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Pushrod Valve Train Geometry


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Hi Wise Ones

Not an issue I have thought about much before (well to be honest never thought about before) but I'm just playing with rocker/pushrod geometry (hence my looking for a spare rocker assy) but looking at both my head and a MK2 head and a couple of gash spitfire ones I have, I notice that looking at the rocker shafts the factory used various shims/spacers/washers plus the coil springs to position/offset the rockers from the pedestals, to I assumed centralise the tip of the rocker on the top of the valve, but however looking closely what they did was a fairly hit and miss job the rocker tips all being offset by up to a few mm difference, i.e. some central, some to left, some to right. Now if your setting up your own rocker shaft with spacers it would be fairly easy to get them just about smack on dead centre, which is what I did donkeys years ago when converting my rocker shaft to solid spacers and shims. However it just occurred to me that where the tappet adjuster screw ball is at the other end of the rocker is in a fixed relationship to the tip, move the tip 2 mm in one direction and the ball also moves 2mm in that direction, thus also moving the angle of the push rod, which could now be a degree or two off vertical.  Which is worst a rocker tip with the valve stem head close to the edge, or a rocker shaft not vertical? does it matter? were the factory spacers to align rocker tips or angle of push rod?

Actually starting to think that pushrod angle isn't important at all, looking at the V8 they are all running over at an angle way off vertical, mind you V8 don't normally rev to the limits we use without some specialist valve work!

Alan

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The geometry of the valve to lifter arrangement can have appreciable effect on the valve opening rate.  Some engines are more sensitive to variations than others.  I seem to remember the Ford Pinto ohc engine had some potentially noticeable effects on valve opening rates depending on how the geometry was messed with...  

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