4/7/2024 0 Comments Valve tappet clearanceIf you then install a 1:5 rocker the clearance must increase to match - 10 thou x 1.2 = 12 thou, and thus the clearance at the cam stays at 8 thou. Clearance at valve 10 thou, with 1:1.25 rockers at cam lobe it is 8 thou. If you have an overhead cam direct on the bucket you are seeing the true clearance - if you have ohv pushrod you cannot see the true clearance and the rocker ratio has to be considered i.e. See if you can get your head under the bonnet of a race car at 8000 rpm on a rolling road and look at the colour of the exhaust - if you can stand the noise! Even if you bring it straight off the track, by the time you measure the clearance it has cooled too much. With all due respect you cannot replicate the conditions of temperature whilst running, by getting the engine really hot in the garage. Add the huge temperature seen by the exhaust valve in race conditions and resulting expansion and you are heading for trouble. set minimal clearance at the wrong point and suddenly your valve does not close properly. For 5 thou extra lift you may alter the opening and closing points by 10 or 15 degrees! Also if you check the base circle on most cams you will find it is rarely completely circular even when it is supposed to be. Closing the clearance ruins the performance because it changes the valve timing significantly. Be really careful about closing valve clearances - you are very likely to reduce the performance and burn a valve.
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