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Is this bike just going to break again?

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Hi,

I have a top end kit on the way for this bike but I am afraid it is just going to break after i put it all back together.

It is a 98 yz 125, I went to order the parts today and the bike shop guy said i should be looking further than just a rebuild by the looks if that piston.

He adviced checking that the spark plug was the right size and other things that i don't understand or know about like reed valves, jetting and running lean.

This is the first time the bike has broken for me but I have only had it for about a month.

The photos i have put up are of the old spark plug and the one i am going to put in that the bike guy told me to.

They look completely different in the serial numbers, what could this mean.

Thank 🙂

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In order of importance:

1. Read the FAQ and download a manual.

2. Read the manual. Specifically in relation to jetting, reeds and the correct spark plug to use.

You should now be able to answer most of your own questions.

3. take the cylinder off and post pictures of it on here before you put it all back together. The guru's will tell you if its ok given your piston looks like it has a hole in it.

Your old spark plug was definitely wrong, so the new one (which I believe is correct, but check the manual) will help as will the new top end.

Edited by tim512

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I have just taken photos of the top left, top right, bottom left, bottom right and exhaust ports of my cylinder.

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All the stuff tim512 said, plus what we can tell from the pics.

The correct plug for a 1998 YZ125 is a BR9ES. The "9" is the heat range, higher the number, the colder the tip of the plug runs. Typically you can go down 1 number (1 range hotter) without issue if you aren't REALLY pushing the bike.

However, your bike had a "5" in there. That thing must have been glowing like the sun in there. The plug can't get the heat out of itself fast enough, and it becomes a glow plug and begins preigniting the air/fuel in the cylinder (it lights off before the spark even occurs). This leads to huge cylinder pressure spikes, and "hammering" of the piston crown. Let it go long enough and it'll eat a hole in the piston, as you now know. That thing HAD to be making some terrible noises when on the gas, there's no way it wasn't pinging/rattling/crackling.

Anyway, with a hole torched through the piston like that I'd argue that you need to pull the bottom end apart to clean out debris and inspect the bearings. Even if no aluminum from the piston got in there you need to look at the bearings, especially the lower rod bearing. Running in preignition or detonation conditions puts huge loads on those parts with the high cylinder pressures.

Generic chart of cylinder pressures with knock. You can see the huge increase in pressure (which means force on everything in there) when ignition is too early, in your case due to a plug that is far too hot (doesn't mean that was your only problem though).

crank.gif

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Looks like it will need to be bored, or at least well honed.

Did you have the bore measured before ordering a piston? Because that cylinder has been bored way out, then a sleeve pressed in. No way to tell by eyeballing it whether it's the stock bore, a size or two over, or even a big bore.

Looking at the mess of aluminum bits in the transfer port I'd definitely say the bottom end NEEDS to come apart to fix this correctly.

Edited by adam728
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Well...

We have mesure the top and bottom of the bore with calipers and it fit tight 54mm all the way around,

how much work is involved in cleaning the bottom end out, will i be able to do this myself or will i need to get someone to do it for me?

Edited by Bowerbird100

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Well...

We have mesure the top and bottom of the bore with calipers and it fit tight 54mm all the way around,

how much work is involved in cleaning the bottom end out, will i be able to do this myself or will i need to get someone to do it for me?

Just change your jets in ur carb to control how rich or lean ur bike will run. Ur lucky to have it sleeved as the cost will be cheaper if you need to step up your bore.

I would put the right spark plug in there first. Install new piston etc... then run the bike. Run the bike for couple hours then remove your spark plug and check out the tip. If the tip is grey/whitish your running too lean. If completely black with thick oil coating, too rich... You want a nice burnt brown color.

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the existing spark plug before it broke the end off it was coated in a thick black grit so i had to clean it with a welding brush, this got rid of it but it came back after a couple hours.

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With the plug heat range that far out and that much damage being done to the engine there is no way to speculate on the jetting. Could be pig rich (hence the black plug) and still preignite due to plug temperature.

With stock bore, and what appears to be untouched ports, I'd say starting with stock jetting would be a safe bet. Check what you have in there now, don't assume it to be stock. Heck, the things been sleeved, and running a plug 4 ranges too hot, so you never know what else has been messed with.

Whether or not you can handle breaking down the bottom end is for you to answer, not us. We don't know your experience or comfort level. Download the manual, read through it, see if you feel it's something you can handle. There's also some links in the FAQ that have both pictoral and videos of doing the bottom end on a YZ250. Won't be exactly the same, but will be very close to what it takes to do a 125. Look through those and decide if it's something you can tackle.

One guy that's never wrenched before might be great at staying organized and following directions and breeze through a bottom end rebuild. Other's claim to have done dozens, but can monkey it up like no one's business, like hammering a crank in, or using case bolt to pull the halves together (BAD!).

Also, for the bore:

Have a shop measure it with a proper bore gauge. Measuring the top and bottom edges tell you nothing, as there is no wear taking place there. Typically you would measure something similar to below, down from the top far enough you are where the rings run, middle of bore, and near the bottom, but still where the piston skirt is loading the bore.

Sleeved cylinders are more apt to wear than plated ones. I can't imagine it survived all that heat and pressure without wearing some. Or in the very least the scratches pictured will be deep enough that once honed out the piston-to-bore clearance will be too large, and require being bored to the next oversize.

i20961.gif

Edited by adam728
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Guys, listen to Adam on this one, he is sharing some good info. With that original plug being at least three heat ranges hotter than recommended, there is no way to tell if the rest of the engine/jetting are even in the ballpark.

To the OP, as Adam said you need to go back to ground zero and set everything back to factory specs, especially in the ignition and jetting departments. Once you have done that, and have a fresh topend on the bike you can start tweaking it to get it to run properly.

One possible reason that someone put such a hot plug in there would have been the bike was fouling the proper plug. Could have been jetting related, worn topend, or even a bad clutch-side crank seal.

If you do decide to crack the cases yourself, you may as well replace all of the bearings and seals while you are in there. Absolutely no sense in cracking the cases just to clean it all out and put it back together with unknown hours on the bearings and seals. New seals and bearings can last for several years depending on how hard you ride and how you maintain the bike.

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You all get to say "i told you so" 🙂

I was convinced by my dad that the bottom end is and will be fine so i left it the way it was and just replaced the top end, while trying to start it after the rebuild the kick start seized.

I took the top end back off including the cylinder, put the bike in gear and rotated the back tire both directions, it seems to make almost a full rotation before seizing then i can rotate the tire the other direction and then it seizes again just bore a full rotation.

After doing this 3 or 4 times it just seizes altogether and wouldn't rotate at all.

Anyone have a link to a tutorial or a step by step guide on how to remove the bottom end from the frame, crack it and clean it?

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Well that certainly sucks.

Do a search on YouTube for bottom end rebuilds. There is a series on there from Rocky Mountain ATV (IIRC) that is pretty detailed on cracking the cases and putting them all back together. I believe they demonstrate using a YZ250 engine, but your bike will be very similar and the basics are the same.

Have you obtained a shop manual for the bike yet?

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The service manual explains it all step by step, I like to buy one to have next to me when I'm working, but u can download one online for cheaper.

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