Pistons and pistons rings - assmbly

Engine removal and overhaul procedures / Pistons and pistons rings - assmbly

1 At this point it is assumed that the pistons have been correctly assembled to their respective connecting rods and that the piston ring side clearances have been checked. If not, refer to the end of Section 5.

2 Before the rings can be fitted to the pistons, the end gaps must be checked with the rings fitted into the cylinder bores.

3 Lay out the piston assemblies and new ring sets on a clean work surface so the components are kept together in their groups during and after end gap checking. Place the crank-case on the work surface on its side, giving access to the top and bottom of the bores.

4 Take the No 1 piston top ring and insert it into the top of the bore. Using the No1 piston as a ram, push the ring close to the bottom of the bore, at the lowest point of the piston travel.

Ensure that it is perfectly square in the bore by pushing firmly against the piston crown.

5 Use a set of feeler blades to measure the gap between the ends of the piston ring; the correct blade will just pass through the gap with a minimal amount of resistance (see illustration). Compare this measurement with that listed in Specifications. Check that you have the correct ring before deciding that a gap is incorrect. Repeat the operation for all twelve rings.

11.5 Measuring the piston ring end gap using a feeler blade

6 If new rings are being fitted, it is unlikely that the end gaps will be too small. If a measurement is found to be undersize, it must be corrected or there is the risk that the ends of the ring may contact each other during operation, possibly resulting in engine damage. This is achieved by gradually filing down the ends of the ring, using a file clamped in a vice. Fit the ring over the file such that both its ends contact opposite faces of the file. Move the ring along the file, removing small amounts of material at a time.

Take great care as the rings are brittle and form sharp edges if they fracture. Remember to keep the rings and piston assemblies in the correct order.

7 When all the piston ring end gaps have been verified, they can be fitted to the pistons.

Work from the lowest ring groove (oil control ring) upwards. Note that the oil control ring comprises two side rails separated by a expander ring. Note also that the two compression rings are different in cross section and so must be fitted in the correct groove and the right way up, using a piston ring fitting tool. Both of the compression rings have marks stamped on one side to indicate the top facing surface. Ensure that these marks face up when the rings are fitted.

8 Distribute the end gaps around the piston, spaced at 120° intervals to the each other.

Note: If the piston ring manufacturer supplies specific fitting instructions with the rings, follow these exclusively.