A simple and cost-effective way to increase valve seat pressures by 10%
These sit between the valve spring and the collet, increasing the spring preload on the seated valve from 35lb/in to 42lb/in inlet and 38lb/in exhaust - useful to consider when rebuilding the cylinder head intended for 'spirited' operating conditions, as a way to achieve good seat pressure with the standard springs.
Gene Berg of 50's VW racing fame first maintained that a stock engine used for racing should have the valves shimmed a further .060" intake and .030" exhaust. Much like Gene and his contemporaries, I build these engines - based on standard components - to run faster and stay more reliable than any other.
Increasing the valve seat pressure helps to mitigate compression losses and counters the inertia of the valve, keeping it under better control at high speeds and preventing valve float/bounce. I've adapted the sizing and placement of these shims to suit the 1KR-FE; the result produces a very robust valvetrain.
These shims should not be stacked
The shim configuration suits the specification of the original camshaft. If a non-standard cam profile is used then you should go with that cam manufacturer's own recommendations - typically you would install a stiffer spring if the cam profile calls for an increased spring rate.