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Bad crank and cracks in bottom end? Help a noob out!

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Hey guys so I'm diving into my yz250. Went to replace the oil seal on the power valve and found the cylinder was chipped on top, half the inner head o-ring missing, and a piece of the Piston top chunked off. So I know my top end is done and needs to be sleeved but I can't tell if my bottom end is still fine!

 

There's some weird surface cracking on the surface right next to the rod, and I can't tell if the crank is bad too. There's some side to side play which is normal but I can almost tilt the rod. No up and down play though that I can tell. 

 

Anyways let me know your opinions on getting this resurfaced and diving deeper into the motor. I'm good at wrenching and working on bikes but this is my first 2 stroke so I'm not sure what to look for. 

20170513_124246.jpg

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First thing I see is yamabond on the base oh boy lol they just casting marks not cracks.  Split and clean metal out. New crank, bearings, seals it needs it👍

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And yes don't sleeve that cylinder.  They never seem to not leak coolent at head oring.  Cause the sleeve makes oring channel bigger.  Have replated or find good used one on eBay 

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Yeah when I say sleeved I really mean sending it to a company who knows what they are doing and seeing what my course of action is. 

I was hoping you guys could help me out with the bottom end though and seeing if I really need to pull it apart. The yamabond was from the base gasket.. previous owner, not me. Obviously gonna clean it as good as I can and reassemble it properly. And ok good I'm glad they're just casting marks. Im thinking the bottom end is still good then?

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If it was mine and any chance of metal went into the bottom end, I would be splitting it and being able to sleep at night.

As Mark said, maybe @Gary YZDOC would rebuild it for you. He's put more Yz's together than most Yamaha employees.

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Still here in SoCal. OC. Still cheaper than a shop in your area. 

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On 2017-5-15 at 1:36 PM, Gunner132 said:

Im thinking the bottom end is still good then?

Best you check it.

How do the main bearings feel?

Did you measure the values D and F in this diagram?

Screen Shot 2017-05-16 at 11.17.50 pm.png

Edited by numroe
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6 hours ago, numroe said:

Best you check it.

How do the main bearings feel?

Did you measure the values D and F in this diagram?

Screen Shot 2017-05-16 at 11.17.50 pm.png

just measured, Side clearance D is at .45mm, small end free play F is harder to measure because of the movement at the base of the rod but when i prevent it from sliding it doesn't go over 1mm, it seems pretty tight and good to me.  Main bearings are solid and smooth, absolutely no movement when i try and wiggle the flywheel.

23 hours ago, zeke383 said:

If it was mine and any chance of metal went into the bottom end, I would be splitting it and being able to sleep at night.

As Mark said, maybe @Gary YZDOC would rebuild it for you. He's put more Yz's together than most Yamaha employees.

See the attached pics of the cylinder, I am always concerned of that sort of thing but there is no marks on the cylinder walls and no metal missing underneath the piston so i think anything that broke off just came straight out the exhaust.  Going to check my powervalve too though for any signs of wear.

21 hours ago, Gary YZDOC said:

Still here in SoCal. OC. Still cheaper than a shop in your area. 

If i get in over my head i will absolutely contact you!  I really enjoy taking things apart and putting them back together so I think i might take on the bottom end myself if i need to, thanks for replying though, i will keep it in mind.

On 5/14/2017 at 7:19 AM, Motox367 said:

And yes don't sleeve that cylinder.  They never seem to not leak coolent at head oring.  Cause the sleeve makes oring channel bigger.  Have replated or find good used one on eBay 

See the attached pics of the chipping on the cylinder, like i said im just going to send it to a company that knows more than me and see what my course of action should be.  

 

20170516_121729.jpg

20170516_121745.jpg

20170516_121759.jpg

20170516_121809.jpg

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Chipping is enough to require welding, bore, re-plating. I send all mine to Langcourt dot com and have them supply Vertex piston in size A. About 245 with round trip shipping. 

Other opinions and company choices will very. 

