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Top end questions - My 1st top end rebuild - Pic intensive

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When I bought the bike (1998 YZ250) the owner said he put 25-30hrs on the bike and that the previous owner had rebuilt the engine 10hrs before selling it. Supposedly there should be 35-40hrs on the top end... The owner was rambling as I looked the bike over and told me he had ridden it about 10 times, each for 6+ hours. Does that math add up? ha! So I figured before I got out too far in the desert I'd rebuild the top end. I ordered the weisco kit and it's on the way. I have today off so I stripped the top end down in preparation of getting the top end Friday. First thing I notice is there are all sorts of loose bolts, nuts, and screws! The spark plug was hand tight! No joke, I put the socket on it, gave it a quick twist for giggles and it came right off! Next, 3 of the head bolts required little effort to break... I mean very little. The power valve linkage bold was hand tight like the spark plug. Not to mention that cover (right side) had 3 hand tight bolts. The screw that holds the bracket in place, had backed out a couple threads! I'm very happy I'm going over this entire bike before I take it on a long ride. Here are the pics, I have questions:

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Here's the top end after I got the head off. That look ok?

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Here's the underside of the head... what are those pits? And is that a normal combustion pattern?

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Here's the cyl wall. Are those groove normal?

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Should honing take that scoring away?

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Here's the piston. That looks like blow by to me... could it be the scoring on the cyl wall causing it? Or the original owners just lied and it's never had a top end rebuild and the rings simply wore out? lol. :)

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The bottom of the piston looks good, golden brown with no evidence of burning of the premix.

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Here's the side of the engine with the plate blocking off the exhaust power valve. Kinda cheesy, I was hoping to change it or paint it or something. Just looks blah to me. Took the cover off to get the clutch cable out of the way. The cover holds the cable in place on this bike.

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As I'm cleaning the engine today I notice I think the right case cover has been polished. I think a little polish will make it look good again. But one thing I noticed was my left cover is plastic. Is that weird? Or was that something they did in 98.

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Looks like you did the right thing in tearing it down. Piston looks past its prime. I would look at honing the cylinder or you may get away with using some #800 grit as the manual suggests.

With the head it looks like something has been floating around in there on a previous occasion. Try and smooth the dents out a bit.

Check your big end bearing for up and down movement while your in there.

Not too bad really.

Good luck.

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Looka t the piston where it is closest to the powervalve.The powervalve on these will eventually close to far and rub against the piston. Look for a shiny, polished line when this happens. You might need to remove a tiny bit of material from the pv to prevent this.

No sweat on the flat powervalve cover. That was a mod for this bike and added to the midrange power. You cna get a regular cover from ebay or searhc aroudn for a carbon fiber sheet and make your own.

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i would judge the dents in the combustion chamber to be from a previous broken piston ring, i would consider it top priority to get the cylinder honed properly.

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I checked the piston on the exhaust side and there is no polished line. To be on the safe side, should I remove some material anyway?

Good to know that the ugly plate will increase mid-range power. I'll keep it, maybe paint it or polish it.

I plan on a good hone. I'm gonna take it to the local machine shop and have them hone and measure it. Anything else I should ask them for?

Is it imperative that I get the dings out of the head? The only way I could see to do that would be to sand them out... and that's gonna increase the combustion chamber size which will decrease compression. If it's not going to hurt anything, I may just leave them. Only thing I could think it may do, is get carbon deposits and heat up during hard rides causing detonation. Thoughts?

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I think I would look at fixing the head. If you have a good machine shop. I bought a lot of used heads for that bike on ebay for cheap. The 97 head has less compression and runs harder on top. Its nice having a 97 and 98 head.

The powervalve will eventually wear enough to hit the piston. Look at the pv closely and move it through its range of motion. It shuts hard and the stopper pieces wear when it slams shut.. Yoru looking under your flat cover. You then remove the round piece and you'll see the two pieces that contact.

I felt safer removing some material. It could cost low end power, but if it did , it was not much.

You can use the downtime to match the powervalve to the exhaust port. Open it up completly and notice how far off it is. mark it with a sharpie and sand it off. Also match the individual sides to each other. If you polish it , the carbon does not stick to it so badly and it stays clean a long time.

Mine would occasionally loosen, so check it every 10-12 hours of operation. Its really easy after you take it apart once. Its a easy design to mess with.

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Only thing I could think it may do, is get carbon deposits and heat up during hard rides causing detonation. Thoughts?

BINGO!!!

It won't even take hard riding.

I had a similar issue in my youth with my 87 CR125

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BINGO!!!

It won't even take hard riding.

I had a similar issue in my youth with my 87 CR125

I had similar head damage on a 500cc 2T engine of mine once. I was concerned about it also so I talked to Eric Gorr about it and even sent him the head. He told me just to make sure there were no sharp or high spots around the indents and that there would be no problems with leaving them.

I've been running the bike at the dunes for a few years now with no problems. I wouldn't worry about it unless you are the type that will have a hard time sleeping at night because of it. Just make sure that the there are no sharp points (they will heat up and cause detonation).

