Nice! I did my first top end back in 1978 on my ‘78 YZ80 using an article from Dirt Bike magazine and a Photocopy from my Yamaha dealer👍😂 I was 12 years old and was so amazed when it fired up and ran great afterwards! One of the most rewarding moments of my life. Sure wish we had RU-vid and videos like this back then!
Hey Robert!! I know that feeling! I remember my first overall rebuild. I was totally amazed that my motor fired up on the literally the first "kick" (but it was a kart, so the way we started them was more of a bump start). Anyway, it is a great feeling, I agree!! Thanks for watching!
Hey Brian! I have an 04 CR125 and it has an A cylinder. When doing a top end rebuild is it important to have an A piston to match the cylinder or is using a B piston also fine?
Hi Jade. So here’s the extent of my knowledge on that. Nona recommends using an A piston with an A cylinder. The reason why..? That I don’t know! The A cylinder is far more common, BTW. Sorry I don’t have more knowledge to share on the subject but I’m sure a little digging on the web should reveal the reasons.
I’m sure there will be some differences, but they are both 2 strokes. However, the PW80 is air cooled. What I would do is get the service manual AND watch the video. The procedures are likely similar but you can confirm all the steps with the manual.
I've spent 2 fuckin hours so far tryna get my cylinder back on and this guy just dose it like that , I'm working on a kx85 and the piston has 2 rings and it's just gets stuck every time even when I'm pushing the top of the piston in the cylinder it just feels like it's getting caught on the rings , maybe I'm not using enough oil? I've also tried zip ties idk what to do
Hey Boogie! Maybe take a break first! Its hard to show some things like sliding the cylinder over the piston and installing wrist pin circlips. It takes patience and some practice. Ive been doing it for over 20 years, so that helps. BUT, compress the ring at a point farthest away from the gap and try sliding the piston in about 3 degrees from 90 degrees. So, the piston is tilted ever so slightly higher on the side farthest from the end gap. Get the side without the end gap to slip inside the cylinder, then compress the ring midway between the high part of the piston that should now be in the cylinder and press they cylinder down a bit more. Working on motors is tedious sometimes. I get frustrated too. I have a new video coming out this weekend where I slide a 3 ring piston into a bore. Maybe it will be easier to see how I do it. If you really cant get it done, I can help you by email. You can do it!!
@@brian.d.mobley thanks man and I did I can back from my storege unit and at chilled out with my girl and went back and put the whole bike back together, I've noticed when I get like that I need to just step away for a hour or 2 and refresh, appreciate the advice it really helped a lot!
@@boogiekush9391 I sometimes struggle with frustration too. But, I’m really glad to read that you got it all sorted out and you’re happy again. Mechanics can be very tedious sometimes.
Awesome video! I’m a 16 year old gear head that just bought a low compression CR125. Wasn’t 100% sure how to order the right size piston. This helps a ton. Keep the great content coming!
OMG the timing of this video could not be better. I just bought a 1997 CR 125 and I’m going to rebuild the top end, I’m definitely going to save this video. I have a question, I’m planning on ordering a top end kit but I’m wondering what year Honda started stamping the cylinders with A and Bs. I read it was either 2000 or 2002 so I do not have to worry about mine being a 1997 correct?
Also do u absolutely have to remove the flapper valves and flapper valve shaft when just swapping the piston ring and gasket? I’ve rebuilt my little nitro engines on my rRemote Control helicopters but obviously never had to mess with power valves or anything. You have no idea how big of help you are, Greatly appreciate it and you got a life long scriber here!!!
Hey Scott! Honestly, Im not sure of the year that Honda started designating cylinders as A or B, BUT in 1997 that naming convention was being used. So, you have either and A or B cylinder and it should be marked on the rearward facing, left side of the cylinder very near the rearward most cylinder bolt. I would recommend getting the corresponding piston that matches your cylinder type. Often, non OEM piston manufacturers only sell an A piston, so if you don’t go OEM, you may have limited choices. There are far more A cylinders out there than B, so odds are that you are fine with an aftermarket A piston sold by someone other than Honda.
