NISSAN VALVETRAIN GEOMETRY
The Nissan S.O.H.C. engines have a complicated valve geometry. If standard valve train geometry is not maintained when modifying the engine, performance can be less than satisfactory and in severe cases, damage to the valve train may occur. The dimensions in the valve train likely to cause serious changes in geometry are: valve stem length (top of stem to spring seat area), base circle diameter of the camshaft and changes in the lift measured at the valve. Any of these dimensional changes can be corrected by changing the thickness of the valve rocker guide located in the top of the valve spring retainer.
When a valve job is performed, we recommend the valve stem be shortened sufficiently to restore standard valve stem dimensions. It is then a simple matter to correct the geometry by installing the recommended thickness valve rocker guide.
The proper thickness valve rocker guide for the above listed cam is:
Set of 8
Set of 12
268N
.117 (Stock)
E911523
E911522
280N
.140
E911513
E911524
288N
.171
E911533
E911532
298N
310N
.207
E911543
E911542
The formula used to arrive at these figures is:
To the standard valve rocker guide thickness (.117) add ½ decrease in camshaft base circle diameter and ½ the increase in lift at the valve (to compute valve lift, multiply cam lift by 1.493) the sum is the thickness of the valve rocker guide that should be used.
If non-standard valve stem length is used, any added length should be subtracted from the valve rocker guide thickness and any loss of length should be added.
The tolerance should not exceed +/-.010
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