Last update I stripped Lorenzo's 1979 XS650 engine down for exploratory surgery. The bike had proved troublesome on the daily commute, and had been relegated to the back of a damp cellar for almost five years, neglected and forgotten. Despite the ravages of time the engine proved fairly straightforward to dismantle, and I can now turn my attention to examining the internals to see what parts need attention before the rebuild.

1. With the cylinder head off the combustion chambers show signs of heavy carbon deposits, particularly on the left cylinder. This looks dry rather than oily, and leads me to wonder if the bike had been running too rich. Before cleaning up the combustion chambers I'll take a closer look at the pistons.

2. The piston crowns also have heavy carbon deposits, but otherwise look in fairly good shape. A few minutes work with some steel wool and Solvol Autosol reveals shiny alloy beneath. The pistons are standard, indicating that the bike has not yet been re-bored. This gives plenty of scope for re-conditioning if required.

3. The ring gap measurements are well within factory tolerances. I always measure the gaps at both the top and the bottom of the stroke. No wear ever occurs at the bottom of the cylinder bore, so an excessive gap here gives a measure of piston ring wear. Any wear on the bore itself will be at the top of the stroke; if the ring gap is appreciably larger here a re-bore might be necessary to recondition the bores. These simple measurements give a quick and accurate indication of both bore and ring condition.

4. Satisfied that the bores and piston rings are serviceable, I can now turn my attention to the valve gear. Each valve is retained in place by two split collets engaged into the top valve spring collar. A valve spring compressor is needed to compress each spring far enough to remove the collets, then the springs can be lifted off and the valves slid downwards out of their guides.

5. Once again, evidence of heavy carbon deposits. The backs of all four valves need a good cleaning up with a rotary wire brush before proceeding. The inlet valve on the left has still to be cleaned off, the one on the right has been cleaned up. Carbon build up like this won't significantly affect the running of the engine, but does nothing to help gas flow and should be removed while the valves are out.

6. The valves need to have a good compression seal in the cylinder head. After many thousands of miles the valve sealing surface, indicated here, will begin to develop small pits as the metal suffers from constant wear and heat. Running the engine with incorrect valve clearances will also contribute to rapid valve pitting.

7. The valve seats in the cylinder head suffer in exactly the same way. With the combustion chambers cleaned up some pitting is evident in the seats. This is the area to watch carefully on older engines designed to be run on leaded fuel. Modern unleaded petrols do not have the additives required to protect these vulnerable areas, and a professional unleaded conversion will include the fitting of new valve seats and new valves made from higher grade hardened steel.

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