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Doing my first top end (YZ250) - suggestions?

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Make sure you put a rag over the crank when your removing and installing the circlips. They drop into the bottom end easily and are a pain to get out. You should use a torque wrench for the cylinder and head. Make sure you measure the ring gap before hand... this is VERY important. If you have access to a hone, its good to hone the cylinder so the rings will seat good.

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already jethro?

wow,

you must be putting in some time on that rig.

good for you!

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Thanks for the info.

To measure the ring gap, I need a feeler gauge? Can I pick one up at an automotive shop or are they particular to bikes? Also, if the ring gap isn't within the spec width, what do I have to do?

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already jethro?

wow,

you must be putting in some time on that rig.

good for you!

Hey Mike,

I figure better safe than sorry - I hope I am not being too conservative. The top end hadn't ever been done when I bought it used and I have put at least 20 hours on it, many of which were riding sand tracks. Today when I warmed it up, it was making gentle soft rattling noise, which I have heard is diagnostic of worn on piston/rings. Still, I am too much of a noob to know so rather than take the risk, I am going to pull it apart. I haven't noticed and drop in performance. It is running strong. Bit nervous about doing to the top end.

I ended up getting the Crossfires - holy jumpin' rednecks they are nice boots. Comfortable from the moment I first put them on, super solid, completely confidence inspiring. Money (lots) well spent, I figure.

Any signs of the melt up there yet?

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An automotive feeler gauge will work fine. Check the gap at the bottom of the cylinder because the wear will be its least at that point. Just use the piston to push the ring into the jug about 2 inches or so. If the gap is too tight, use a fine file and take off the minimum you can get away with. I usually set the gap in the middle of the specs.

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Hey Mike,

I figure better safe than sorry - I hope I am not being too conservative. The top end hadn't ever been done when I bought it used and I have put at least 20 hours on it, many of which were riding sand tracks. Today when I warmed it up, it was making gentle soft rattling noise, which I have heard is diagnostic of worn on piston/rings. Still, I am too much of a noob to know so rather than take the risk, I am going to pull it apart. I haven't noticed and drop in performance. It is running strong. Bit nervous about doing to the top end.

I ended up getting the Crossfires - holy jumpin' rednecks they are nice boots. Comfortable from the moment I first put them on, super solid, completely confidence inspiring. Money (lots) well spent, I figure.

Any signs of the melt up there yet?

It was certainly time from the sounds of it. Better safe than sorry. Be sure to mic the cylinder at the top, middle and bottom of the jug to make sure it is square throughout it's bore. I am doing my '02 YZ250 and Rm134's top and bottom ends right now myself.

Glad you are enjoying the crossfire's too. It is tough getting the best equipment because you are now spoiled and won't be able to stand anything less.

No sign of any melting just yet. We had a nasty low blow though late last week and it dumped nearly a foot at my house. We are well above 14 hours of daylight now, gaining several minutes per day. We should be able to put knobs to dirt by the end of the month.

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02YZ426 and NOMADAK,

Thanks for the info. Would you mind clarifying a few things for me? I am completely new to all this.

What is the jug?

What would I be filing, the ring?

What does mic mean and what is making sure it is square?

It looks like I need to hit the books and brush up on terminology.

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No sign of any melting just yet. We had a nasty low blow though late last week and it dumped nearly a foot at my house. We are well above 14 hours of daylight now, gaining several minutes per day. We should be able to put knobs to dirt by the end of the month.

Whoa. I imagine it is going to be a long month. I forgot you uber-northerners get so much light during the summer. That would be sweet riding into the wee hours of the morning.

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Man, wish I was there to walk you through this one.

I thought you had some buddies around there who rode.

Are any of them mechanically adept? Know a mechanic from a shop who likes beer?

Did you get a service manual with the bike?

The cylinder=the jug.

Mic'ing the cylinder at the top, middle and bottom will tell you how much wear the cylinder has sustained. The main wear area will usually be in the middle area of the cylinder as that is where the piston is spending twice as much time as it is at either end. Each of those sets of numbers should be relatively close to each other and will tell you if the cylinder is still within tolerance or still square. What you are measuring is the total circumference and any differences in the top, middle, bottom numbers.

It is going to be tough sitting for another 30 something days, no doubt. Hitting the gym, trying to get rid of the winter flab, getting all the big maintenance out of the way for the upcoming season. It is a ritual.

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Man, wish I was there to walk you through this one.

I thought you had some buddies around there who rode.

Are any of them mechanically adept? Know a mechanic from a shop who likes beer?

Did you get a service manual with the bike?

The cylinder=the jug.

Mic'ing the cylinder at the top, middle and bottom will tell you how much wear the cylinder has sustained. The main wear area will usually be in the middle area of the cylinder as that is where the piston is spending twice as much time as it is at either end. Each of those sets of numbers should be relatively close to each other and will tell you if the cylinder is still within tolerance or still square. What you are measuring is the total circumference and any differences in the top, middle, bottom numbers.

It is going to be tough sitting for another 30 something days, no doubt. Hitting the gym, trying to get rid of the winter flab, getting all the big maintenance out of the way for the upcoming season. It is a ritual.

