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How to know when to replace the Piston/Cylinder-First 2 Stroke/Dirt Bike

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I picked up a 1999 YZ250 midway through last season as my first bike. I rode it (softly) last season as I learned and started this season. I noticed that there was excessive oil spray around the exhaust and where the cylinder meets the case halves, so I tore the bike down and put the engine on a bench for a rebuild. I am not unfamilar to engine rebuilds, but 2 strokes and dirt bikes (and bikes in general really) are new to me. I included some pictures below to show the state of my components. I was wondering how I could tell if/when certain parts need replacing or rework. I know that the carbon scoring under the cylinder rings on the piston indicate that replacement piston rings are needed at the least. Otherwise, the piston seems in decent shape outside of some scratching on the side.

IMG_20181017_101023.thumb.jpg.4c697e4bd228da2d233cdf31e079b6a8.jpgIMG_20181017_101003.thumb.jpg.c5c191bb36b8750cacd234d920606825.jpgIMG_20181017_101034.thumb.jpg.bfee59e328e16aa3bf13156853a95ecf.jpg

The Cylinder has some scratching near the intake and exhaust ports, as well as nick on part of what I think is called the "skirt" (still learning the terminology). This would seem to need at least work done to it, be it boring (if thats done on bikes) sleeving (a concept I have seen but don't understand too well) or replacing.IMG_20181017_101052.thumb.jpg.9f571ea8f183ac75dacd9e16b31d5efd.jpgIMG_20181017_101121.jpg.a475b5ed14063a9e16b409755f194675.jpg

Anyway, thanks for the advice all, I appreciate the help.

Edited by Caleb Schmucker

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Hour meter and service intervals are fine and all for something that I know the current state of, but given this is a used and rather old bike I don't know the current status or how it was cared for previously. After this first major service I will be using my hour meter to track service intervals, but for this one, I need to know what needs replacing.
I guess on that basis the piston should just be swapped. But for the cylinder, is it useable, need fixing (hone, sleeve), or need a full on replace?

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The general rule of thumb is...if you can feel the scratches with your fingernail they are too deep and the cylinder needs to be re-plated.  Or if you can see the plating worn through to the base aluminum as it APPEARS to be in your picture, then it needs to be re-plated.

With respect to the piston, they are cheap and easy to source, if you have it apart you may as well replace it IMO as you do not know how many and what type of hours it has on it.

Going forward, there really is no way to know when to replace it exactly, as the only way to know is to run it to failure, at which point it gets much more expensive and difficult than replacing on a predetermined schedule.

If you are a recreational rider that spends most of your time well below WOT and max RPM type situations, and you run quality oil at a reasonable ratio, and you keep your filter clean and properly oiled, then 50-75 hours is entirely safe.  100 is likely safe too, and I believe even 200 would be attainable, but I would not recommend that, as I don't want anyone to say that I recommended it...lol.  And - don't guess.  Use an hour meter.

I changed my last one at 75 hours, and other than the carbon marks, it was difficult to see any difference between it and the new one on the bench.  However, as a piston wears, it develops more clearance to the cylinder wall, which increases knocking and impact to the cylinder plating, which I believe shortens the life of your cylinder.  So for this reason, it is not smart or economically logical to stretch out your top end replacement intervals.

 

 

Edited by Doc_speeder
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1 minute ago, Doc_speeder said:

The general rule of thumb is...if you can feel the scratches with your fingernail they are too deep and the cylinder needs to be re-plated. 

That is the case with this Cylinder, I can feel the grooves for sure. Thank you so much for the additional info provided. Now, sorry for my inexperience, but what _is_ re-plating? Is this something I can do on my own or is this a shop job? Is it better to replace in full? Thanks again for the help!

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Definitely not a DIY job.  The Nikasil coating needs to be applied/refinished.  Millenium Technologies I think is one company that does this, there are others you can find if you use the search feature.

