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Cylinder\New piston questions - 1990 YZ125

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Kind of new to the whole dirt bike scene and definitely a newbie at rebuilding engines. I'm going to install a piston kit. The bike was bought off of craigslist and I have no idea what the inside of the cylinder looks like yet. I know the factory bore was 56.0mm and the service limit is 56.1mm. Here are my questions. Thanks for looking.

 

1. If I pull the cylinder off and it's wore in a way that is more than 56.1mm, Is there usually enough material to bore to the next sized piston? (For example, Wiseco offers piston sizes 56, 56.5 and 57mm.)

 

2. If I do have to bore to the next size, Will I have to change my jetting or anything else to accommodate for the change in piston size?

 

3. If I had to bore to the next piston size, what should the clearance be around the piston? Say the piston is 56.5... then should the cylinder be 56.503? That's kind of specific but I've read that there should be .003 of clearance so it's not too tight. 

 

4. Will I need to worry about adjusting the ring gap when the piston kit arrives or will it come ready to install right out of the package? 

 

I know that some of this stuff is probably defined really well in the manual but I haven't got the chance to purchase on yet. I've been trying to find an electronic copy if anyone knows of a good place to point me. Thanks again in advance for any help you guys can offer. I really appreciate it. 

 

Any other feedback is also appreciated. I'll take any tips for rebuilding that you have. 

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A 1990 has a hard plated cylinder wall stock you don't bore it out typically it is replated. If there is cylinder wall damage it be a big bore and still need a replate unless you go with a sleeve insert and that's a waste of time and cash really.

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Thanks for answering. If you don't bore out the cylinder, why does Wiseco sell three different sizes? It seems kind of pointless for them to sell all three but you have to have the cylinder replated every time you go over the service limit? This is why I'm a noob. Haha. Also, if you can't bore/hone it, how do you get the 57mm piston that they sell to fit? There has to be a way to bore/hone this stuff I would think...but idk...

 

So just to clarify, I have to get the cylinder re-plated after every top end rebuild if the cyliner wall is out-of-round or wore to the point it's no longer within spec? 

 

This is starting to sound expensive. Thanks again

 

Edited cause I had more to add. :)

Edited by pnwyz125

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No the reason why they went to aluminum sleeve's with hard plating was because they cool better then  a steel liner and the hard plating wears for a much much longer period then a steel liner would. If you keep your top end in good shape and don't let the piston wear out or have a bad failure happen the hard plating will go like 10x  longer as a steel liner ever would in a small bore motor. Steel liner is old school old tech aluminum liners with hard plating is newer tech much better.

 

They sale the bigger bore pistons for two reasons for those who want to get more cc's and go bigger bore or since the hard plating isn't very thick say you damaged your cylinder wall bore it out 1mm to the next size and replate the cylinder. There is a cheaper method of having the aluminum sleeve pressed out and having a steel liner put in place. But it's just that a cheap fix that will serve it's purpose providing the steel liner is matched up to the port windows correctly and 99% they are not! which can screw up the power band big time, bike will generally run a lil hotter you may be able to get one or two top ends outta of the steel liner before it needs to be bored over to the next size and steel liner the piston will wear a bit faster. 

 

So when you compare the costs in the end hard plating wins! it will actually save you money in the long run too you can also look on ebay for other cylinders to fix that year I think it would be `89 thru `93 would fit but I believe that the `90 was the best of those years.

 

http://www.goforwardmotion.com/pdf/modeltuningtipsYamaha.pdf - scroll down to the proper section for the `90 - `93 will tell ya how to dail in the bike.

Edited by SmokinJoe90

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