Motorcycle 2 Stroke Top End Rebuild – Piston Replacement


Hi I’m Eric with Rocky Mountain ATV/MC.com
and today I’m going to show you how to rebuild a two stroke top end. We’re gonna need an
owners manual, some common hand tools, a Tusk Top End Gasket Set, Wiseco Piston and some
contact cleaner. We’re going to start with the tank off, the subframe removed, the antifreeze
drained and it’s always really important to start with a spotlessly clean bike. You want
to make sure you don’t have any dirt or anything that can fall down into the top end. We’re
going to start by taking off the pipe, and we’re going to use our Tusk Spring Puller
to remove the springs. Go ahead and pull the pipe off. Next up we want to take the top
motor mount headstay off, the spark plug. it helps remove the compression so the piston
will go up and down pretty easy. Some radiator hoses, always like to have an extra rag there
in case there’s any coolant left in the lines. We’re going to go ahead and remove the head
now. As you pull that off, some models are going to have o-rings, others are going to
have a gasket. This model has o-rings, make sure and get those out. There’s two little
dowels on the sides that help center the head, make sure and pull those out so we don’t drop
them down into the cylinder. We’re going to pull the power valve cover off now. Inside
there we have our linkage and on Yamaha’s it has a little groove right there that you
place a holding tool in so when you loosen the linkage bolt it doesn’t damage the power
valve assembly. Go ahead and pull that off. There’s a spacer behind there also. Next up
we’re going to loosen our cylinder bolts, some of there are hard to get to. A lot of
times you can get to them with an end wrench though. We need to move the carburetor up
out of the way. It’s kinda nice just to lay it over the top and then you don’t have to
undo the cables and everything. Just get it out of your way. On this model we need to
move that CDI box out of the way but on other models it might be necessary to move the radiators
or anything else that might be in your way. It might be necessary to tap the side of the
cylinder with a mallet to loosen it up from the base gasket. Go ahead and pull the cylinder
off, kinda pull the piston down out of the way and then move it out of the way. Now we’re
going to go ahead and put a rag in the bottom end, and pull our wristpin clip out. Grab
a socket, suitable size and push that wristpin out. When you pull that out make sure you’re
being careful not to drop anything. There’s your wristpin bearing. Now is a great time
to refer to your service manual for wear tolerances and parts inspection for assembly. As we begin
installation the first thing we need to do is check our ring end gap. And to do that
we are going to install a ring in the cylinder and measure with a feeler gauge the gap. Wiseco
recommends .0013 of an inch, always refer to your piston or ring instructions to get
the correct gap. If it is too small you can take a small file and file away a little bit
of the ring and open up the gap a little bit. Next we’re going to install the ring on the
piston and you start by on one end get it in the groove and then use your thumb to kind
of spread the ring very carefully without scratching the top of the piston, and slide
the ring all the way in. Now pay special attention to that centering pin, make sure you have
that ring centered on there when you install the cylinder. Next up is our circlip, you
want to make sure and twist that on, not bend it. If you bend it, it won’t stay in place
and it will fall out. Now we’re going to go ahead and use a little raw premix and lubricate
the rod and the bearing. Now when we put the piston on you need to pay special attention,
there’s normally an arrow or a marking on top of the piston. Wiseco’s normally have
the arrow marking the exhaust but always refer to your piston manufacturers instructions.
We’re going to go ahead and put that on and then we are going to slide the wristpin in,
make sure everything lines up nice. And then next we’re going to go ahead and install the
circlip on the other side. Make sure that rag is still in the bottom so if you do drop
the circlip, it doesn’t fall in the bottom end. Now we’re going to install our centering
dowels and our base gasket. Make sure the surfaces are clean so you have a good sealing
surface. There again you can see the sealing surface on the cylinder. We’re going to use
a little bit more raw premix on the bottom on that beveled edge of our cylinder to held
the rings slide in a little better. Now we need to compress the rings as we slide that
beveled edge on, make sure that the rings are centered on the centering pin, and you
can feel them kind of slide down and into place. Make sure and take some time right
here, get everything lined up. Now is a great time to make sure the piston goes up and down
nice and easy. There’s no snags, you can feel that it’s installed correctly. Now we’re going
to go ahead and put on our base cylinder bolts in a criss cross pattern so there’s no warpage.
It’s always good to have your piston all the way to the top so there’s no way you can drop
anything down. We have our o-rings installed, we’re gonna drop our cylinder head on. We
put on our bolts and once again there’s our copper washers. And then we’re going to put
our cylinder bolts on and tighten them in a criss cross pattern once again so there’s
no warpage. Always refer to your service manual for torque specifications. Now we’re going
to reassemble our power valve exactly how we took it off. There’s a spacer, the link,
make sure that goes on it’s groove correctly. We’re going to put our holding tool in so
we don’t damage the power valve when we tighten the bolt. Next we’re going to put the cover
on, there’s a little gasket down at the bottom, you need to make sure it’s lined up correctly.
Install our bolts. Next up we want to make sure we’ve got our spark plug, our radiator
hoses, our cylinder head stay. We’re going to put our pipe back on with our Tusk Spring
Puller. As you can see we have our CDI box back installed. Spark plug cap, carburetor.
We’ve filled it with antifreeze. Now it’s time to put the tank on, seat, subframe. Go
ahead and refer to your owners manual for proper break in procedure and let’s go ride!

