Manual - Chapter 1 Engine

30. Reassembling the engine unit: refitting the pistons and connecting rods

  1. Fit the oil scraper ring inner section to its groove in each of the pistons followed by the outer section; their end gaps should be 180. apart. The second (lower) compression ring has a tapered inner edge on its top surface; this surface may also have the word TOP etched in it. The top compression ring is a plain rectangular cross-section which has the word TOP etched into its top surface and was found to be chrome-plated. Fit the two compression rings as indicated by their markings and identified by their cross-sections, space their end gaps at 120. from the scraper ring outer section end gap and from each other.
  2. Fit each piston to its connecting rod and refit the gudgeon pin, warming the piston if necessary to permit this. Fit new gudgeon pin circlip(s) ensuring that they are settled fully in their grooves with the open ends well away from the slot provided for removal. Use the marks or notes made on dismantling (see Section 10) to ensure that the piston is positioned correctly on the rod; if a new rod is being fitted note that it's 'upper' or intake surface is indicated by the slots for the bearing shell locating tangs or oilways. From this can be found the connecting rod's front (cam chain) face which is to be aligned with the arrow in the piston crown.
  3. Fit the bearing shells to the connecting rod and cap ensuring that each shell is pressed firmly into place with its locating tang fitted into the slot in the rod or cap. Try to avoid touching the bearing surface, and lubricate the shells and crankpins liberally with clean oil. then liberally oil the cylinder bore, piston and rings.
  4. Position the crankshaft at BDC for the assembly being refitted and slip the lengths of protective tubing over the threads of the connecting rod bolts. Fit a piston ring compressor (use any suitably-sized car-type compressor available from most auto accessory shops) to the piston, checking first that the rings are correctly arranged as described above. Checking that the piston/rod assembly is the correct one for that cylinder and that it is refitted the correct way round - arrow stamped in the piston crown facing towards the front (cam chain) end of the engine, connecting rod bearing shell locating tangs/oilways upwards (intake side) - insert the assembly into the bore from the cylinder head side.
  5. Carefully insert the piston into the bore until the ring compressor is seated, then tap the piston gently but firmly through the compressor until it is fully in the bore. At the same time, guide the connecting rod big-end over the crankpin.
  6. Again lubricate the bearing surfaces, remove the tubing from the bolts, then refit the big-end bearing cap ensuring that its shell is correctly seated and that the locating tang/oilway is uppermost. Tighten the cap retaining nuts evenly until the cap is seated, then tighten them to the specified amount.
  7. The big-end bearing cap retaining nuts are tightened in two stages. The first, to preload the bearing shells, is carried out using an ordinary torque wrench set to the specified pressure. The second stage is carried out using a method not yet in common use in the motorcycle world, ie to tighten the nuts by rotating them further through a specified angle. To measure this either use a degree disc (see accompanying photograph). or cut a circular piece of cardboard, pierce a hole in its centre and mark a heavy zero line from the centre to the edge along the true radius, then use a protractor to mark exactly the required angle anticlockwise from the zero line. Fit the disc to the tightening tool as shown and align the zero line with any convenient fixed point on the engine unit, then tighten the nut until the 80. mark aligns with the fixed point. Hold the engine unit steady (an assistant would be useful) and be very careful not to move the card on the tool during tightening or the angle measurement will be lost.

    Note: This method is very precise and effective in tightening fasteners but must be used with great care and accuracy if the nuts are to be correctly tightened; the pressures involved are quite high and the permissible tolerances minimal.

  8. Repeat this procedure to refit the remaining assemblies. Check for free crankshaft rotation at each stage; some stiffness will be inevitable, especially if the bearing shells have been renewed, but the crankshaft should be able to rotate with reasonable ease.
  9. Refit the cylinder head as described in the following Section.


30.1 Compression ring top surfaces can be identified by markings and cross-sections, see text

30.2a Ensure correct piston is refitted to each connecting rod - piston may require heating to permit refitting

30.2b Ensure retaining circlips are correctly positioned, as shown

30.3 Bearing shell locating tangs must engage with cap or rod grooves - oil liberally

30.4a Arrow stamped in piston crown points to front (cam chain) end of engine - note larger (intake) valve cutaway upperrmost

30.4b Marks on connecting rod and cap should ensure correct refitting of all components

30.5 Use car-type ring compressor when refitting piston/rod assemblies

30.6 Oil liberally bearing surfaces before refitting bearing caps

30.7a Tightening big-end bearing cap retaining nuts - first stage is by torque wrench ...

30.7b ... second stage is by degrees of rotation - see text