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Ah, will do. Thank you for the direction! The only other question i have is if that nick in the bottom of the skirt makes this piston non-serviceable. I can send it out, but dont want to if its too far gone.

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6 minutes ago, Caleb Schmucker said:

Ah, will do. Thank you for the direction! The only other question i have is if that nick in the bottom of the skirt makes this piston non-serviceable. I can send it out, but dont want to if its too far gone.

I wouldn't expect that to be a problem.

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I would definitely replace the piston and rings and replate/hone the cylinder. if you were keen on pulling the entire engine apart it wouldn't be a bad idea to replace the main bearings etc, but should be able to go without it.

 

also, I wouldn't recommend doing it yourself as its easy to stuff up, and the equipment needed will probably cost more than getting someone to do it

Edited by Max Meredith

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I was stating what is next. 

Sorry, guess I should have said your piston and cylinder is toast.

I use Power Seal for my weld and plate company and turn around right now is 3 weeks.

They are very reasonably priced and do excellent work.

Here is what I would do with a 99 engine that you have.

Get an OEM crank, bearings, and seals for your 99-00, or you can use a 01-02 (different, but the same)

Get the cylinder re-plated, sending an OEM or equal piston (size A) with it to insure quality fit.

 

 

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2 hours ago, Max Meredith said:

also, I wouldn't recommend doing it yourself as its easy to stuff up, and the equipment needed will probably cost more than getting someone to do it

What sort of tools? I cant see anything I'd need specialty, so long as I dont get into the case halves (past which, no idea if I would or would not need there). I have a fairly expansive selection of tools, enough to rebuilt a car engine, so as long as it isn't beyond that I think I would be fine.

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I guess one thing I wonder is how people torque their front cylinder bolts, which are not accessible by socket. Do you use a crows foot?

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7 minutes ago, Caleb Schmucker said:

What sort of tools? I cant see anything I'd need specialty, so long as I dont get into the case halves (past which, no idea if I would or would not need there). I have a fairly expansive selection of tools, enough to rebuilt a car engine, so as long as it isn't beyond that I think I would be fine.

I have 6-7 specific: Torque wrench w/Motion pro adapter and always the assorted sockets etc., Engine lock up, flywheel puller, case splitter, crank puller, clutch lock.

5 minutes ago, Caleb Schmucker said:

I guess one thing I wonder is how people torque their front cylinder bolts, which are not accessible by socket. Do you use a crows foot?

Yes, or Motion Pro sells one. Just calculate the change on a torque wrench.

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41 minutes ago, Caleb Schmucker said:

What sort of tools? I cant see anything I'd need specialty, so long as I dont get into the case halves (past which, no idea if I would or would not need there). I have a fairly expansive selection of tools, enough to rebuilt a car engine, so as long as it isn't beyond that I think I would be fine.

you can do the rebuilt easy if you have tools such as blind bearing pullers and a torch (for heating case to remove bearings) etc, but plating your cylinder yourself would be where you run the risk of messing things up

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Oh, i didnt intend on that part haha. Another question for you all: What is the difference between sleeving and plating and whats the advantage to either?

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11 minutes ago, Caleb Schmucker said:

Oh, i didnt intend on that part haha. Another question for you all: What is the difference between sleeving and plating and whats the advantage to either?

I'm only 15 so don't have much experience with either, but i think plating is re-doing the nikasil coating, and sleeving is replacing entire cylinder 'wall', but you shouldn't need that as scratches don't look too deep, might need to bore and re-plate the cylinder 1mm oversized at the most to get rid of any deep scratches, and you can get a 1mm oversized piston and rings easily as well. 

Edited by Max Meredith

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Ya man you gotta get it wound up pin it and listen to her in the air if she is like ring dadaingdadingdingding then you can rip hard another day but id she says boawadadadawawa when you grip it and she dont rip it then is time for a new piston namura is the best with 10:1 ryobi oil and at fresh go jooce that extra oil will seal up the ring gap and fight anuther day bro

  • Haha 2

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