How to Install a New Top End on a 2 Stroke Dirt Bike • YZ125 Rebuild Ep. 4 • WIN THIS BIKE!!

in this video we're gonna show you how to put a new top end in a 125 two-stroke first things first prepare the engine to take out the top end now I'm doing this really primitive with a brick to prop up the motor I didn't have an engine stand on me at the time and most people are gonna do this with the engine inside the bike which I usually prefer but since we're doing the rebuild we already had the motor out and it made it a little bit easier to film clean up any dirt debris anything that's gonna get in the way we're popping up the cylinder and then we can mow to our way on through this rebuilding remove cylinder head so the first thing you're gonna do is pop off the spark plug you can do this after you have the head off but it is way more of a hassle and go ahead and loosen the cylinder head bolts I did that in a crisscross pattern which is more important to do when you're tightening I just did it when I'm loosening as well and make sure to take off the washers organize everything you're gonna be putting on new washers but it's still nice to have the old parts nice and organized so you can find everything efficiently and pop off the head disconnect the power valve pretty simple step you're just gonna go ahead and take these eight millimeter bolts out pop this cover off you'll expose a ten millimeter bolt which you'll take off there is a washer watch out so you don't lose that and just get your fingers in there kind of jiggle it loose it'll pull out a little bit and then there's also dis collar take that off make sure you don't lose it remove the cylinder there's four cylinder bolts one is in this power valve chamber a little bit hard to access make sure that you have a wrench and then I also use a hammer just to break loose and get that initial turn and again just like the cylinder head I always like to loosen bolts opposite each other it's just a habit but it's a good one to get into and completely remove the bolts there are no washers on these and then just a little back and forth and you should be able to wiggle it up and pull it clean off remove the piston before you do anything stick some clean paper towels down into your bottom and above the crankshaft trust me you do not want to learn this the hard way when you drop one of these circlips there's a little open spot in the piston and just use the needle nose and take out only one side to start click what you need and then your finger should be just enough to push that wrist pin all the way through and then remove the piston and always take time to just notice is there anything abnormally warned about the piston it'll give you a little idea if anything's been going on with your motor and then use a razor blade just to assist you in taking the cylinder gasket off try to get as under the gasket as you possibly can so you're not kind of splitting it and making more of a mess time to check the crankshaft and the cylinder now there are different ways to check the healthier bottom end I'm using a real simple side-to-side an up and down motion to see if the crankshaft bearings feel good they did they felt tight very little free play I'm also looking for crosshairs that were evident in the cylinder meaning that I felt confident enough to move on without having to hone or replay the cylinder now that said I know the history of this bike and know that it had very little hours on it if you're at all concerned about your crankshaft health or your cylinder please have a professional take a look at it continuing on just gonna clean using air cleaner spray and towels just making sure that the cylinder gets a nice thorough cleaning replacing the gaskets on the top have some motor oil handy so that you can coat the inside of the cylinder as well as the piston ring in the next step [Music] now it's time to install your shiny new piston and really let your bike know that it's loved I like to lay everything out make sure that you're not missing anything before you move on this is a quick glimpse of old versus new parts install the cylinder gasket this is the piston ring there is a marking with the letter T as you see right here that is the top of the piston ring very important that you keep the T on the top go ahead and apply a light coat in a motor oil to the ring before you install it start with one side put it in the groove and then not bending it too much but it does have a little expansion to get it up over the top of the piston and then go ahead and align your piston ring with this little gap on the top of your piston there's also indication marks on the very top of your piston that arrow should be going towards the front of the bike towards your exhaust coat the wrist pin bearing with motor oil as well as the wrist pin and stall that through with the arrow pointing towards the front of the bike and in the circlip insert one end use a needle nose and just kind of compress it and tuck it down in there and then just finish pushing it in do notice that the gap in the circlip is opposing the gap in the piston you don't want those two gaps to align and repeat on the other side also double check to make sure that your paper towels are good you do not want to drop the circlip down in the bottom end and learn the hard way time to install the cylinder remove the paper towels from the bottom and double check to make sure the piston ring gap is aligned with that alignment gap at the top of your piston and then compress the piston ring with one hand and have the cylinder in the other and just very small Wiggles make sure that your dowel pins are in place mine never came out and the cylinder should drop down onto the bottom end nice and smoothly if you have any friction whatsoever right here you really want to double check and make sure that you're aligning that ring tighten down and torque before cylinder bolts at 22 foot-pounds check your owner's manual for your bike connect your power valve make sure to put that little spacing collar back in there connect your arm and then replace the bolt with the washer the torque on this is very light just a few foot-pounds by the way go ahead and go online Google your bike you're gonna find some kind of online manual with torque specifications this is what I've been referencing and replace the power valve cover gasket make sure to seat the front of a cover gasket on to that existing rubber piece that's already on the motor and replace your three bolts install the cylinder head so I'm just giving it a final clean before I put it on getting rid of the existing carbon and such that was on fire from before a little air to clean out all the holes install the cylinder head onto the cylinder that new gaskets were already in place install new brush washers and go ahead and put on the bolts tighten these in a crisscross pattern the torque on these is going to be 20 foot pounds and it is important so make sure to reference your own owner's manual to make sure you're getting that right check the reads and install the spark plug this is not a necessary step for checking the reads but since we're so far into it I figured it's a good time to just check the hell to this if you've ever had debris come through your air filter which is not good it can chip your read so I always look at the corner and the edges make sure they're legitimate and these look great so I just went ahead and pop those back in new reads can be an awesome upgrade for a 125 two-stroke but we just wanted to keep this primarily stock and make it nice and fresh there was already a new spark plug in this bike so I'm just knocking a little bit of a carbon off with the wire brush blowing it off pop it back in thank you so much for watching episode number four of our yz125 build giveaway there's more videos on your right click to subscribe and enter the giveaway we are super excited to give away this bike see you in episode number five

Channel: The Mx Factory

Views: 129,235

Rating: 4.9485531 out of 5

Keywords: 2 stroke dirt bike, 2 stroke, 2 stroke engine, dirt bike top end rebuild, dirt bike top end, dirt bike piston replacement, dirt bike piston ring installation, motocross top end rebuild, dirt bike rebuild, dirt bike restoration, dirt bike teardown, the mx factory, yz 125, 2 stroke top end rebuild, 2 stroke rebuild, two stroke engine, yamaha yz125, how to rebuild, how to rebuild a dirt bike engine, how to install a piston 2 stroke, motocross bike build, dirt bike repair

Id: RjiUeO77vww

Length: 7min 20sec (440 seconds)

Published: Mon Dec 30 2019