Crank is not turning piston on my 1977 yamaha champ 80

could have problems with crank shaft bearings , should have had the crank ground and main bearings line bored , did you adjust the crank shaft thrust bearing for end play, after the crank shaft main bearings are torqued down you should be able to rotate it by hand .

I think someone is seized to the shaft bearing of the crankshaft.Download the heads and roll the crankshaft clockwise and back to see if pistons move when you turn the shaft or receive clearance and then leave

Sounds like you have a bearing failure, eithe big end or crank shaft bearing. When they go they quite often knacker the piston and cylinder too as it can put them off square. Remove cylinder and the problem will be easy to locate as you can see the big/small end bearings and check for cylinder damage. Often the cost of repair far outweighs the value of the equipment.

Let me give you some info. I guess you are talking about the bearing next to the fly wheel. The crankshaft has to come completely out. the crank can not be removed until you have removed the rods from the crank shaft pin. Most of the time if a bearing goes bad you need to replace the rods and crank shaft pin. I 've never done it but if you can get the rods out of the way the crank will come out. My recommendation would be, remove the fly wheel, remove the inner cooler, remove any small tubing lines, Remove the front frame cover, Remove the 2 bolts holding the crank pin bushing (don't forget to lock wire these bolts back together during assembly), Remove the cylinder, head, piston and rod together as a unit. Don't let the piston come out of the cylinder or move around to much unless replacing the rods. Once you get both LP and HP cylinders, heads etc off you can remove the front bearing cap exposing the bearing and a snap ring. Take a piece of wood or brass and from the crank shaft counter weight side (rod side)knock the crank shaft toward the fly wheel side until the snap ring can be removed. Once the snap ring is removed knock the crankshaft back toward the rod side and continue until out of the bearing races. Then remove the crank. The outer bearing can then be pressed off with the retainer at one time. Remove the sleeve. To remove the large bearing(back bearing) you need to be very careful and cut it off. Don't mess up the crank shaft. I've tried several ways to remove the back bearing but the easiest is cutting it off. To answer your question yes you have to disassemble to change bearings.

can you turn the crank shaft over by hand? if not the big end has sized up the piston is normaly the last thing to get stuck up,,,

BALANCE SHAFT LOOSE?? CAM CHAIN TENSIONER FAULTY?

The top end comes off then the left and right side covers, flywheel, clutch and shift shaft. Now a case splitter is used to split the center case halves. To replace the bearing you will need a hydraulic press to disassemble and reassemble the crank halves. If the bearing is seized you should replace the bearing, crank pin and the connecting rod. The crank must be realigned using twin dial gages. Personally, I suggest a new crank assembly be purchased from www.babbittsonline.com ( $241 ). I would replace the crank end bearings at the same time and also the sprocket shaft outer bearing and seal. Please rate my answer.

A service manual ( if you can find one ) is what you need. The second thing you need is repair parts. They are pretty hard to find. Babbitt's, a well known distributor, only has parts left in stock for the top end. Go to their website, www.babbittsonline.com/parts/viewbybrand/parts.aspx
and you can see what is left. There's also a diagram of the crankcase center halves. If you can't get the repair parts, spending hours doing an engine teardown is fruitless.

The teardown will require a flywheel puller, a case splitter, an impact driver, and a torque wrench (somewhat optional). Disassemble the top end then remove the left and right side case covers. Remove the flywheel and related electrics, the clutch, crankshaft drive gear, oil pump, kick starter, shift shaft assembly, shift drum end cap and index pins. Now remove the screws on the center case and you can then split the case. This gives you access to replace shift forks, gears, and bearings. Note that crank seals can be replaced without splitting the case.

I am not trying to be a smart *** here, but re-assembly is just the reverse of dis-assembly. This is why a manual would help. Another potential source for parts is to Google motorcycle salvage yamaha .
The normal things you would need to replace inside the center case would be bent shift forks (if shifting is the problem), busted gears, crank bearings and seals, and sprocket shaft bearing and seal. All else can be replaced by removing just the side covers.

I assume you are using the original left and right crankcase halves. If not there is the problem. Case halves must be a matched set or this type of problem often shows up. Note the positioning pins in the diagram below. Are they both there? There is an chance the crank may be out of alignment (dropped at the factory, Yamaha parts dept., shipping center, somewhere...). Get it checked at a dealership. Please rate my answer. Thanks!