100 comments

  • should any air leak from cylinder power valves?

  • Very good job.Out of 100 marks i give you only 75 and cut 25 because you are wearing ring in finger and watch in wrist.Thats wrong while working in technical line.OK.You seems expert.

  • Rocky Mountain ATV MC

    you need to measure the ring in the cylinder – measure the ring end gap. Rings are faily inexpensive. If you have it tore down, I suggest new rings anyway.

  • Have you heard about the weight burning system that lies hidden in your body. Go and Google Fat Blast Formula to bring it out.

  • I was rebuilding the top end on my 1999 yz 250 2 stroke and I took the tag out to early, II ended up dropping the circlip in the crank case so I started slowly rotating the crank shaft and sure as shit, the circlip was stuck on the shaft from the oil on it!! :DDD so I put the rag back in and finished, next day I practiced on the track and me chain let go. But still, close call.

  • Rocky Mountain ATV MC

    That is a close call! I've had that happen to me and we were lucky enough to easily get it back out but a rag is always a good idea!

  • I had a rebuild on my 2004 crf 450 ( piston kit and con rod assembly ) and was wondering if I should have good compression?

  • Rocky Mountain ATV MC

    Each bike will be different so you'll have to refer to your service manual for compression specs but yes, where it was just rebuilt it should have plenty of compression unless something else was going on…

  • i have the same bike, does it matter what way the rings are facing up or down? PLEASE say no because i just finished putting the bike back together!
    thanks

  • Great video, just getting a few tips before attempting first 2t piston/ring change on sons 2010 KX85.

  • Rocky Mountain ATV MC

    Sounds good, if you run into any questions during the rebuild be sure to let me know!

  • can someone plaese help im taking my topend apart but i have come across a problem u no the cylinder base nuts that clamp cylinder down and how there is one nut inside the powervalve linkage cover well the stud loosens when u loosen the nut amd cant be removed?

  • Rocky Mountain ATV MC

    If you have the other 3 nuts removed and the stud is just loosening with the nut, you could try and raise the cylinder up as you are loosening the nut and removing the nut and stud the same time you remove the cylinder. If you can't get that to work you'll have to try and clamp the stud (possibly with two nuts) and try and break the nut free on the stud and then remove it…

  • thank u so much worked great. removing the nut by locking two worked a treat thanks

  • Rocky Mountain ATV MC

    Perfect! Glad you got it figured out!

  • Is it hard to do this on ur own ?

  • Rocky Mountain ATV MC

    No, this job is fairly simple and can be easily completed by just about anyone as long as the service manual is followed closely. You can also use this video as reference as well and if you run into any questions just ask.

  • Thanks alot i really appreciate ur help thanks

  • Rocky Mountain ATV MC

    Yamaha has divided the pistons into 4 slightly different sizes (A,B,C & D). The difference in these classes is only .01mm so what we recommend is if you are replacing your top end while reusing your used cylinder go the the "D" sized pistons. The Wiseco piston will work no matter what piston your are replacing or whether its in the a,b,c or d class since they are so close. The only time you would want to worry about the class is if you were installing a brand new cylinder. This bike is a 2004.

