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Changing a 4-Stroke Piston


KJ790

So today I decided to throw a new piston (high compression of course) in my 250F. Many people are afraid of getting into an engine at all, but it really isn't very hard as long as you take your time and make sure everything is clean.

First thing you need to do is wash your bike, and wash it well. No matter how well you wash it, there will be a lot of dirt still on it when you start taking stuff off. The goal is to get every area around the engine spotless. You don't want any dirt falling into your engine while it's open! Once you remove the pipe, gas tank, shrouds, and head stay you will most likely find a lot of dirt on top of the valve cover and on the frame.

2007_06280004.jpg

In this picture you can see what I'm talking about. The valve cover is covered with dirt still. I like to use a shop vac and a small screw driver to pick at the dirt and suck it away (so it doesn't just fall somewhere else on the bike. You once the chunks are cleaned up you can use a damp cloth to get it spotless. Don't remove anything else until the frame and valve cover are spotless.

Next you can drain the water out of the radiators and remove the radiator hose that goes to the head. Next you can go ahead and remove the valve cover. At this point you can get out a feeler guage and check the valve clearances if you want.

Next I loosened the clamp between the carb and the head, then removed the top bolt on the subframe while loosening the other two bolts that hold the subframe to the frame. This allows you to tilt the subframe back, pulling the carb out of the way. Remember, my goal is to remove the least amount I have to. If you want to completely remove the subframe and the carb, go right ahead.

On many bikes, the hose connecting the two radiators is very close to the head. To deal with this I removed the bottom bolts on the radiators and loosened the top ones, then just rotated the radiators forwards:

2007_06280013.jpg

Next, tie a piece of wire around the cam chain to make sure that it doesn't fall down into the bottom end when you remove the cams. Unbolt any oil lines going to the head. Remove the cam chain tensioner. Before you remove the cams, cover the area where the cam chain runs through with a rag so that the cam clips cannot fall down in it should they fly out as they often do. Now you can remove the cams and unbolt the head.

2007_06280006.jpg

Next you can unbolt the cylinder and pull it up allowing the piston to slide down out of it. Cover the crank area around the rod with a clean rag or paper towel to keep anything from falling in there (such as dirt or wrist pin clips).

2007_06280008.jpg

Next, remove one c-clip and slide the wrist pin out to remove the piston. Clean all gasket surfaces, hone the cylinder, measure everything and make sure that all of the measurements are within spec (given in the owner's manual).

Once you make sure everything is in spec, you can put one c-clip in the new piston. Make sure it fits in the grove completely. Put the new rings on the new piston, making sure the gaps are aligned as your manual shows (normally opposite sides of the piston from each other). Put the piston on the rod and slide the new wrist pin through, make sure to coat the wrist pin in oil before putting it in. Make sure the piston is in the correct way (most have a mark showing the front side). Then install the second c-clip. Make sure that it fits all the way into the groove. this is very important, if the c-clip falls out while the engine is running it will cost you a lot of money. Make sure both c-clips have their openings facing down. Install a new base gasket.

2007_06280009.jpg

Now you can slide the cylinder over the piston, using one hand to squeeze the rings together. Make sure the rings stay in their correct grooves. Use a torque wrench to install all bolts according to the owner's manual specifications. Now you can put on a new head gasket and put the head back on. Make sure the engine is at top dead center (marked on the flywheel) and put the cams in. Make sure that the marks on the cams are in the correct places for the engine at top dead center (your manual will show you what to look for). On my bike each cam gear has a dot that should be horizontal and flush with the top of the head. Install the cam chain tightener and double check the timing. Many people neglect to do this and have problems. When the tensioner is installed, it often rotates the intake cam backwards, revealing that the intake timing is off one tooth. If this is te case, take the tensioner back out, remove the cam cap and turn the cam to the correct timing.

2007_06280012.jpg

Now you can put the valve cover back on and reinstall any oil lines. Be careful with banjo joints. In my case the banjo bolt couldn't hold the torque recommended by the manufacturer

2007_06280011.jpg

These do not need to be very tight, just enough that they won't leak. Obviously mine was too weak to hold the torque that Yamaha recommended, so the new one was put in using less torque to be sure that I wouldn't have that problem again.

Reinstall the headstay, radiator hose, and radiators. You can now pull the subframe back and reconnect the carb to the head. Fill the radiators and put on the exhaust and plastic and you are now ready to break in your fresh top end.


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