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YZ250 Piston Locks When Cylinder Nuts are Tightened

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I'm working on rebuilding a 2001 YZ250 from the crank up and am having a problem with the top end going together.

When I snug down the cylinder, the piston locks up with, or without the rings on it. I can also move piston around in the cylinder but it won't go up and down.

There's 2 5/16" on both sides of the rod, so the crank appears to be centered.

Specs:

New Hot Rods crank and bearings

New Wiseco piston

Cylinder was serviced by Millenium Technologies. 

 

Has anyone had this problem before or know what it might be??

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Is it possible that you got the wrong connecting rod?  So when the cylinder is snugged down the rod is too long and is hitting the bottom of the cylinder?

 

Did you measure the piston/bore clearance?  Does the piston move freely in the cylinder with the piston off the rod and the cylinder off the motor?

 

Weird one.

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I actually just figured it out. The flywheels on the hot rods crank must be slightly bigger than stock because the bottom of the cylinder skirt actually touches down on the crank. I'm going to file down the skirt a bit and hope that takes care of it.

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I actually just figured it out. The flywheels on the hot rods crank must be slightly bigger than stock because the bottom of the cylinder skirt actually touches down on the crank. I'm going to file down the skirt a bit and hope that takes care of it.

I guess you mean the crank webs?

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Well I was able to clearance the cylinder so the crank doesn't rub any more but now the piston seems to be catching around the exhaust port/power valve. It does this with or without the rings installed and no part of the PV sticks into the cylinder

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I bought the bike already torn apart. The last guy seized the connecting rod on the crank and bought the new crank assembly, so I used the one he bought without a second thought.

It came with an 01' title and everything to this point has gone together perfectly.

Ring end gap is .016 on top and .02 on the bottom, but again it still hangs up in that one spot without the rings installed.

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There's some wrong parts in there.

x2.  There is no way you should have had to grind anything to make it fit if the right parts are in it.  Now that you have, I'd be very concerned about the integrity of the cylinder...

Edited by Doc_speeder

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Photos would help with this thread.

 

I was looking at bikes on CL the other day. The guy had an 04 250 with a 96 motor in it it. 

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Some of your parts have to be wrong or its put together wrong

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Sounds like a crank issue to me, where its not rotating properly to get the the piston cycling correctly, really hard to say without seeing it or having pics...are you sure the pistons facing the right direction?

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really hard to say without seeing it or having pics

I don't have pictures right now, but can take some tonight. Is there anything to describe whether its an 01' engine? What do you want to see pictures of?

 

The arrow on the piston is facing the exhaust port.

 

The cylinder is fine. I just took a small file to the corners of the skirt on the intake side and the material I took off was below the plating.

 

Everything else in the bottom end has gone together and works perfectly. Would that be the case if the crank was wrong? TDC and BDC both look to be perfect as well.

 

Also it only sticks at 1 specific crank angle. If the rod is pointing towards the intake side it runs smooth, but when the rod comes around and is pointing to the exhaust side it seems to hang up around the power valve ports. It does this in either rotating direction.

 

 

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Rotating the crank hangs up at one point if the rod is pointed in a certain direction?

Does the crank rotate complete revolutions?

Edited by PALMER84ONE

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Rotating the crank hangs up at one point if the rod is pointed in a certain direction?

Does the crank rotate complete revolutions?

 

Rotating the full assembly by the flywheel. The crank itself rotates just fine. It seems to be the piston (with or without rings on it) that sticks in that one spot.

I was just trying to say that out of the 2 times it passes that spot in a full crankshaft revolution, it only hangs up one of the times, but in the same spot relative to crank position.

I hope this makes sense. Its kind of hard to explain. I'll take a bunch of pictures tonight.

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Is the PV sticking out enough for the piston to catch?

 

Unfortunately, no. That was serviced while millenium had it as well.

There's about 1 mm between the power valve and the inside cylinder wall

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If nothing else, did you loose something down in the crank and it only catches on one way rotation. Just seems very odd your have issues.

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The crank runs free without the cylinder on it, so I don't think anything got down in the case.

 

And I had it backwards earlier. It only does it on the stroke where the rod is angled towards the intake side.

I'm really hoping I'm missing something stupid here...

 

Here's some pictures

 

BDC

20160425_174013.jpg

 

TDC + Crankcase ID 

20160425_174106.jpg

 

Where it's sticking

20160425_174215.jpg

 

Cyl ID

20160425_174048.jpg

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