  • Im replacing piston in my 2008 kx 85 2 stroke and i was wondering how much different you guys think it would be because it is our first time rebuilding it by ourselves

  • Rocky Mountain ATV MC

    It will be a little different than what's shown in this video obviously because it's a kawasaki and it's an 85. The power valve linkage will be a little different so be aware of that but as long as you follow along closely with your service manual and reference to this video you should be fine. If you have any questions feel free to call in so we can help you with it

  • Nicely done! Thanks for sharing…

  • i dont think that bike is an 05 Yamaha came out with the aluminum frame in 05 this one is steel

  • How long would a top end last if you only ride dessert trails. How many hours? Would it need to be changed more often. I don't ride at tracks. I ride like every other week.

  • How do you know what size your piston is? How do you tell if the piston fill fit the cylinder??

  • Just out of interest how long does it take to complete the procedure roughly?
    Thanks for the very informative video!

  • hi cool video…one question…what could happen if I instal the piston in the other direction…I mean intake side to exhaust…just wondering..thanx

  • very easy nice thanks you rocky….

  • you are really the boss mister rocky i love it 

  • Decent video , thumbs up

  • Thank you,very good video

  • ده بستون ثنائي يعني راحت عليه .

  • can someone tell me what i should use for lubrication the cylinder? Is Castrol Power1 Racing (2Stroke) ok for that? Or should it be any special oil?

  • can't for the life of me get the piston to slide in like this one did…

  • Is the bottom ring supposed to close all the way just like the top ring? My second ring has a good size gap when trying to close and its impossible to get into the cylinder, any suggestions?

  • Had a top end rebuild on my TZR 125 it starts and revs fine in neutral but has no power when i try to ride it just kind of spluters any ideas ?

  • I have a question. I have a 1984 Kawasaki KDX250 and I need a FLYWHEEL MAGNET ROTOR and I can not find it online anywhere at all. Will another later model work on my bike to get it up and running again?

  • 4:25 use your flat top piston (or if the dome is symmetrical) to push the ring into the cylinder which ensures the ring is normal (perpendicular) to the cylinder before checking gap.

  • why would a new never run top end seize up when I rotate it around

  • Cinnamon Holt

    How would you break in a cr125

  • Hey would a little earlier piston then the bike I have work also? I have a 1992 yz250 and I didn't know if the 1994 piston would work or not.

  • Does a 1997 cr250 top end come apart the same way as this?

  • I have a 2000 rm 250 and I just bought it. The guy that slod it to me said he just did a rebuild on the top end. It blew up the next day and I took it apart to find a brand new wiesco piston and the jug has been Mic at a standard bore and the piston is the right size and all but it doesn't have lubrication holes in the piston exhaust side.could this be the problem ?

  • Frederic Jordan

    Hi , thanks for what you guys doing, it's awsome!
    I have done a compression test on my yz250 2014. I just bought it and the bike got a hour meter who's indicate 23 hours. My compression test result is 170 psi. The part's guy at the Yamaha told me that a really good compression is around 150 psi and you are good between 120 and 130 psi. But now that i've done a research. I've figured that this is maybe all wrong. I have read that at less than 180 psi you might need a new top end.
    That'll be apreciate if you guys can help me about it, or even just…what is your thought about the compression test? Is that really a good tool to know what's going on inside….
    I think i can trust my compression tester, i mean i did not buy the cheapest. Oh and by the way, im french so sorry for the poor english.

  • Frederic Jordan

    +Rocky mountain

    Ok thanks for the fast repply and tips! Your right. Actually i just ordered a top end. Don't wanna gamble.

  • Got a 94 kx125 that was leaking coolant from the radiator. (rock might of hit it and caused a little hole that was leaking) and like an idiot i kept riding.  Then the bike just died and would not start again.  What happens when your bike overheats like that.  Would i be okay to just rebuild the top end and replace the radiator or could it have wrecked something else?  Any advice is appreciated.  Thanks

  • If my Yz 125 has no top end power, could it be just the rings? Or should I replace the top end?