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53 minutes ago, Gunner132 said:

just measured, Side clearance D is at .45mm, small end free play F is harder to measure because of the movement at the base of the rod but when i prevent it from sliding it doesn't go over 1mm, it seems pretty tight and good to me.  Main bearings are solid and smooth, absolutely no movement when i try and wiggle the flywheel.

See the attached pics of the cylinder, I am always concerned of that sort of thing but there is no marks on the cylinder walls and no metal missing underneath the piston so i think anything that broke off just came straight out the exhaust.  Going to check my powervalve too though for any signs of wear.

If i get in over my head i will absolutely contact you!  I really enjoy taking things apart and putting them back together so I think i might take on the bottom end myself if i need to, thanks for replying though, i will keep it in mind.

See the attached pics of the chipping on the cylinder, like i said im just going to send it to a company that knows more than me and see what my course of action should be.  

 

20170516_121729.jpg

20170516_121745.jpg

20170516_121759.jpg

20170516_121809.jpg

I've drilled alot of pistons but never a modern yz250 cause there's no bridge.  I'm seeing holes in that piston someone had to drill those or something Idk about lol what kind of piston is that do you know?  Doesn't look to be ported or anything.  Hmmmm

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1 hour ago, Motox367 said:

I've drilled alot of pistons but never a modern yz250 cause there's no bridge.  I'm seeing holes in that piston someone had to drill those or something Idk about lol what kind of piston is that do you know?  Doesn't look to be ported or anything.  Hmmmm

Feel like i just got pregnancy results back... Shes a big bore!  Ugh that probably explains why im having issues in the first place.  Looks like a maxrpms 71mm piston to make it a yz285.

I even asked the seller if it was big bore or anything like that and he said no! In his defense he didn't seem to know much, but the price was right.  

In that case, I know that requires head mods and powervalve mods so it looks like im stuck at this bore for now.  Little pricey to reset it back to stock.  Good eye!

2 hours ago, Gary YZDOC said:

Chipping is enough to require welding, bore, re-plating. I send all mine to Langcourt dot com and have them supply Vertex piston in size A. About 245 with round trip shipping. 

Other opinions and company choices will very. 

Hey if they earn your business then i'm sure they do good work.  Sounds like you're no stranger to these bikes.  Any idea if they can do the same type of thing to an overbore cylinder?  Looks like its 71mm judging from the piston.

20170516_151305[1].jpg

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49 minutes ago, Gunner132 said:

Feel like i just got pregnancy results back... Shes a big bore!  Ugh that probably explains why im having issues in the first place.  Looks like a maxrpms 71mm piston to make it a yz285.

I even asked the seller if it was big bore or anything like that and he said no! In his defense he didn't seem to know much, but the price was right.  

In that case, I know that requires head mods and powervalve mods so it looks like im stuck at this bore for now.  Little pricey to reset it back to stock.  Good eye!

Hey if they earn your business then i'm sure they do good work.  Sounds like you're no stranger to these bikes.  Any idea if they can do the same type of thing to an overbore cylinder?  Looks like its 71mm judging from the piston.

20170516_151305[1].jpg

They can do and provide whatever you like. So you have a BB on that 250. 👍

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54 minutes ago, Gunner132 said:

Feel like i just got pregnancy results back... Shes a big bore!  Ugh that probably explains why im having issues in the first place.  Looks like a maxrpms 71mm piston to make it a yz285.

I even asked the seller if it was big bore or anything like that and he said no! In his defense he didn't seem to know much, but the price was right.  

In that case, I know that requires head mods and powervalve mods so it looks like im stuck at this bore for now.  Little pricey to reset it back to stock.  Good eye!

Hey if they earn your business then i'm sure they do good work.  Sounds like you're no stranger to these bikes.  Any idea if they can do the same type of thing to an overbore cylinder?  Looks like its 71mm judging from the piston.

