Crankshaft and bearings - examination and renovation

1 Examine the bearing surfaces of the crankshaft for scratches or scoring and, using a micrometer, check each journal and crankpin for ovality. Where this is found to be in excess of 0.0254 mm (0.001 in) the crankshaft will have to be reground and undersize bearings fitted.

2 Crankshaft regrinding should be carried out by a suitable engineering works, who will normally supply the matching undersize main and big-end shell bearings.

3 Note that undersize bearings may already have been fitted, either in production or by previous repairer. Check the markings on the backs of the old bearing shells, and if in doubt take them along when buying new ones.

Production undersizes are also indicated by paint marks as follows:
White line on main bearing cap - parent bore 0.40 mm oversize
Green line on crankshaft front counterweight - main bearing journals 0.25 mm undersize
Green spot on counterweight - big-end bearing journals 0.25 mm undersize 4 If the crankshaft endfloat is more than the maximum specified amount, new thrustwashers should be fitted to the centre main bearings. These are usually supplied together with the main and big-end bearings on a reground crankshaft.

5 An accurate method of determining bearing wear is by the use of Plastigage. The crankshaft is located in the main bearings (and big-end bearings if necessary) and the Plastigage filament located across the journal which must be dry. The cap is then fitted and the bolts/nuts tightened to the specified torque. On removal of the cap the width of the filaments is checked against a scale which shows the bearing running clearance. This clearance is then compared with that given in the Specifications (see illustration).

35.5 Checking the width of the filament against the scale of the packet
35.5 Checking the width of the filament against the scale of the packet

6 If the spigot bearing in the rear of the crankshaft requires renewal, extract it with a suitable puller. Alternatively fill it with heavy grease and use a close fitting metal dowel driven into the centre of the bearing. Drive the new bearing into the crankshaft with a soft metal drift.

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