Piston, cylinder and head fitting

I checked the gap on my piston rings prior to fitting and found they were a little too tight. Some light filing increased the gap to over 0.2mm, the minimum recommended. The brown rings seen on the walls of the cylinder in this photo are build-ups of oil where the rings had been kept in storage:




The easiest way to get it all together is to place the piston with rings into the cylinder while on your workbench. Make sure the arrow on the piston head points towards the exhaust port, and make sure you've lightly oiled your cylinder and beneath your rings with 2-stroke oil. Tease the cylinder far in enough to retain the rings.

Fit your bottom gasket and long cylinder studs. Grease the gasket and cylinder studs to help fitting and future removal, then offer the cylinder/piston assembly up to the studs and slide down until the piston reaches the conrod. Now you can fit the circlips, gudgeon pin, and bearing (lightly oiled) so that the piston is attached. Then slide the cylinder all the way home.




I used a high-temp gasket sealant for the head gasket. Make sure you only use a little as it'll ooze out the side when you bolt it all down!

Here's a top tip, before you fit the head screw in a spark plug all the way and wipe away any burr or deposits - better this ends up on a rag than in your cylinder.

Tighten the head bolts in a diagonal pattern to about 2kg-m.

Cylinder head, cylinder, piston

Head and cylinder removal is very straightforward. Four nuts on the head hold it and the cylinder against the block with long rods threaded at each end, passing through the cylinder casing. When you've removed the head and cylinder, the rods can be removed by threading two nuts on the upper thread, and then turning using the innermost nut.

To release the piston from the conrod, simply remove the two circlips holding in the gudgeon pin, then push the gudgeon pin out from one side using a suitable bar or socket. Below you can see the gudgeon pin, circlips, bearing, piston and the conrod hanging loosely from the crank:



It looks like my piston has had a hard life, with lots of deposits on the top, and what look like scores or pitting on the wall: