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Need a quick hand with first topend..

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hi, just trying my first topend, first a bit of info - 2001 yz125, bough approx 18 months ago, owner said it had jsut had a rebuild when i bought it (usually i woudnt beleve anyone who im buying a bike or car off but it was a mate of my brothers). i probably only have ever taken it for a ride approx 15-20 times since i have owned it, and havent touched it in about 4 months, just got around to starting the top end today, when i have road it apart from the last couple of times it wasnt riding it very hard as i was just beginning.

i am just trying to get an opionon from someone who has done this before, befor i go ahead buying parts that i may not need/or need that i miss when doing an order. from my limited mechanical knowlege i have never experienced any engine problems from the bike except a couple of fouled plugs over the 20 or so rides (which i think was me mixing to much oil in teh fuel - i usualy mix 30:1 and use good castrol oil) and also there is usually i line of spooge running down the silencer (too much oil, not riding hard enough?)









firstly does anyone know what type/brand of piston this is? all i can find is 5135D stamped on the top? no toher numbers? also is the dark buildup on top normal?

from the pics does the cylinder look ok? i cant see any scratches ect looks ok to me

i have put up a video of teh connecting rod, i knwo its not good quality but i noticed a bit of play and was trying to wiggle it around and see if anyone can notice if there is to much play or not? i know a little is ok but how much is 'a little' am i not sure

also for a bad note, there was a burn mark (i think) in the bottom end, is this really bad or not? engine still turns over ok

so lets just say everything here looks ok, the next step is to get the cylinder checked so the piston i buy will not have to be oversize?

thanks for any help 👍

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i cant really tellf from the video, but side to side movement is normal, up and down movment is bad. Need to rebuild crank if there is up and down movement or replace. If the cylinder has no scores, gouges, and cant find any with your finger nail, hone the cylinder to deglaze it. Or use a sotch pad or sos pad with soap and hot water and scrub in there, rinse with hot water and dry it. For your jetting, the spooge that comes out of the silencer, is apparently jetting. To rich. I would go smaller on the main one step at a time and see how you like it, what is your current jetting? Make sure your on the pipe with these picky 125s. Check the float height and make sure its right, can mess jetting up. Those spots on the crank are normal. That piston looks new/ run very few hrs. What kind of riding do you do?

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first off congrats on your pick up... now to your questions, if your having problems with jetting which from the sounds of things is your biggest issue. Put a fresh top end in it, and if you dont have much experience with jetting get a jd kit, it will take 98% of the guess work away from the equation. A 125 needs to stay on the pipe, they are not designed to put around on! No one on here will be able to diagnose many problems from your pictures, we need to see the sides and the top of the piston. 30:1 is a decent ratio for a 125 but the bike needs to be jetted specifically for that ratio. My first guess would be that it still has stock jetting in the bike and these bikes are jetted rich from the factory.

As far as your crank goes as long as there is no up and down play chances are your crank is perfectly fine.

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When your checking for up and down play on the rod, make sure that you check for it at different crankshaft positions. So check with the rod straight up at TDC, then check at BDC, and inbetween both ways. If there is up and down play at all in any of these positions, it's a good warning sign that the bearing will blow, sooner or later.

Your cylinder looks ok from the pics, but you need to feel it and make sure that there are no scratches. You should not feel any with your finger nail, sometimes there are blowby marks that looks like scratches, but are just stains. The finger nail check is a good indicator of a scored cylinder.

I'd give that cylinder a full disassembly and gasoline bath for a night, scrub it down and get it clean so you can see things more clearly.

Steel wool and some carb cleaner or any cleaner for that matter on the head (the domed area) will clean that up really nice.

That blue burn mark on the crank is part of the heat treatment forging process at the factory, perfectly normal. All cranks come like that. It's usually on both sides of the crank at TDC.

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yeh thanks all i am not at home at the moment but i will check for up and down play tommorow,

i am not that worried about jetting for the moment, i am nto doing the rebuild because of lost performance but becuase it has been 18 monnths and for peace of mind

i showed my mate who does jetskis/boats and he says the piston has small marks on it and also at teh top of the head but that is most likely from a aprevious rebuild.

am i right in saying this is a nikasil cylinder? so i bath it in fuel and give it a clean, is it ok to do the powervalve at the same time? i have read conflicting stuff on the 'deglazing', just use a scotch pad and fuel on the inside of the cylinder?

he said next step would be take it to the engine shop and get it measured up?

thanks all





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  • 2 weeks later...

just need a quick confrimation, got cylinder bathing in some fuel now, what do i use to de-glaze, i have a steel wool, scotch pad and 600 grit wet and dry sandpaper? and do i deglaze it in - fuel, wd40 or a hot soapy bath?

after cleaning the piston i found a part number and it is a normal size (54mm) piston from wiesco. now the next question, from what i have read -

oem pistons come in a,b,c, or d size? b being a tiny bit bigger then a, c being a tiny bit bigger then b.. ect ect? and this is for a normal un-bored cylinder?

and after market pistons only come in 54, 55, 56mm ect?

now the stamp on my cylinder is rubbed off, would i be better just to get another normal (54mm) wiesco or pro-x kit, or get a sized oem?

also out of wiesco/pro-x/yamaha ect what would be the best brand for me, not after the latest and greatest performance but after reliability?


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The difference in sizes between the a,b,c,and d are very small. Most will order the largest size, a "d", to install in a used cylinder.

Yes, often the aftermarket pistons only come in one size and thats usually sized close to the "b" or "c" spec piston.

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i would just buy another wiseco the same size. I just did a top end on my 04 125, piston wias in bit better condition, but compression was being lost.

Here's my piston fomr my KTM that might need replacing ha:smirk:

(those are scratches by the way)


All i did was remove the piston and head, stick the new one end after putting some gas on it, put the head back on, simple as that.

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Just use the scotch brite pad on the cylinder and do not worry about crosshatch marks. The cylinder is nikasil and does not need to be honed. Honing is only for non-nikasil cylinders. Remove the power valve and clean it all up. Use the same kind of wiseco piston and put it all back together. You do not have that much blow by on that old piston so everything should go back together well and you will just have to heat cycle the engine after the rebuild.

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well visited the bike shop just to get a price for the rebuild parts, happened to have the cylinder on me and showed them, advised it had actually had a sleeve put in it. took it to the engine shop they confirmed this to, left it with them to be measured and honed... if anyone can see why the wiesco piston kit wont be the best for me please advise.... thanks

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