Manual - Chapter 1 Engine

10. Dismantling the engine unit: removing the pistons and connecting rods

  1. If the engine is in the frame, the cylinder head must be removed first, with all preliminary dismantling work that this entails. See Section 9.
  2. Rotate the crankshaft by means of an Allen key applied to the ignition rotor flange retaining bolt. On 75 models one piston/connecting rod assembly will have to be dealt with at a time, with the crankshaft being rotated first to bottom dead centre (BDC) and then to top dead centre (TDC) as described for each assembly. On 100 models two piston/connecting rod assemblies can be removed or refitted at the same time, either the inner pair (2 and 3) or the outer pair (1 and 4).
  3. Note: Before disturbing any component, make careful written notes of exactly how each component can be identified, also how its original fitted position can be indicated. Slowly rotate the crankshaft looking for paint spots, marks made by the manufacturer and any other identifying features; note all these and if necessary make your own. At the very least obtain three or four (as appropriate) containers in which the components of each piston/connecting rod assembly can be stored separately and clearly marked.
  4. The component parts of the machine featured in the accompanying photographs were identified as follows. The pistons had larger valve cutaways on the intake side and an arrow stamped in the crown of each pointing towards the front (cam chain) of the engine. The big-end bearing caps all had red paint spots and the bearing shell lubricating channels/locating tangs were aligned against each other on the 'upper' intake side of the engine. The connecting rod small-end bearing oil way was also on the intake side and each connecting rod had a single spot of blue paint (indicating its weight group) on its rear face, ie towards the gearbox. Lastly, each rod and cap had a two-digit number etched into the flat-machined surface of its 'upper' intake side as follows: cylinder 1 marked 40, cylinder 2 marked 42, cylinder 3 marked 44, cylinder 4 marked 46. To be safe, a hammer and a small punch was used to mark the intake side of each rod and cap, making one mark on each for cylinder 1, two for 2 and so on. Check for similar marks on the machine being overhauled.
  5. On 75 models position the crankshaft so that any piston is at the bottom of its stroke (BDC); on 100 models position the two middle pistons at BDC.
  6. Working evenly, by one turn at a time, unscrew the two nuts securing each connecting rod big-end bearing cap and withdraw the cap; these will be very tight and will require a few taps from a soft-faced mallet to release them. When the caps have been removed, it is good practice to prevent the risk of the bolt threads marking any bearing surface by slipping a length of rubber or plastic tubing over each.
  7. Rotate the crankshaft until the piston is at the top of its stroke and use a wooden dowel or similar to push the piston out of the cylinder towards the cylinder head. Store all the components of each piston/rod assembly in a separate container and mark it with the cylinder number to avoid any risk of swapping components and promoting excessive wear by mis-matching part-worn components.
  8. Repeat the procedure for the remaining assemblies.
  9. Before separating a piston from its connecting rod, ensure that marks are made or identified which will ensure that they are correctly refitted. For example in the case given in paragraph 4 above, the arrow indicating the direction of piston installation was pointing 'away' from the blue paint spot on the connecting rod's rear face. Us a a pointed implement to prise out the gudgeon pin retaining circlip, then press out the gudgeon pin and withdraw the piston. If the pin is a very tight fit, immerse the piston in boiling water (taking care to prevent any risk of personal injury when heating components or when handling them), thus causing the aluminium alloy piston to expand faster than the steel pin. Discard the circlip; these should never be re-used.
  10. Remove the piston rings carefully, by expanding them sufficiently with the thumbs to pass over the piston. If necessary use three thin strips of metal to ease them from their grooves (see illustration). The rings are very brittle, and must not be handled roughly. Note which groove each ring came out of, and which way up on each piston. The two-piece oil scraper ring must be removed in two parts; first the outer section which is removed in the same way as the compression rings, and then the coil spring-type inner section.

10.4 Mark connecting rods as described before removal, to ensure correct fitting.

10.6 Unscrew nuts and remove big-end bearing caps, noting how shell locating tangs are positioned.

10.9 Removing gudgeon pin circlips from piston - always renew disturbed circlips.

10.10 Oil scraper ring is two-piece design - remove in stages, as shown.

Fig. 1.4 Method of removing gummed piston rings.