yz 125 top end rebuild question

[yz125rider]

i am rebuilding my yz 125. but i am confused what size piston to get. i know the standard size is 54 mm but do you have to go one size larger like 54.5 mm because the cylinder walls wear down? If someone could reply with help asap that would be great!

[mj4trax]

yz125 has a plated cylinder.  If the plating is worn enough for a bigger piston, then you'd need to replate anyway.

In other words, stick with the 54 mm piston.  If your ring gap is wrong with new piston, or you have any bare aluminum showing through, or deep grooves, you'll want to send it out for replating. 

There are some people who will get into the A,B,C piston sizing where you can get a 54.01 mm piston, etc. ...  and there is some logic to this, but I've never bothered, and never had an issue.   

[yz125rider]

ok thanks alot !.. it has 100 hours on the top end too

[VintageBlueSmoke]

I recommend sending the cylinder to Cycle Playground.     Since I just got back from the post office to send mine and I REALLY REALLY want them to take good care of me.

But yeas, you should measure the bottom of your cylinder and get the right piston for that. assuming the plating is still good. 100 hours is long for a 125 but not unheard of.

[yz125rider]

sorry for the noobish questions but what would it look like if it still had the plating vs not having any plating left.

[mj4trax]

You'll notice that where the cylinder is plated, the color is darker than the rest of the cylinder.  The aluminum undeneath will be a bit lighter. 

Look at the exhaust bridge, and just below the exhaust port on that yz as this is the first place that will break through if the plating is worn through.  Since the aluminum is softer, it will "smear" in vertical lines up and down the cylinder.

The one thing to be a little careful with is that sometimes the piston skirt can leave some aluminum on the cylinder, in this case, the lighter colored areas will be raised up from the the plated surface rather than gouged out.  I recommend using a dullened dental pick to "feel" the surface a little.

Regarding plating shops, I've only sent my stuff to Millennium Technologies in Plymouth, Wi. 

I honestly don't know that they are any better than any other shop out there, but I've been happy and I like to stick with what I personally know works. 

[Stusmoke]

What everyone else says basically. If you're really concerned about the piston size, you can either buy a cylinder bore gauge and measure it yourself or take your cylinder to a local shop and get them to.

[VintageBlueSmoke]

Also, I have been told that a lot of the big engine builders use Millennium Technologies (and add a hefty surcharge...or tell you they did it in-house).

[yz125rider]

yes i have heard alot of good things about millenium so i would probably take it there if anywhere. after the cylinder is replated do you have to remove any burs or is it ready to be dropped back onto the engine. thanks everyone for helping me out.

[mj4trax]

I've never found any burrs on Millennium's work to remove, and I check (recommend for you to do the same) 

I do clean the cylinder thouroughly with soap and water then coat the bore with fresh oil prior to assembly.

I recommend to keep a journal of exactly what your lower, mid and upper ring gaps are during assembly.  Then when you tear down for inspection later, you have a good idea of how much wear you are seeing, and where it is.     

[yz125rider]

ok thank you!