  • you forgot to mention that many rings have a up/down side to pay attention too, side with lettering or numbers on it always faces up

  • Can anyone tell me if streaking on the cylinder is to bad cause I just put piston in it and still runs exelent but cant see cross hatching on the cylinder anymore when I look in the cylinder, It look good when I did it

  • Im just wondering how long it will last or is it gonna blow, cause im A class rider if blows in the air Aint gona be good in certain situations

  • Christopher Bassit

    I'm new to this. I'm fourteen and need to learn. For you guys that are more educated with this I have a few questions.
    1. What is a bottom end rebuild
    2. What is a top end rebuild
    3. Are they similar
    4. How long will this take at most
    5. How often will a yz125 under moderate riding conditions have to be rebuilt.

    Also if you know a good channel to learn that will help. I'm in no rush but I really bees to learn within a year.

  • looks like the chrome is coming thru so much for ring end gap.

  • Just did my first top end today after watching this video. Thanks!

  • If a cylinder has scratches on the walls will a hone take them out or will it have to be re sleeved or finished? My buddy just blew his rm 125 up and we havent tore it down yet but im hoping for the best. Also, when 2 strokes "blow up" aka he was runnin wide open in a field and she bogged and quit, does it generally mess the crank or rod up? Opposed to a top end just wearing out over time? Thanks

  • what is raw pre mix used on the wristpin bearing

  • Great video! Is it necessary to have the cylinder machined? I didn't recall you saying if that was done or not . Thanks.

  • Rodriguez Russell

    i have a 06 yz 250 rebuilding top and bottom end but the previous owner of the bike add a steady offroad flywheel. how do i remove it.

  • I have a newb question.. On my 03 KX125 my powervalve cover is made of a flexible material and it doesn't make a complete seal on the engine.. I bought this bike not running and plan on rebuilding the entire engine and I want to know should I be looking to buy a new cover or should I just put it back on.

  • Do you have to change the o rings?

  • Frederic Jordan

    +Rocky Mountain ATV MC Hi guys, ive done my top end for a second times already and it went pretty well. But ive got an issue, it's hard to describe but i ll try my best ( im french) Bike is a yamaha yz 250 2 stroke. piston is oem, ive got a pc works pipe and shorty silencer a fly wheel weight and a boyesen waterpump Thats about it for engine's mods. when ive done my top end i did clean the whole carb and ive jetted the bike for procircuit recomandation witches 175 main and 48 pilot, 2 air screw turns at the see level. (last season my bike ran great jetted like this than i put a new air filter new spark plug.) My first ride was awsome the bike was running strong and i felt like my yz had never ran that great before. Fuel mixture for break in was around 28/1, it was a little chili outside 7 degrees. than the day after was colder around 2 to 4 degrees, and that day it was like day and night.. it ran like shit.. first it did not start cuz the plug flooded. So ive drained the gas tank and change the spark plug and back to 40/1 mix. bike worked but couldnt feel any powerband.. it was all or nothing and i had to shift the gears super quick cuz the bike went in high rev so fast.. i turn out the air screw to make it ritcher but no real difference noticeable. when i got home i changed the jets for ritcher (stock 180 main 50 pilot) around 1600 feet from see level.. (Rode in a higher place) the bike work a tiny better but still have the issue. the rev goes from nothing to stratospher right away. the sparkplug look not bad.. I dont recognize my bike, i cant even pop a wheely.. its really weird. i was wandering if its possible thats my piston's rings are fucked..?? Sorry for this long text.. That'd be really sweet if you guys could help me somehow.. i did purchased some stuff from you but the canadian dollar is so low right now lol

  • Is this the same for a cr85

  • I have a 2000 cr250. if I have a plated cylinder I shouldn't hone it correct? do you recommend the green scotchbrite method for the cylinder instead of hone?

  • I noticed you guys didn't drill a hole In the piston skirt. do I need to for a 2000 cr250?

  • Newbie question… Why take the head off? Why not just take off the cylinder?

  • WOW!! dude why are you using junk paper base gaskets?

  • Justin Danatos

    i am losing power on my 99 yamaha rt100 2 stroke throughout all gears…specifically once the motor begins to heat up a bit, its incredibly hard to kick over as well and the kicker goes down about 1/2 way before grabbing. im assuming it is piston/rings but would like an expert opinion if you wouldnt mind giving me one. and if your company has a link for an rt100 top end kit it would be much appreciated. thanks

  • how often should you replace the piston on a yamaha blaster and how do you know if it time to do so

  • When I do this, should I also hone the cylinder? My understanding was that you should hone it so the new rings will seat, but the video makes no mention of honing.

