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Benchtop Basics 101>Episode 6>

Benchtop Basics No. 6

Engine top end installation


Installation tips

Assembly order is the reverse of disassembly.  Inspect the top end rod bearing before re-assembly, and replace it if necessary.  Don’t forget to install a base gasket (see below for how to set squish band clearances), and the top O-ring set.  Use lightweight machine oil for assembly lubrication.
Make note of the orientation arrow on the piston.  It should face the exhaust port.  Also make note of the location dowel pin in the ring groove.  This pinmustbe oriented with the ring gap, or ring breakage and/or piston and/or cylinder damage may occur upon re-assembly.  Compress the ring with your fingers, and slide the cylinder down onto the piston and ring, using the chamfered guide skirts at the bottom of the cylinder to help ease the assembly on.  If you should ever need to replace the piston, make sure to replace it with the correct letter designation to match the cylinder (A, B, C, D).  You will find the letter stamped on the top of the piston, opposite the arrow, and on the bottom of the cylinder jug (stamped on the gasket surface); a "e;B"e; piston fits a "e;B"e; cylinder, and a "e;C"e; piston fits a "e;C"e; cylinder, etc.

Setting squish band clearance

The cylinder gasket top end set includes 2 O-ring seals for the head, and 3 different thickness base gaskets.
Assemble the cylinder first with the medium thickness gasket, and place a piece of light electrical solder (Radio Shack .032” PC-board solder will do just fine) cut to just slightly smaller than the bore size, across the piston top along the piston pin axis as shown below:

Assemble the head, then gradually and evenly torque the head nuts to 0.8 Kgm (5.78 lb/ft or 69.36 lb/in).
Using the pull starter, cycle the motor through TDC (Top Dead Center) once or twice with the spark plug absent, to compress the solder.  Remove the solder and measure the solder’s thickness at the ends, where it will have been compressed by the cylinder head’s squish band (if you don’t have a dial caliper or micrometer, just take the solder to someone who does).
Optimal performance is achieved with a clearance of .5 mm (approximately 0.020 inch) at the squish band.
Replace the base gasket with one of a different thickness, if necessary, to achieve this clearance before final assembly.
The 50cc kit comes with a rubber-isolated mounting bracket, which should be installed under the top two head nuts before final assembly, offset facing downward.  Use the standard frame spacers that came on your bike to remount the engine.
If you have replaced an older, short spark tower head 40cc top end with a 50cc big bore kit, also don’t forget to use the longer thread plug (Champion N2C or C-55 racing plug), or a dramatic loss of power will result.

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This information is provided in good faith as a guide. We are not responsible for any typographical or informational errors or omissions.  Due to the many mechanical and technical variables, this information is to be used only as a guide, and is in no way guaranteed to be accurate or applicable to your engine, your bike, or your world in any way.

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