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Yz125 to 144c bore kit?

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Hi I have a yz125 2017 model and am wanting to bore it out to a 144cc. What way should i go? Should i use the athena kit and do it my self or send it in to someone. Thanks!


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What's your intended use, once done? MX, XC, trail riding... etc. 

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If you just send out your cylinder to have it done, you still have to grind away at your cases some. I did mine myself, but it depends on your level of comfort doing this. That was with an Eric Gorr, with the athena, I am not sure if you have to do this, but I would assume you do. Eric did have a video on how to do it properly. Having EG or one of the other companies do it they will port the cylinder as well, vs. the athena.

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Shift up a gear......Sorry had to. I went from a 450 to a 125 this year and it was the best decision I made. Got the fun back and I am not much slower either......there are strengths a 125 has that have been forgotten about if you are willing to shift like a ballerina.....which is half the fun.

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@johnk408 i expected tp have to grind the cases some and personally i would be fine doing it my self. My main concern is cost and things like that. If its cheaper to do it my self i would gladly do it.

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@minkerides33 yeah i have tried to shift more and speed issue is mainly from my lack of experienced but i felt that as i got better and better i would eventually need a little more. Just a little bit more so i could keep up with the 250f's haha. I agree with you though but im trying to think about the long run. Lol. Ya made my day!

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Shift up a gear......Sorry had to. I went from a 450 to a 125 this year and it was the best decision I made. Got the fun back and I am not much slower either......there are strengths a 125 has that have been forgotten about if you are willing to shift like a ballerina.....which is half the fun.

@minkerides33 yeah i have tried to shift more and speed issue is mainly from my lack of experienced but i felt that as i got better and better i would eventually need a little more. Just a little bit more so i could keep up with the 250f's haha. I agree with you though but im trying to think about the long run. Lol. Ya made my day! Also ignore the fact that i didnt see a reply button till just now.
If you just send out your cylinder to have it done, you still have to grind away at your cases some. I did mine myself, but it depends on your level of comfort doing this. That was with an Eric Gorr, with the athena, I am not sure if you have to do this, but I would assume you do. Eric did have a video on how to do it properly. Having EG or one of the other companies do it they will port the cylinder as well, vs. the athena.
@johnk408 i expected tp have to grind the cases some and personally i would be fine doing it my self. My main concern is cost and things like that. If its cheaper to do it my self i would gladly do it. Also ignore the fact i didnt see a reply button

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For a strong MX engine I would recommend modifying your cylinder. I can build a better 144 MX cylinder from the stock cylinder than starting with the Athena.

I use POWERSEAL to strip and bore the cylinder. Then have them send it back to me and I do my port work. Then I send them back with the piston and power valves and they fit it all together and plate the bore. Once I cut the cases and get it all together I cut the head to go with it.

I think they can even cut the head if you like. You'd have to check with them on that.

Edited by 218

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For a strong MX engine I would recommend modifying your cylinder. I can build a better 144 MX cylinder from the stock cylinder than starting with the Athena.
I use POWERSEAL to strip and bore the cylinder. Then have them send it back to me and I do my port work. Then I send them back with the piston and power valves and they fit it all together. Once I cut the cases and get it all together I cut the head to go with it.
I think they can even cut the head if you like. You'd have to check with them on that.

@218 thanks! I appreciate the response and i will look into it. About how much do you think it would be around price wise?

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I think I had about $400.00 in the last one I did, with shipping cost, for their work.

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I think I had about $400.00 in the last one I did, with shipping cost, for their work.

Thats pretty good price for what ive seen kits going for

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There was a post on here about the Athena kit, not good.... they seam to have trouble with the o-ring between the cyl & head if I remember.

We had great luck with a Eric Gorr YZ144 kit, the case grinding is pretty easy.

I hear the Max Power 139 kit is very good too!

Good Luck!

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A reoccurring (it seems) issue with a 144 is getting the jetting dial-in. I've read posts here where owners will chase jetting issues for so long that they eventually give up and go back to the stock cylinder. :excuseme:

Obviously some 144 owners have their setup entirely sorted-out and are happy with it; I'm just saying to expect more than just a "Bolt-on & Go" conversion.

Good luck, and keep us informed with what you do. :thumbsup:

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2 hours ago, bendo401 said:


Thats pretty good price for what ive seen kits going for

That's because I'm telling you a little secret on how guys like Gorr and many shops do their big bore kits. They may use a different plating service, but basically it's the same routine.

If you came to me to build you a 144 this is what I would do, then add on the porting, head work, case cutting and reassembly, so the price would be about double that. But, you are paying for an engine that's ready to run, and run correctly, in my case.

If you aren't a cylinder plating company, you are doing it like this. Porting and cutting the head is pretty much all Gorr is doing. The rest is done by the cylinder plating company.

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@yamaha7654 hey just checled them out. They look pretty good for what i want. Kinda cool that they are having your own personal pipe built for yoyr engine after allowing you to port your engine when they do they mods to it. Looks pretty nice. Whats your experience with them?

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Spend your money and just buy a tooth or two bigger rear sprocket,reed cage and some radiator braces, skid plate.  Stuff you really need. Big bore is not needed sry to say I bet you'll not finish any better 

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Many big bore kits use a head gasket instead of o-rings. Add price of new gasket each time you Check cylinder.
Big bore pistons cost substantially more. Make my mistake with jetting and it won't take long before first piston is finished.
If you want to save money forget the big bore.

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