Barrel

Cylinder barrel, base gasket and studs

The cylinder block for the 750 is common throughout its Commando history; the only difference being in the colour selected for the finish. The block was launched in silver, changed to black prior to mid 1969, then returned to silver in mid 1969 until the introduction of the Combat. The Combat was painted black , but all standard Commandos remained silver until the demise of its production in 1975. The block is as it was in the first introduced model of the 500cc range in the late 40s. The design never changed; built in cast iron, it is heavy and therefore wear is slow if an oil supply is maintained and you let the engine warm to temperature prior to hard acceleration.

Careful inspection and cleaning is required to remove the usual dirt and grit from the fins and old gasket material from the top and bottom. Inspection should be given to the threads in the top which hold the cylinder head down as they have a tendency to stretch and protrude upward. This in effect could prevent a proper sealing of the head gasket and premature failure would be imminent. Also, check after removal from the cases for "shuffling' areas (mainly around the back side). This will be noted as darker grey areas on the cases. This is common to 750 Commandos and is one of the reasons they leak oil around the base. Ensuring the mating surfaces are flat and well cleaned prior to reassembly is the key to preventing this from causing any problems.

If you require a rebore this can be accomplished quite easily from a reputable auto parts store, or machining facility. State the proper size of rebore and make sure the measurement goes on the service record. There are not many machinists around who have seen a Norton barrel and any who have probably don't retain the measurement for very long. This is critical for a proper job. Measuring to find if a rebore is required is another matter. There are several ways to check this yourself, but without the correct tools the figures will not be absolutely accurate. Again, if you have access to a good machinist let him/her do the measurement as it will prove to be extremely accurate.

Bead blasting the barrels for repainting is an option but I tend to agree with most experts in this area in that bead blasting tends to leave residual glass beads in the metal surface. For the barrels this is not so critical providing the holes have been plugged to prevent entry into the oilways and pushrod tunnel. Use a high temperature paint for the finish. Lastly, check all oil passageways for obstructions and clean them out with pipe cleaners or pieces of rag looped through a strong wire. This is also a method for the pushrod tunnels. One last item to note is that as the cylinder barrels are made of cast iron, they will tend to begin rusting almost immediately upon the removal of the paint and the rebore. It is therefore good pratice to receive these from the blast shop as soon as possible to coat them with paint, or from the rebore shop and smear them with a light coat of oil.

J.B. Nicholson - Modern Motorcycle Mechanics

Cylinder Block Measuring and Reboring:- Cylinder wear can be fairly accurately determined without micrometer equipment by placing a piston ring in the lower unworn end of the cylinder, measuring the gap with a feeler gauge and then shoving the ring with a piston to the top end of the ring travel in the cylinder and measuring the gap in this postion. The difference in joint gap measured at these two positions will represent approximately three times the cylinder diameter wear. For example, if the ring gap at the lower end is 0.010" and the top end measures 0.040", the difference of 0.030" circumference increase indicates approximately 0.010" diameter wear, and reboring and oversize piston fitting is indicated. Generally cylinder wear of over 0.008" calls for reboring for fully satisfactory results.

Norton Service Release Notes