Cylinder and Piston

The cylinder, piston and piston rings turned out to be the easiest part of the restoration and did not need any work. All the tolerances were within those specified in Table 2 above. The piston pin could easily be removed with a little bit of help from my Makita heat gun blowing hot air over the piston. The manual says to remove the piston pin circlip on the flywheel side and remove the pin towards the flywheel. By retaining the timing side circlip in its groove, it ensures that when the piston is refitted, the piston pin goes in from the flywheel side - the piston's front-back alignment does not change and the piston mates correctly with the corresponding surfaces on the cylinder. You do not need to run the piston in again.

The photo above shows the piston before I took it out and it also shows a rear stud that when I later tightened the head, shared off! Working from the bottom of the piston, the bottom compression ring went in first, followed by the oil scraper after I made sure the oil bypass holes under the ring were clean. Next to go on were the top two compression rings near the piston crown. Pushing the rings into the bore and centering them with the bottom of the piston skirt showed that all end-gaps were at the recommended 0.30mm. I staggered the three compression rings 120 deg around the periphery of the bore though in use rings move around. The manual says to mark the rings and the positions so that when reassembled the cylinder won't need to be run in; however, at some point in the restoration, rings got mixed up. Oh well...

I reused the piston pin circlips (gasp!) making sure to stuff the crankcase mouth with a rag. Once at the side of a now vanished track, half an hour before final practice, while changing a seized piston I dropped a circlip into the case and no amount of probing with a magnet could dislodge it. A weekend to forget!

I cut out a 0.25 to 0.30mm thick paper gasket by marking out the cylinder mouth and studs. Using my gasket punches I cut out holes for the cylinder studs and smeared it with some red grease to hold it in place. I support the base of the piston on two wood flats to keep it perpendicular and then slowly rest the heavy cylinder on the first ring, a little nudge here and there with a thin screwdriver, one ring done, some more nudges on the second compression ring, now the oil scraper, finally the bottom compression ring, and here we go, remove the wood flats and the cylinder slides smoothly into the crankcase mouth. A jaunty twist of the wrist on the flywheel and the cylinder rises up with the piston!! One brawny arm resting on the top of the cylinder and I hear the satisfying sound of the piston rising and falling smoothly in the bore.