@@scottbuck9348 Regarding the flapper valve service, its a free country, so you don’t have to! But, its not hard to do and it gets harder and harder to get them out and clean them the longer you wait. Its really not a hard job and it makes a noticeable difference in performance when they are clean and working right. I’d just suck it up and do it since you have the cylinder out. And THANK YOU for the comments and for subscribing! Ive become obsessed with videography and editing so Im glad I was able to help someone with one of my video projects!
Hi Jake. If you are using a matched piston and cylinder, you really dont need to measure the piston. For instance, on a Honda, if you have an "A" cylinder and install and "A" piston, you are good to go. But, I did go over measuring pretty extensively in this video if you need all the details: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-B76wuDaao4w.html
Honestly, I’d recommend skipping any kit and just buy OEM replacement parts from the manufacturer. There aren’t many parts to buy and you can trust the factory quality. I don’t buy kits anymore.
Hey YLWR6! All you have to do is turn the petcock to the off position. I know it sounds overly simple, but thats it. It may drip a wee bit of gas, but not much. If you get stuck or need help, dont hesitate to reach out.
Hey Kevin! So glad it helped! I had fun making that video. Got a new camera which helped to make the image quality better too. Thanks for the taking the time to share your experience with the video.
@@brian.d.mobley I’m jealous. My garage goes from being decent to looking like a hurricane hit it when I’m wrenching on something. Excellent video btw! I’m about to tackle on top end rebuild on an RM 80 so this helps a lot!
@@davemccage7918 Mine gets messy sometimes too. But I try to at least put my tools back after I’m done with them so at least those stay organized. But it’s hard to always keep everything tidy, so I feel ya! Good luck on the Yamaha!
Is it normal to have alot of piston movements at TDC? I have new topend bearing & big end. I'm Using old piston and rings because my cylinder needs service and didn't want to hone & new rings. Unfortunately I didn't notice any of the movements from piston at tdc.
Hey Aussie. In a word, no. But you can measure the outside diameter of your piston and the inside dimension of the cylinder to make sure all are within the manufacturers specs.
Ok thanks mate, just to be safe I think i will check. At the very top of sleeve I can see a uneven line where the new factory nicseal is & the worn nicseal. I was thinking that is because of piston rocking?
It’s essentially the same procedure but the engine featured in this video is a 2007. Best practice is to supplement your knowledge base with the factory service manual.
Hey I have a 2003 cr125 and it runs good no bog or anything and it will rev out but I feel like (I know) it’s not as snappy or fast as it should be I think my power valves are having problems any idea what could be my problem? Just did top end, new lectron carb also. Cylinder is brand new.
Hi ONebulaz! Like a lot of problems, there can be many reasons. If the power valves are not clean, they can affect performance. I have had such poor luck and spend so much time trying to help people analyze their problems that I just find its basically useless unless I have the motor in front of me, its almost impossible to say.
@@burstable252 It’s a free country, so you don’t have to. I’ll just say that if you don’t want to do the job as recommended by the manufacturer, you’re risking the reliability of your repair.
Hi Chris. A ball honing using a hone made for the ID of the cylinder is a good idea. Be sure to THOROUGHLY clean the cylinder after the procedure. Good question.
thanks for the great video just got my hands on an 02 cr125r and the carb is leaky and clogged and the top end needs urgent rebuilding! i’m new to riding but i love it so much and i want to start wrenching on the bike! bought tons of tools for the bike and the proper manual and other stuff im eager to start and vids like these help a ton! thanks a lot!
Congrats on the bike and hats off to you for deciding to work on the bike yourself. It’s a lot of fun and very satisfying working on mechanical things!
It’s really not possible for me to say without having the head in my possession for evaluation. If you’re unsure, take it to a motorcycle dealership and have it evaluated by an experienced mechanic.
Hello , what would be the minimum compression it should have ? I just bought a 2006 Honda cr125r and it won't turn on . I did a compression test an it shows it has 120psi.
Hey Eddie. Off the top of my head, I don’t recall. Do you have the service manual? If not, I can email one to you in PDF. All that good info is in the manual. If you don’t have it, you need it!
