How do you rebuild Honda 300ex engine?

Answer 1

first off don't go to the dealer for this. it is fairly easy. you will need an online manual or a regular manual though. first you have to take off the top end. to do this remove the tank the plastics basically everything you need to get to the top of the motor. take off the exhaust and the carb. then you have to take off the covers to the valve adjustments. then you can take off all the screws on the head. once that is done use a ruber mallet or soimething similar to just loosen the seal because it is form a gasket basically gluing it on. when you do this then you can pull it up with your hands using the wholes for the valve adjustments. then there are 4 nuts and 2 bolts holding on the head. you will get to them soon. but first, you have to take out the cam. to do this take off the two small covers on the left side of the motor. one has a bolt on it to rotate the crank and the other is a sight hole for timing. which you will need when puting it together. there are two screws holding on the sprocket to the timing chain. remove the one being carefull not to drop it down. then rotate the motor until the other screw is easy to get too. take it off and don drop it. the sprocket will drop down a little. just enough to get the chain off and then you can pull the cam out. then you can finally take out the 4 nuts and 2 bolts in the head. two are on the outside part of the head and two are internal. take them out and then move on to the bolts. they go all the way through the cylinder to the case i believe. its a ten mm Allen head not hex. after those are out you can pull up the head using a rubber mallet as your only assistance. do not use a screw driver! the head barely clears the frame so be carefull not to scratch anything.you have to go down and out the front right with the head. then comes the cylinder. it has two 8mm bolts holding it on. take them out and your good to just pull the cylinder up. now you just have to get the piston out. there are clips on both sides of the piston. take one out and get a good size socket to push the pin out. then the piston can be removed. you can eather buy new rings or buy a new piston which is recommended depending on what was wrong with the bike. now basically take the process backwards in putting it together. put one clip in the piston before you go close to the bike because you don't want that in the bottom end. then put the pin through the piston and the conecting rod. get a rag and stuff it into the bottom end to stop enything from going down into it. out the second clip in and you are done with that.now you can get to honing your cylinder. you can buy one from sears for 25 bucks. good investment they can be used for a long time. once you hone it then you can put the cylinder back on and be sure to clean off any of the old gasket and put a new one in. be sure that the timing chain is on the sprocket on the crank. pull the chain up and make sure it doesnt fall down use a zip tie or somethig to hold it up. now for the valves this is fairly simple if you have experience with it. you will need valve seating compound and a valve lapping tool. you will also need a valve spring compressor. do one at a time. take all of them out but put one in at a time. you will have to do seals which is the hardest part. you can use needle nose plyers to pull them off but you will need a small pick to put them back on. take the metal rings off of them and put the rubber pieces on all of the spots. then put the rings on with the pick. now to lap the valves. put a small amount of valve seating compound in and put the valve lapping tool on the bottom of the valve and spin it back and forth with your hands and go up and down with it and when you are done doing this with every valve then you can wipe out all of the ports. put one valve in and compress the springs and put the two clip pieces in. then do this with all of them. now you can put this back on going up through the same spot it came out of. put the head gasket on and the timing chain guides in place and pull the chain through( helps to have two people for this part. pull it up and keep it up just like before. put in all the bolts including the ones on the cylinder on the bottom. take the timing chain tensioner out for easier adjustments. now put the cam in the same way you took it out. put the cam lobes facing down and turn the bottom end. this is were you look into the sight whole on the left side of the motor. turn it until you see the single t. there are a few different things on there including a double tt. wonce you find this you can bolt on the sprocket with the timing chain on it. there are two lines on the sprocket that have to be in line with the head they are on the right side of the motor. once these are inline with the chain on and the bottom end seems to be lined up with the t then put the one screw in on the cam. then rotate the crank and put in the second bolt in. then put the chain tensioner back in and see if its lines up corectly if not start over with taking the bolts out on the cam and moving the chain on the sprocket a few teeth. once this is all done you can put the valve cover on. put the little seal on the right side in and put oem gasket sealer on the valve cover side . pull the rocker arms up and put it on lining it up with the pins. put in all your screws and torque them to about ten poundes or less. it seems like its not enough but if you go more you risk striping the wholes out. now you can do your valve adjustment. make sure the t is lined up and that the rocker arms are loose on both sides. if not then turn the motor one whole rotation. then use a feeler gauge that is 0.40 i believe. loosen the ten mm nuts on the rocker arm. use locking pliers and put it on the top part of the adjuster put the feeler guage between the valve and the rocker. make it so it has just a small tug. now tighten up the ten mm bolt when holding the adjuster in place do the same for all of them. now you can shake it and you will hear a clicking noise that sound means its correct. now you can put the covers back on and put all of your other parts back on like oil line carb and exhaust tank plastics anything you took off. depending on the sealer wait how ever long it says and then you can finally start it up. it should fire if everything is correct. including order of piston rings and direction which should come on the paper that is sent to you with the piston. make sure to turn the choke on for first start up change your oil if anything was broken and your ready to ride. 4 strokes don't really need a break in process but for the first few hours of riding just take it easy giving it a few breaks in between then you can get on it. this is the same for basically every 300ex so everyone can use this. if you want me to send you a manual online email me at Eric.farkas15@gmail.com