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Discussion Starter #1 (Edited)
Got a complete bottom end and cylinder head combination off the same low mile 09 TW. Thus far i am thinking I will build the 58mm crank kit and the yfm250 cylinder sleeved per. Sebastians list with the 74mm LA-Sleeve and the oem Yamaha 9.5 to 1 piston. Final bore still subject to change. I kind of like the 9.5 to 1 idea for not making it as fuel sensitive. Coach me on the best logic here based on experience.
 

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The 6 speed conversion would suit you better, and beef up the clutch while you’re at it

This should give you a bike (correctly geared) that will be capable of around 80mph — with brakes that can’t stop it from 65mph, and frame geometry that becomes unstable at 70 (on knobblies)

Talk to Joemama about extended swing-arms (which sorts out the frame) - the braking situation is a little more complicated ……. ;)
 

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Discussion Starter #7
Well put, fortunately I don't want to go that fast any more so it should work out perfect. Little more lower gear for crawling around and a little more grunt coming up to speed and a little less effort when going 55-60 with my fat ass on top of it. Already done the stretched swing arm, hoping to install it Sunday.
 

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Discussion Starter #8
I am finding small shops boring cylinders all over the place still but nobody in the area yet that can or wants to deal with removing the old sleeve and replacing it. Kind of perplexed, a lot of the good old engine shops around my area are gone these days as we move into a crate motor era. Talked to a slick old engine guy that has been a friend for decades, he built a few drag race engine back in the day for me but retired now. He give me some old timers tricks for extracting the old sleeve in my shop with the things I have to work with. I think I will head that direction and see if I can present one of the local boring shops with an empty aluminum cylinder and a sleeve and piston kit to work with instead. Wish me luck.
 

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I may be way off here. Was thinking of some folks having the leak around the base of the cylinder. Putting the bigger jug on and add the high compression piston might be a bit much. Still I am excited to hear how it goes with the 250 changeout, costs and such. Wouldn't mind doing this to one of mine, I have run out of power a couple times and just had enough to get me over the hill.
 

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Discussion Starter #10 (Edited)
I may be way off here. Was thinking of some folks having the leak around the base of the cylinder. Putting the bigger jug on and add the high compression piston might be a bit much. Still I am excited to hear how it goes with the 250 change out, costs and such. Wouldn't mind doing this to one of mine, I have run out of power a couple times and just had enough to get me over the hill.
My first choice is to go with the OEM yamaha 9.5 to 1 74mm piston combination that Sebastian has listed, if not I will stop with a 71mm readily available through wiseco since Brian says his is not fuel sensitive, I was concerned about that prior.

If there is a known issue sealing up this biggest bore option that is already known I really would like to hear about it before heading down this road. I was thinking the 9.5 to 1 74mm might be the less problematic of the 2 options -vs- the 71mm at higher compression at 10.25 to 1.
 

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Discussion Starter #11
I am considering 3 different 6 speed gear sets, from xt225 and or ttr225 and wondering what the opinion of those with 6 speeds or have considered doing this conversion are about the ratio choice. What are your thoughts on the 2 different ratios available for this deal. The XT 225 has a deeper 1st gear than the ttr225 and both are deeper than the TW with is already deep gear set for taking off from an intersection. Seems maybe XT for crawling off road and the ttr might be more friendly for the road riding ??? Am I on the right track with my thinking ??? Chart below is someone elses work, I just hung on to it for the future.

 

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Discussion Starter #14
I like the gear ratio spread on the XT!

Dang it... now I'm going to be thinking about an engine swap! :BangHead:
That low first would just be even more annoying on road but maybe real nice off road at a crawl which is my speed these days. Maybe 2nd is low enough to take off thru and intersection ??
 

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My first choice is to go with the OEM yamaha 9.5 to 1 74mm piston combination that Sebastian has listed, if not I will stop with a 71mm readily available through wiseco since Brian says his is not fuel sensitive, I was concerned about that prior.

If there is a known issue sealing up this biggest bore option that is already known I really would like to hear about it before heading down this road. I was thinking the 9.5 to 1 74mm might be the less problematic of the 2 options -vs- the 71mm at higher compression at 10.25 to 1.
Don't want to scare you off this upgrade, just something to think about. With 25% more trying to lift that jug off the case on the compression stroke it might be too much. Is an interesting thread.

