The valve arrangement, and by extension the design of the cylinder heads, on 4-stroke motorcycles has evolved over many years. The early cylinder heads were typically made from cast iron and were a simple shape offering a place for the gases to be compressed and, via a spark plug, offering an ignition point for the said gases. Early heads did not have valves located in them as these were positioned in the cylinder barrel; a configuration referred to as side valve due to the valves being located to the side of the cylinder.
Another early valve arrangement was the F-Head, seen on such engines as Harley Davidson’s first engine in 1902/3. The F-Head design incorporated the inlet valve over the piston, while the exhaust was mounted side valve style adjacent to the cylinder.
The development of the cylinder head passed from side valves, to overhead valves, to overhead cams and valves of current designs. But regardless of the design, every cylinder head and valve system will at some time require service or maintenance.
High mileage engines generally need their valves re-seating and their seals (where fitted) replacing. However, occasionally, both the valve seats and guides may need to be serviced or replaced as required. These two jobs are normally entrusted to an automotive machine shop that will have the necessary machinery and skilled workers to complete these jobs.
For the home mechanic, servicing the cylinder head will generally be limited to decoking the combustion chamber and re-seating the valves.
Assuming the cylinder head has been removed from the motorcycle, the mechanic should place it on a bench in the upside down position, in other words with the combustion chambers uppermost (see note). He or she should then carefully fill the combustion chambers with automatic transmission fluid and allow this to soak into the carbon deposits overnight.
Note: If the cylinder head is of the OHC type, the mechanic should remove the cams after removing the head from the motorcycle before doing any service work.
Scraping off the Carbon Deposit
After the oil has soaked into the carbon, surplus oil should be drained off and the soaked carbon deposits should be scraped off using a wooden lollipop stick or similar. (Note: Do not use screw drivers or other steel tools for this job as these will damage aluminum cylinder heads).
After the head has been decoked and thoroughly cleaned, the valves should be removed ready for re-seating (this process should be done one valve at a time on multi valve heads so that the valves get placed back in their original location).
Before re-seating the valves, the valve seat and mating surface of the valve should be examined. There should be no pitting or cracking in either item.
Resealing the Valves
The mechanic should place the valve into its respective guide having oiled the valve stem. He should then smear a small amount of valve grinding paste onto the valve’s seating surface. Next an electric drill with a variable speed trigger should be located onto the top of the valve stem. The mechanic should now rotate the valve relatively slowly and bring it into contact with the seat–lifting and returning to the seat a few times will ensure a uniform finish. (Note: Re-grinding the valve seats in this way must be done after new valve guides have been fitted where applicable).
After each application of paste and subsequent grinding, the mechanic should inspect the mating surfaces to ensure a continuous ring around the seat. A thorough cleaning will be required before moving on to replacing any rubber seals (some machines use a seal on the inlet valve stem under the spring), and the springs etc.
To test the effectiveness of the seal, the mechanic should file some chalk onto the valve faces inside the combustion chamber, and then spray WD40 (or its equivalent) into the respective port. A slight weeping is normal and can be seen as a damp patch emanating from the valve edge. A poor seal will allow the fluid to come past the valve quickly dampening the entire area around the valve.