The spring load is important to prevent the valves from 'floating' at less that the engine's rpm limit. It is not about preventing the valves from leaking. Even a very weak spring will close a lapped and stationary valve and prevent it from leaking.
A spring load tester, the equipment and not the person, will read-out the lbs of force it takes to compress the spring at two different hts. One height is when the spring is installed and the valve is closed, and is called the seat load, and the other is when the valve is fully opened. The load at each of the two specified hts is important, though the open load figure is most important.
Any machine shop that works on heads will (should) have one.
The Bentley manual (page 116-56 thru 60) does not provide these specifications. It uses a down and dirty method, says to line up all the springs (on a very flat surface), place a straight edge across the top and replace any spring that is short. That is a quick (and cheap) method for a repair shop. That method will often not find a weak spring. A weakened spring can still retain full ht yet allow valve float. All the good performance books and magazines will tell you so. That is why a spring load tester should be used. If BMW will not supply height and load specs a machine shop with a load tester will still be able to find any weak springs.
Back-cutting will not affect long term reliablity. The valve stem is the weakest part of the valve, not the valve head. And like Bobdmac said it will lighten the valve, which will increase long term reliability, if it affects reliablity at all.
Sorry for the long post, but in my opinion its better to be clear and thorough than short.
540, Dirrezas (4) @ 275/35-18 ea, Eibach springs & bars, Bilstein Sports, Dinan Stage 2, slotted ATEs, Hi-Temp HTP pads, SS Lines, UUC-SS, White/Tan, Bling-free & Stealthy
Last edited by franka; 03-03-2011 at 08:08 AM.