As far as the leaking gasket where the jug sits on the case, those have been on stock bikes that I have read.
 

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My current engine set up is as follows ..... TTR225 cases, TTR225 steel sleeved cylinder with a 70.5mm wiseco 10.25-1 piston, XT225 crank & balancer, modified TW output shaft and the six speed TTR225 transmission, TW200 side covers and flywheel.

I really like how this has worked out so far. The six speed trans gives it more of a dirt bike feel and the extra displacement and compression helps the bike throughout the power range.
 

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My current engine set up is as follows ..... TTR225 cases, TTR225 steel sleeved cylinder with a 70.5mm wiseco 10.25-1 piston, XT225 crank & balancer, modified TW output shaft and the six speed TTR225 transmission, TW200 side covers and flywheel.

I really like how this has worked out so far. The six speed trans gives it more of a dirt bike feel and the extra displacement and compression helps the bike throughout the power range.
This is the same set up I am running except a 71mm Wiseco with a stock bore YFM250 cylinder. We ride at sea level and use premium fuel. It is on the edge of detonation when hot and being lugged down too far.... For most normal riding, the HI COMP piston would be fine, especially as you go up in altitude....
 

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I am finding small shops boring cylinders all over the place still but nobody in the area yet that can or wants to deal with removing the old sleeve and replacing it. Kind of perplexed, a lot of the good old engine shops around my area are gone these days as we move into a crate motor era. Talked to a slick old engine guy that has been a friend for decades, he built a few drag race engine back in the day for me but retired now. He give me some old timers tricks for extracting the old sleeve in my shop with the things I have to work with. I think I will head that direction and see if I can present one of the local boring shops with an empty aluminum cylinder and a sleeve and piston kit to work with instead. Wish me luck.
https://www.ebay.ca/itm/Yamaha-Moto-4-250-YFM250-Cylinder-Piston-Gasket-Kit-1989-1991/381812082762?hash=item58e5c6084a:g:cbQAAOSwwrtZxXOp&vxp=mtr

I have been eyeing these kits up lately .... super cheap low compression china knock offset up for the YFM225 & YFM250.
 

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Discussion Starter #19 (Edited)
Still undecided on the cylinder and bore, mostly cause I am striking out on any local shops being able to do the bore and re-sleeve, all just a bunch of fly by night bore shops. May end up at 70.5 or 71 since it is easy and proven. With the time and cost factors living in a rural area, it almost makes sense to just order it all new oem from one of the Yamaha parts discounters.

Is the stock cylinder being bored to 70mm as Sebastian suggests is ok, thought of as being less robust than the 225 cylinder being stock at 70mm, has anyone had any issues with it or actually tried it. Never mind, dug out the answers here in Elimes engine thread.
 

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The cooling fins are the same! I went with a yfm250 cylinder piston kit! All found on Ebay! You can use the low compression piston or sell it to recoup money for the high comp Wiseco piston as I did.
Item number 381169823810 $88.98 free shipping!

Wiseco high comp piston kit!
4312m07100 71mm with all parts. Piston, rings, pin and clips.
Wiseco piston $88.62 free shipping right now! 229cc 10.25 to 1 ccompression ratio!
Great quality with gaskets ready to bolt on.

We have a great shop here I am taking my old jug to for the 74mm sleeve. I also have ported, replaced my valves and the guides in the head and ordered my Webcam largest cam kit springs and all. I am making my own tig welded 1.5 inch od exhaust header like the factory pipe but larger from mandrel bent exhaust tube bought at Summit Racing that is a hooker header product. The muffler is being changed to a Procircuit SY02250-SA for a larger lighter muffler for better flow and spark compliance. Found a 36mm carb for a 400cc motor that has a pilot, main and a intermediate fuel jet to help with better fuel control. Ordered the thunder products Intelejet and torque wing. The possibilities are just about endless. I bought an extra ttr225 motor to build a stroked 74 mm bore engine also. I got a 2001 TTR225 for 90.00 that I am restoring and improving everthing from the ground up to put lights on and turn it into an all around universal bike I can use just about everywhere. At 61 just playing is more fun now than when I raced motocross for 20 years. I am having a blast and the kids and grand kids too!
 
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