Me too, me too.

The one guy I rode with who was a mechanic has moved away so I am SOL. The rest are all new to bikes as well.

I worked as a bike mechanic and used to seriously be into racing RC dork cars so I do have some mechanical apptitude (arguably, perhaps).

Got what your saying - if the cylinder is at all not square through out the stroke, it will need to be honed (or bored?). Is mic some sort of measurement method? What are the chances the cylinder won't be square? I was under the assumption that the cylinder wall coatings used these days are so tough that the pistons and rings will wear far before there is any significant wear to the cylinder walls.

And what's the scoop with the rings? If the feeler gauge shows that the gap isn't between the specs, the rings will have to be changed or somehow reduced in circumference?

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The cylinder walls are coated with Nikasil, very tough stuff. Yes, pistons and rings will wear before the Nikasil does generally but a seizure or debris can leave damage to the coating. Nikasil is a silica/glass based substance mated to the liner. Yamaha produces OEM pistons in sizes A (stock), B, C and D. 4 different sizes. Most folks mic the cylinder, then go from there based on those readings and get the next larger/appropriate sized piston. Several people go from piston "A" to piston "D" bypassing "B" and "C". Wiseco does the same.

The mic is a measuring gauge. A picture of one should be listed in your manual. I don't have one myself, one of my racing buddies owns a shop and mic's my cylinders for me. He also does my honing too. Very convenient. You should probably fine a good machine shop locally to do this for you.

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Gotcha. I read about that - the A-D designation for pistons. I hope the one I bought fits. The salesman didn't say anything about sizes. I just told him I needed a topend kit for my bike and that's what he gave me.

When I pick up a feeler gauge, is there any particular specs it should have (range of measurement)?

Thanks again - the collection of beers I owe you for all your help must be setting some sort of record.

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Oh, and one other thing (I'm hijacking my own thread). I need a new rear tire. I read your post about your experience with tires. You had great things to say about the Dunlops, despite them wearing out quickly. For a weekend warrior like me who doesn't need top performance at the expense of longevity, what would you recommend? In mountainbiking, I was always a die-hard fan of Michelins so I was thinking of trying some out. The Kenda Millevilles that came with the bike seem ok but now that they are wearing out, the knobs are ripping off.

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He probably gave you the stock sized piston.......and that may be ok depending on what you actually find.

You can always exchange if it isn't what you need. Keep your receipts.

Not sure what you mean by the feeler gauge comment????

You don't owe me anything, you've always been a pleasure to talk with. I just hope we get to ride together one day.

We all have to start somewhere. One day you'll be the guru to someone else. Just pay it forward.

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Myself, I'm staying with Dunlops. I have been on them exclusively for 2 1/2 years now. I don't remember what another tire feels like anymore. I did try a Milleville, I didn't even get to enjoy it because I broke the metal apart in the bead just trying to install the darn thing. Once you have used several different brands of tires, you will get a feel for what each one offers. It depends on how much you are going to be riding and on what terrain you are going to use it on; that is going to make the biggest difference in how the tire lasts and performs.

If you can find a set of 952's, that is what I would choose for you.

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Well, Jason, I did my first ride of the season today and the Maxxis Maxxcross front tire was quite nice, much better than that stock Dunlop that came on my bike. Maxxis I'm told by many, are a great "bang for your buck" tire, sort of like in mountain biking too.

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Dang - I tried getting into it last night but got too scared of wrecking something. I need to wash the whole chassis area, especially the area under the tank, before I get the top end opened up. Also, the I should wait until the apartment landlord gets finished his project in the work room so I can spread my stuff all over the place.

I am going to start a new thread with photos of the whole process. And I am sure, tons of dumb noob questions.

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Everyone starts somewhere Jethro. Better to learn than to pay someone else to do it for you. This is normal maintenance and will surely need to be done again further down the road. No time like the present to start. I'm sure it is well within your abilities to complete it properly.

Cleaning things up before hand is a great start. Any dirt falling down in that bottom end could be disatrous. Things can't be clean enough before you begin. Under the tank area is very important. I would take the whole tank and seat off to have more room to work and visualize. Not absolutely necessary but certainly easier to do a top end that way. Clean around the down tube of the frame and where some of the electronics reside right above the motor too.

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Everyone starts somewhere Jethro. Better to learn than to pay someone else to do it for you. This is normal maintenance and will surely need to be done again further down the road. No time like the present to start. I'm sure it is well within your abilities to complete it properly.

Cleaning things up before hand is a great start. Any dirt falling down in that bottom end could be disatrous. Things can't be clean enough before you begin. Under the tank area is very important. I would take the whole tank and seat off to have more room to work and visualize. Not absolutely necessary but certainly easier to do a top end that way. Clean around the down tube of the frame and where some of the electronics reside right above the motor too.

For sure, I agree. I just need to get it cleaned up and make a little time. If I run into troubles, I'll post photos.

First question - when putting everything back together again, how do you manage to use a torque wrench for some of the head and cylinder bolts that have almost zero clearance from the case? I'll try to get a photo....

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