Perfect demonstration. I have never rebuilt any engine and usually when i need help with something for my bike i gotta watch a million videos and look in the manual but you sir got it all in one and it only took me 2-1/2 hours to rebuild my top end purely based off this video.
Great video! One question. Do 1994 Honda CR125R 2 stroke have timing? I removed my kick starter because the spring broke and the piston moved. My concern is that make be it needs to be timed. I might be wrong, but wanted to ask you. I appreciate it 👍
Hi Gustavo. Replacing the broken spring on your kickstart lever will not be adversely affected by the movement of your piston. Good luck on your repair. Thanks for watching!
Did you replace any of the shiny looking nuts on the head and the place where the cylinder meets the crankcase, like the four studs? I’m trying to find replacement flange nuts and they all have serrations on the bottom from what I can find. I’m kind of against using a nut and washer to replace them considering how the original parts were flange nuts. Any help is appreciated!
Hey Jrob. I did replace the studs and the nuts. I got all the OEM parts from Rocky Mountain ATV. I don’t recall if the OEM nuts were serrated on the underside of the flange, however. Thanks for watching!!
About to attempt my first Top end Rebuild on my 03 Cr125r ofc I bought it looked nice ran pretty good but ofc I took it apart the piston looks fried, cylinder wall looks fine, carb needs rebuild, I’m doing that and the Jetting…. She outa be a rippa when I’m done and honestly I can say I’m proud that I’ve gotten as far as I have, thanks man I’m 17 and need to know this!!!!
That warms my heart to read that young guys like you still want to learn to use your hands and actually create something. The MAIN reason I started this channel was to pass on what I’ve learned in the hope that the younger generation might learn and benefit. Keep wrenchin’!
@@brian.d.mobleyhey man I got my bike going big thanks to you I’m so happy it started up and ran better than before, almost like the feeling of working on it more than riding it🤣💯💯
Do you have a parts list by any chance? Any specialty tools you think would be helpful? I’m going to attempt this for the first time and was wondering. Thanks!
Hey Bill! The parts needed are really dependent on the year model and make of your bike. If you dont have the factory service manual for your bike, that should be step one. If you haven’t purchased the parts yet, contact a company like Rocky Mountain ATV or Motosport and just ask someone in their sales department whats needed for you specific bike. As for special tools, the only one is a torque wrench. If you get stuck, message again and I’ll help you through it.
If the piston is seized in the barrel (maybe broken ring) what the best way to minimise the damage when taking it off? I'm thinking ill have to take the head off and use the kick start to get the piston as low in the barrel as possible then tap on the piston whilst lifting the barrel?
Hey Dr. Paca. I don’t know if a good way to do the procedure about which you are inquiring with the motor still in the chassis. The best way is with the motor out using some sort of press. I’ve used the crank remover press for this purpose. I show the procedure in a more recent video where I disassembled a CRF70 motor. It had a piston in it that had become so corroded with oxidation that it was seized. Hitting the top of the piston with a mallet is not good practice as you can damage the lower end rod bearing and even throw the crank out of balance using blunt force. I wish I had better news.
@@brian.d.mobley Thanks for the reply Brian. The piston will move with the kick start but scraping a bit, guess some damage to the barrel is unavoidable. I'll avoid hitting the piston as much as possible. Thanks again
@@drpacman9996 If the piston will move, you have hope. You can probably get the cylinder off. The best way to avoid damage to the cylinder that’s practical for a home mechanic is to try sliding very thin feeler gauges (maybe 3) between the ring and the bore to compress the ring slightly. This could be challenging and time consuming, but with patience, has worked for me. But, I always had the motor out of the chassis.
@@brian.d.mobley Thanks. I'm not gonna rush anything as the season here is kinda over. I'll try the feeler gauge trick and hopefully be able to hone the barrel rather than re-plate
Hey Growgamer! I like vapor blasting. But, you can soak them in something like Marvel Mystery Oil or brake cleaner for a day or two and then gently run a tap into the hole.
