1990 YZ80 Top End Assembly

Assembly of the Piston and Cylinder of the YZ80

Laying out the top end assembly components.
4 brass head stud washers, 4 head stud nuts, connecting rod needle bearings, gasket set (top head gasket, cylinder base gasket, reed valve gasket and a new exhaust O-ring),  New wrist pin (or piston pin), 2 c-clips for wrist pin, piston, piston ring, Warren 2 cycle oil for assembly lubrication, cylinder head, reed valves, carb base, vent hose.



The cylinder has been bored and honed to size and decked (decking makes sure that cylinder head mounting surface is flat for good sealing). I failed to show the bottom of the cylinder so you can see the chamfering which allows easier installation of the piston and ring into the cylinder. Even though the cylinder looks good and clean you need to wash it thoroughly with dishwashing soap to make sure all the honing grit and minute metal shavings are gone. You can be sure it's clean when a clean dry rag wiped in the cylinder comes out clean.

Next, install the piston ring into the top of the cylinder before installing it on the piston. You need to do this to check your ring end gap.

To square the ring in the cylinder use the top of the piston to carefully push it (ring) down a quarter of an inch to half and inch in the bore.

Remove the piston and use feeler gauges to check the ring gap. Minimum ring gap is .004" thousandths of an inch per inch of bore. (A one inch bore would be .004" a 2 inch bore would be .008" and a 3 inch bore would be .012". In other words multiply the size of bore X .004" to get your ring end gap.
In our case the bore is right around 1.9288 so we round it up to 2 inches X .004" and we get .008" ring gap.

If you need to open up the ring gap, place a small metal file in a vise to hold it secure. File in one direction only (don't double file which is staying in contact and rubbing the ring up and down the file without losing contact with file). Take your time and check often.

Using a ring expander tool install the ring on the piston in the groove making sure ring gap is at the small pin in the ring groove. You can install a ring without using the tool but you can break the ring if not careful, twisting the ring during installation will more likely break it. Use the tool. The Wiseco racing piston for our YZ80 only has one ring. Some pistons have two or three rings.

Take some  2 cycle oil and coat the cylinder walls using your finger. Barely visible in the photo can be seen the cross hatch pattern left from honing cylinder walls. Set the cylinder aside.

You don't have to but generally I like to install one of the piston pin clips in the piston before final installation. These clips can be a bugger to wrestle with so having one already done aids in the installation of final assembly of piston onto the rod.

Coat with 2 cycle oil and install the needle bearing race into the rod (not shown) carefully place the piston over the rod (making sure piston is in correct orientation). Coat piston pin with 2 cycle oil and insert through piston and rod until it stops against the clip on other side. This is where the one clip already installed in piston helps out. When pin is seated all the way in you should see the other groove for the second clip. Now install the other c-clip into the groove by putting one open end inside the groove and hold it tight against the piston while using a small flat blade screw driver to gently pry the remaining part of clip into the groove. Make sure it is seated completely in the groove using the screw driver to push it snug up to the piston pin, you should hear it snap in.

Below you can see the base gasket sticking out from under the blue shop towel. You can put it on after the piston is on the rod but I like to do it before so I don't forget as I did by not showing it earlier to you. The shop towel is to catch any parts down in engine during assembly of the piston to the rod. Those little clips like to spring off sometimes when you try to get them in the groove. you don't want that bugger going down in the engine.

Carefully slide the cylinder over the studs and down on the piston. Pinch the ring into the groove and slide cylinder down. The chamfer on underside of bore aids in guiding the ring in the cylinder. Don't force it, take your time. As soon as the ring enters the cylinder the piston will glide right in.
Next install the head gasket noting in the repair manual the correct orientation to insure coolant can flow properly. Also note the small arrow engraved in the piston top at left side of piston in picture. This is so you know the piston has to be installed with arrow pointing towards front of engine. If not then your ports in cylinder and ports in piston itself will not line up, you will quickly burn the engine up if not installed in correct orientation. More over the engine probably won't even start.

The cylinder head has two (On this YZ) hollow locating pins, they do not allow coolant to flow. The gasket holes where the locating pins go are not large enough on this set at least. The paper was cut away from hole circumference where the pins go to allow head to seat using a slightly larger sharp drill bit that is the same size as the outside diameter of those locating pins.

Install the head on ( you may need to tap it with a rubber or plastic tipped hammer. Do not use a metal hammer please). Install the four copper washers on the studs and then the nuts of course. Use a torque wrench to tighten nuts in two stages. Tighten all in a crisscross pattern to 9 ft lbs, then tighten all four again to 18 ft lbs in the second stage. If the head is not going down evenly stop and check to see what might be causing it to bind.  Check the operation of the piston for binding using the kick starter by hand. It should move with ease without a spark plug installed. It should move up and down in cylinder nice and smooth. if it does then your good to go.