® Adjusting valves at other than top dead center compression

User forums perpetuate a fallacy that predates the Internet, the notion that putting each cylinder at something other than top dead center compression (TDCC) is preferred while adjusting valve clearances.

There actually was a very unique case in the 1960s involving BSA's big singles (500, 441) that had cams with (presumably mistakenly-made) eccentric base circles. Most BSA manuals advocated putting the cam lobes in certain non-traditional positions to overcome this defect while adjusting clearances.

However, I know of no other instance in which it is advisable to deviate from the standard TDCC position for valve clearances. Yes, Honda promoted in their vintage sohc four manuals doing all four cylinders' valves with only two turns of the crankshaft, meaning half the valves were adjusted at places other than TDCC. But two things. One, Honda has never made cams with non-concentric base circles so that is not the reason. The reason is simply ease of maintenance. And two, Honda got away with this because the cams in those machines were about as agressive as the cam in a leaf-blower. Ridiculously mild, with only 5mm valve lift.

In the early Superbike era a few racers started promoting non-TDCC valve adjustment in connection with Z1 production level racing. Whether in attempts to impress, or because the aftermarket cams used in those bikes suffered from eccentricity, I am not sure. However, nearly fifty years later we are still hearing about non-TDCC valve adjustment on those old bikes and even on newer ones. Except for the reasons just enumerated, this is mere attention-getting, plain and simple, and the stock-in-trade of Internet user forums.


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