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Took the jug to the local machine shop, whom I have a lot of respect for, and asked him about honing the cyl. He told me it was a plated cyl and honing wasn't needed. He said he could do it, but didn't see the point. He also told me a few things about pistons and this bike. First graded pistons are better than the forged pistons. Forged pistons take a more careful warmup than a standard piston, which I knew, but he said I'd really need to watch out that I didn't cold sieze the engine. I've always allowed for very long warm ups to get the bike ready to ride... and never romped on the throttle when it was cold. At this point I was worried about doing the right piston, but assured me that the weisco forged piston was just fine for the desert riding I was going to do. Second he pointed out that my cyl is a "B" cyl, for which there are A, B, and C cyls which all corespond to a size. Third he told me the forged piston should be a little loose in this jug because of the B code, but still not a problem. He made it sound like for desert riding it was ok, but if I was out racing on a track with the throttle pinned all the time, it might become an issue. I asked him about the scoring on the cyl walls and he said it looks like it's due the side to side movement of the piston in the cyl. He said it was pretty normal for 2 stroke that is 11 years old. He looked the cyl over and said it looked pretty good. He also pointed out the nicks in the power valve due to worn out rings. He did a quick mic of the bore and said it was roughly 68mm. I asked him about the head, and he told me to sand the high points down to stop detonation and everything should be ok. I told him I picked up a head for $50, he told me to run the new one, but the old one shouldn't have a problem. The nicks could have been caused from a ring failure, side electrode of a spark plug, or from something in the bottom end that was pushed to the top.

I talked with him about a total rebuild, which would cost $500 for new bearings, seals, rod, ect. I may do that on my next top end rebuild. I'm running out of $$$ to spend right now. He agreed and told me to run this top end and come back when I needed the bottom end.

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Looks like someone tried to hone the cylinder, but the crosshatch is all wacky. Did the mechanic also tell you the "graded" pistons, -cast- piston, don't last as long 'safely' as the forged pistons. Safely as in ... don't have the "money" for a rebuild right now but will do it as soon as I can....type of thing. Too many people with cast pistons have ended up breaking the piston with a few too many hours on it. But its a "tighter" piston initially. The cylinder has to be honed with a diamond hone to etch the coating. He probably didn't have one, he could have made 25-35 bucks if so. Try to get someone to not just roughly check the bore but get the measurement to the .001" to verify wear on the lower cylinder area. Overall most everything he said is correct. Remember this bike has a race engine in it and tolerance do count on everything.....to be safe as well as fun. :)

Techmc

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Took the jug to the local machine shop, whom I have a lot of respect for, and asked him about honing the cyl. He told me it was a plated cyl and honing wasn't needed. He said he could do it, but didn't see the point. He also told me a few things about pistons and this bike. First graded pistons are better than the forged pistons. Forged pistons take a more careful warmup than a standard piston, which I knew, but he said I'd really need to watch out that I didn't cold sieze the engine. I've always allowed for very long warm ups to get the bike ready to ride... and never romped on the throttle when it was cold. At this point I was worried about doing the right piston, but assured me that the weisco forged piston was just fine for the desert riding I was going to do. Second he pointed out that my cyl is a "B" cyl, for which there are A, B, and C cyls which all corespond to a size. Third he told me the forged piston should be a little loose in this jug because of the B code, but still not a problem. He made it sound like for desert riding it was ok, but if I was out racing on a track with the throttle pinned all the time, it might become an issue. I asked him about the scoring on the cyl walls and he said it looks like it's due the side to side movement of the piston in the cyl. He said it was pretty normal for 2 stroke that is 11 years old. He looked the cyl over and said it looked pretty good. He also pointed out the nicks in the power valve due to worn out rings. He did a quick mic of the bore and said it was roughly 68mm. I asked him about the head, and he told me to sand the high points down to stop detonation and everything should be ok. I told him I picked up a head for $50, he told me to run the new one, but the old one shouldn't have a problem. The nicks could have been caused from a ring failure, side electrode of a spark plug, or from something in the bottom end that was pushed to the top.

I talked with him about a total rebuild, which would cost $500 for new bearings, seals, rod, ect. I may do that on my next top end rebuild. I'm running out of $$$ to spend right now. He agreed and told me to run this top end and come back when I needed the bottom end.

He gave you some decent info and some "opinions". Most of the opinions are not backed with fact.

The cast pistons vs forged piston was an opinion. The desert running vs Motocross (I assume) in insane. I wonder what puts more stress and runtime on a motor? A 15 minute motocross race or a 4 hour offroad race?

The worn rings will not cause the piston to hit the powervalve. Looking at the assembly and using some logic will show this. The powervalve hits the piston only at the exhaust port, when its in the closed position. It hits because the stoppers are worn.

If somethign from the bottom end came up and caused this damage, odds are you'll know it after running the bike for a few minutes. If you have a bearing going bad and spitting out sections, consider this minor damage a warning. Its sure to come apart in a big way.

We have a really good, local machine shop that does some incredible work. However, the same place will spend all afternoon trying to convince you a sleeve is a better option than replating a cylinder. Even if no oversize pistons are available. Why? Because he can do the entire sleeve job and pocket a few hundred. He has to send the plating out and makes very little doing so.

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It really isn't too bad for what you will use it for. You're gonna be mosty +50% throttle so the scoring on the cylinder wall probably won't give you much problems. The piston and rings are the wear parts and the cylinder is plated so if you hone it, there is a chance that you will get aggressive and make it worse. It looks like that's what happened before, they broke a ring or a reed pedel or cold siezed the top-end (ring grooves on the top of the cylinder wall) and tried to hone too aggressively and spun a bunch of scratches. Since the head was loose, I'd be curious if it was warped; Place it flat on a piece of glass and see if it rocks or if there is a gap. If you can, while it is flat on the glass, fill the squishband with water thru the sparkplug hole and see if it leaks.

Otherwise,

Clean the carbon off everything, smooth out the burrs on the head squishband, put it back together and enjoy.

Great pics, by the way!

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The saga continues... lol.

I got my top end kit today. Opened up the box and, to my surprise, found two pistons. They sent me a Banshee kit. Doh! So I'm off to return it and get the proper yz kit. :)

Yeah, when I looked at the movement, I didn't think the rings would hit the PV. He did tell me to clean the PV, which I had planned on, and i'm gonna remove some of the material as RC stated. I too figured it something came up from the bottom, it would be catastrophic.

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