20170516_151305[1].jpg

Interesting to see all them lube holes . Maybe contact max powers for top end kit to send with your cylinder.  You definitely wanna do the bottom end.  I'd strongly suggest oemparts . That thing will be badass. Man if you were close I'd make that thing rip lol

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4 hours ago, Gunner132 said:

small end free play F is harder to measure because of the movement at the base of the rod but when i prevent it from sliding it doesn't go over 1mm

The small end check is done to measure the condition of the big end bearing.

The values I showed are for the new parts. Wear limit is 2.0mm for the measurement labeled F.

1 hour ago, Gunner132 said:

 Looks like a maxrpms 71mm piston to make it a yz285.

Strange peppering marks on the piston crown. Detonation signs perhaps.

Also some damage on the top edge facing the exhaust port.  Perhaps too much free-play in the movement of the main exhaust valve. So it touched the piston.  Check the squish clearance is under 1.5mm and compression not too high for your fuel and deto should be no more.

The big bore is great, as long as you can get pistons the right size.  Just get the cylinder replated and send them you new piston for size reference.

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3 hours ago, Motox367 said:

Interesting to see all them lube holes . Maybe contact max powers for top end kit to send with your cylinder.  You definitely wanna do the bottom end.  I'd strongly suggest oemparts . That thing will be badass. Man if you were close I'd make that thing rip lol

Yeah actually I think thats who I will call first is maxrpms to see what they can do for me.  Its their kit after all!

2 hours ago, numroe said:

The small end check is done to measure the condition of the big end bearing.

The values I showed are for the new parts. Wear limit is 2.0mm for the measurement labeled F.

Strange peppering marks on the piston crown. Detonation signs perhaps.

Also some damage on the top edge facing the exhaust port.  Perhaps too much free-play in the movement of the main exhaust valve. So it touched the piston.  Check the squish clearance is under 1.5mm and compression not too high for your fuel and deto should be no more.

The big bore is great, as long as you can get pistons the right size.  Just get the cylinder replated and send them you new piston for size reference.

if 2mm is the wear limit then i'm golden!  And yeah i saw those marks too.  She was definitely not running right before i tore into it.  Thought it was reeds at first and the only reason i took the cylinder off at all was because i was going to replace my leaky oil seal on the powervalve (premix all over the water pump, lots of threads on here about my exact issue).  

Im thinking that i will just send the whole cylinder and head and piston to maxrpms and let them massage the powervalve so the clearance issue doesn't occur again.  Also get them to make sure the head is the right so i can run pump gas.

 

 

Okay so everyone is saying its time for a bottom end, but for what reasons?  Theres no material that made it past the rings, as the cylinder walls are in good condition.  The crank is in spec and bearings feel good.  Im not afraid to split the cases but I just dont think it needs it. 

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Two strokes are mechanically so simple. So in the bottom end, I see only four things to worry about:

1. Big end bearing health.
2. Main bearings health
3. Crank/rod clearance with spec.
4. Cases not leaking or damaged.

You cannot check the runout with the crank installed, so when riding it you'll find it is either smooth or vibrates.

If it was mine, I'd remove the clutch and then turn the crank by hand (holding the rod) and carefully feel the condition of the main bearings. I'd probably squirt some fuel down the bearing lube holes to wash the bearings dry of oil (then suck the fuel out) and check the main bearing feel again. Since bad bearings can feel ok with oil on them.  If they feel + sound fine and you are sure you're below 2.0mm on the rob big end free-play check, then fix the top end and ride it.

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I only say replace bottom end from looks of that piston.  Little bite ofplay your sucking air . You'regoing to pay a good buck for the top end I'd just do both for peice of mind. But yes its possibly could still be good up to you. Still it's going to have very fine aluminium particles in it. Only way to properly clean is to split. If you want that thing to run like new then do bottom also.  But it's your choice ofcourse . I'd atleast slpit clean and replace bearings and seals. You don't realize the performance loss in a engine cause it gradually goes down. It's amazing what new bearings and seals will do. Half the problems on this site are related to that but they "feel good" until you take them out and shake them suckers lol 

Edited by Motox367

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