  • you will be triggered

    followed instructions based on video, after installation my motorcycle now fly at 600kph

  • u make it look so easy any advice for a novice

  • I got a 1988 yamaha ysr50 2stroke five speed and its got 8,450 miles on it since last year in August it had low compression and took it to a mechanic then he gave me estimates on wat was wrong wit it so I got the parts but just need a mechanic and tools to help but if it has low compression and bought seals and gaskets for it u think it would fix the compression

  • hello I'm wondering if you can help me plz I have a 95 CR80 I have a wossner piston and rings I just need to know what way to put the ring it has writing on one side writing up or down ?

  • Kellysexy Mechanic

    You forgot that you MUST use a Torque Wrench for all the bolt and nuts!!!

  • What year is this bike that you are working on??

  • Nice job with the video. I Always enjoy the good deals online, just bought some accessories yesterday for my 06 Crf250r. Wish you could upload a rebuild on a 08 crf250r so I can confidently jump into my rebuild.

  • great and clear video thumbs up :)!

  • wiseco pistons are terrible , like worst piston u can put in ur bike. vertex makes way better pistons and oem quality if not better.

  • Do you guys have any tips on how to take the cylinder off without coolant running out of it into the crank? Every time I do a top end there is always a little bit of coolant left in the cylinder and when you take it off that bit of coolant runs out. Any tips on how to avoid this???

  • No torque wrench?

  • do you have to tork the jug bolts?

  • very very helpful video thank you. On the Suzuki RM 250 you have to remove the radiators which is mentioned in the video. I order all my parts on this website they do a great job !!!!

  • Do you have to break it in or do heat cycles??? Or do you just rip it??

  • Hi there Ive just done a top end rebuild on a suzuki 125rm.. well I seam to be having fouled plug issue.. and I was just wounder if it's my rings,either it isn't in the correct position.. would it be alryt top strip the bike to check it or would I have to buy new gaskets? I've got my bike all at factory settings runs alryt just yeah plug fouling after a while? ANY IDEAS OR TIPS

  • Do you have to hone out the cylinder?

  • Great vid, good tips.

  • Informative, maybe you could do a new KTM/Husky 300. By the way is it Wise- CO or WIS-EC-O ?

  • I want to know if you used a Tourque wrench? I noticed in this and your YZ bottom end rebuild you didn’t show the use of one.

  • I just got a 97 rm80 standard bore it needs bored. It has a sleeve already. Can it be bored without resleeving the jug?

  • Thank YOU!!! You and one other YT video saved me time, money, appointment setting… and a trip to the local mechanic! 😀

  • I am VERY close to buying a new YZ 125 and going into BEAST MODE! That said, I changed piston and rings on my '86 rm 80 a few times, but this looks A LOT more complicated. Is this harder or easier than it looks? How long should this take a novice?Thanks in advance, y'all. Great video!

  • Please reply – I have a yz125, the only thing that puts me off this job is the idea on bores and what size piston i need. is it not as simple as just measuring the old piston and ordering one to match? i dont have the tools to start doing stuff to the cylinder. on my currently cylinder it has a letter B on it. i know that is a reference to something about the piston

  • hello , the pieces are also sent to italy?

  • I have a 96 Yamaha blaster the cylinder is non nikasil plated can I use an oem size top end kit and just hone it ?

  • Hey, I’m having trouble getting my wrist pin out, I’ve tried a socket, and I have the other circlip out, but it won’t push through, any suggestions?

  • Hi what is the reason for opening the power valve housing up when doin a piston rebuild? I only ask as I don't no so no silly comments I've a dt125 04 plate I need new piston for

  • Power valve?

  • Anyone know where I can find the proper torque spec for a 1998 Skidoo Tundra top end 4 bolts?
    Any advance is much appreciated, Thx!

  • Etonfield Vainerere

    Thanks for the video

  • Good vid!!

  • Raw premixed? Is that mixed gas or just the oil?

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