Hi Food Critic. Excellent question! Like a lot of things, there are schools of thought. There are those who support the idea of a break in period such as low to medium revs for 10-15 minutes. Let the motor cool. Ride again for 15 minutes with a couple of full throttle runs in the last few minutes. Let the motor cool. Run it hard for 15 minutes. Let the motor cool. Then retorque the cylinder and head nuts and your good to go. Other suggest that the nikasil cylinder wall is so hard that it doesn’t “seat” with the ring at all so there is no need to observe “break in” procedures. Its hard to argue with the latter, but I just think that taking it slow and doing a few break in cycles can’t hurt and, Honda recommends some type of slow break in period. So, it’s debatable, but I just do it the conservative way. Great question! Thanks for watching!
@@brian.d.mobley awesome. Great video and great information. One more question, do you have a video or could you create one accessing, cleaning, adjusting, etc. the carburetor on a Cr125?
@@foodcritic1399 Hi. I could. It would be a fun one to make also, but I am committed to so many other projects right now that it would be hard to fit that into my schedule. But, I’ll keep it in mind. I’m sure a lot of people would find it useful and it would be fun to make.
@@chrisrieger9201 It’s all a function of how deep you want to dive in order to do the job right. It’s possible that employing less precision can still result in a well functioning motor, but you risk more if you don’t take the time to confirm all the dimensions and clearances.
I have a question about the valves I had a bore done and seem like my valves keep hitting the top of piston .! When it's bored ? Should they shave the valves too BC that seem not the same size or do I replace the slider valves to the bore size ?
Hi John: The procedure covered in this video is for 2 stroke motors that do not have valves. Maybe look around for a video on 4 strokes?? Thank you for watching!
Hey Miguel! That is awesome. I started out working on mechanical things at around age 8. I started by pulling bicycles out of a bayou nearby that had probably been stolen and trashed! I took them all apart, cleaned them, painted them and rebuilt them. I sold a lot of them too. I love to work with my hands and I love to learn new things. Stay curious and stay with it!
Hey Marcus! The procedure is basically the same, but the CR80 doesnt have a power valve. I also have a couple of videos on how to Teardown and Rebuild a CR85 motor. You can alway look there too. But, basically, the procedures are the same on the CR80 and the CR125.
How much play should be in the rod and piston? Or should there be any at all? I just took apart my 2001 cr125 and can’t find anything wrong besides that, but it makes a horrible top end noise when runnng
HI Ashton. There should not be a lot of lateral runout on a wrist pin. The service manual will specify the range. I doubt this is the cause of the noise though. Without having the motor in front of me, its really hard to diagnose problems with motors, however.
@@brian.d.mobley ya I looked a t it some more and the crank bearing is shot I won’t know what all else needs replaced till I get a case puller in. Thank you tho video help out a ton
@@ashtonross5376 Its fun to split the cases and dig in there! If you havent already purchased a splitter, this is the one I use and its been great: amzn.to/3jNTDzs
I have a 2004 Honda Cr 125 and I was wondering if it will be the same to your 2007 if I rebuild my top end? Also, my 125 has not been running good and to be honest, I’m not that smart mechanically. It keeps fouling plugs with the plugs being black with carbon buildup and once I replace it, it runs for maybe 20 minutes at most and then is fouled again. I run 50:1 from that’s what the guy that sold it to me told me he runs. I live in Alaska and I’m not necessarily going to race but just learn how to ride a 2 stroke for the first time. He also said that he recently rebuilt the top end but I can push the kickstart lever all the way down with my hand, and there is some resistance but not a ton. So after telling you all that, what do you is wrong with it but just reading all that, and what is your advice to how to start to fix it? Please reply if you can, I could really use the help. This video is awesome!! 🤘✌️
Hey Jake! The 04 is very similar with the exception of the RC Valve being servo driven. (I did another vid on the RC valve if you need it). Plug fouling is usually due to using the wrong plug and/or wrong jets. Check your owners manual for those. That kickstart sounds normal. I can cycle all mine by hand also. One think I DO NOT recommend is running a ratio of 50:1. The factory recommends 32:1 and I’ve been running that for decades. Ratios are hotly debated, but you won’t lose performance by using 32:1 and you will do a lot to ensure the longevity of your motor.
