sorry if I sound like a killjoy but I don't think you will be happy with end result.



It's your choice though.




No need to apologize for killing my joy, I appreciate all of your advice! Since I must tear down the the top end to replace the base gasket it wont be much more trouble to bore my cylinder and start fresh rather than just re-ring it. The question is how big and what piston? It will be a more expensive route, how much depends on which piston and where I get it from. My major motivation is the old adage 'there is no replacement for displacement'. Willing to spend a bit of money but not willing to pull the engine and split the cases I see this as my opportunity. I do not disagree with your long stroke preference, but in my case I will be retaining the stock exhaust, carb, airbox, ect. So would not a short stroke big bore engine retain the factory power band characteristics and piston speed? I am concerned that a 70mm bore would be to thin for the tw200 liner although I have seen it suggested by some to be fine. Hope for some more input on that one and which head gasket to use with a 70mm piston, tw200 or xt225 ?



thanks for all the help so far, keep it coming

regards, mac
 

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My personal thoughts on this are, unless you are going to use a long stroke crank from a 225 engine with your over bore then it will not be worth the expense or trouble. It's the increased stroke that gives the extra performance more than anything.



Sorry if I sound like a killjoy but I don't think you will be happy with end result.



It's your choice though.




Yes, ideally should be placed a crankshaft 225 (58mm and short rod 105,5 mm)



 

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If you have the time and a little $ the 225 cylinder bored to 71mm is worth the trouble 71mm X 55.7mm stroke= 221cc's. I have done this without any re-jetting and it is a nice improvement over stock. Nothing like a little extra power to enjoy! Wiseco Piston YTM/YFM 225 #4312M07100. The Wiseco piston has a small raised dome but the TW piston is domed also. My 2 cents.



Ronnydog
 

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Discussion Starter #16
Thanks much everyone for all the motor building advise. I know enough about motors to get myself in trouble. So I like Ronnydog's 221cc plan. I think I would use a 225 cylinder and Yamaha piston, factory parts / factory reliability. Tell me though, the higher weight of the bigger pistons, does that cause vibration or balance issues? I'm assuming that is more to do with the rotating mass of the crank and balancer?



regards, mac
 

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Thanks much everyone for all the motor building advise. I know enough about motors to get myself in trouble. So I like Ronnydog's 221cc plan. I think I would use a 225 cylinder and Yamaha piston, factory parts / factory reliability. Tell me though, the higher weight of the bigger pistons, does that cause vibration or balance issues? I'm assuming that is more to do with the rotating mass of the crank and balancer?



regards, mac




Yes, vibration will be intolerable. As soon as you finish building the engine you must send it the me so that I can properly wear it out and dispose of it in an environmentally appropriate manner.
 

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Discussion Starter #18
Yes, vibration will be intolerable. As soon as you finish building the engine you must send it the me so that I can properly wear it out and dispose of it in an environmentally appropriate manner.


Thanks for the offer qwerty, but I am perfectly capable of "properly wearing it out" myself. However for a modest core charge and oversize shipping fee and oversize handling fee I would be happy to let you dispose of it in an "environmentally appropriate manner".



thank you kindly, mac
 

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So, you have a TW200 and you want to change to a different piston with a different weight. But I'am sure, you don't have to spend a mind on possible vibrations.



I have a TW125 upgraded to 200ccm, but I haven't changed the balancers.

I am still using the much more lighter 125 balancer and I don't have "intolerable vibrations".

The vibrations are a bit higher, but's not a bad feeling for the driver.
 
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