Crankcase and cylinders

Crankcase and cylinders

The crankcase is an aluminium casting suitably ribbed to ensure correct air cooling. The main bearing and camshaft bearing bores are machined as also are the tappet seats, gearbox companion flange and timing gear cover mounting flange.

The cast iron cylinders are finned radially to increase the cooling air surface and are located symmetrically on the crankcase, each being held by four studs. The cylinders are installed by sliding into the crankcase bores and finally held in place by the cylinder head (see FIGS 1:8 and 1:21).

Inspect the cylinder bores for score marks, wear and any other defects or damage. The clearance between the piston maximum diameter and the cylinder bore should be checked to ensure that it is within the maximum wear limit of .0059 inch.

It is essential that the diameter measurements are taken at two different heights in the cylinder bore along both the longitudinal and transverse axles as shown in FIG 1:22. It is recommended that to zero the internal micrometer Fiat ring gauge C.672 is used (see FIG 1:23).

If bore wear or ovality is between .0059 and .0079 inch the cylinder bore may be honed. Should however the wear limit of .0079 inch be exceeded then the bores must be recut.

Honing or reboring cylinder bores

This operation should bring the bore size to correspond to the oversize piston sizes in order to obtain the correct clearance between the piston and cylinder. These limits are given in Technical Data. It will be observed that the cylinders are divided into three classes depending on the bore diameter. The classes are identified by the letters 'A', 'B1 and 'C, one of which will be stamped on the mating face with the cylinder head as shown in FIG 1:21. Pistons are divided into three classes to correspond with the cylinder bore sizes. Naturally the piston and bore must belong to the same class. The maximum available piston oversize is .0236 inch.

Pistons and rings for the Model 500 sports engine are not available in oversize dimensions so if the cylinder bore diameter is above the maximum wear limit new parts must be fitted.