Any clearance on a hot rod motor is critical. If you have coil bind it will destroy your cam and if not enough spring pressure for the lift can cause valve float. Specifications and clearance issues are a must for all well built motors.
A flathead is a V8 engine and installed height is as important as it would be in any other circumstance.
it is absolutely critical. no less important than proper valve to head clearance, proper oil clearances, or ring end gap, or...... how important to YOU is building your engine to survive its specific use?
Thanks for all of your responses to the poll. I agree wth the 11 (73.3%) as to very critical and for the 3 (20% not critical) and the 1 (6.67% don't worry about it) I offer the following information. I've done a lot of research on this subject and know that the original spring heights recommended by Ford were 37 to 40 #'s closed spring pressure with 0.213" installed height on the 32-50 valve setups and 0.189" installed heights on the 51-53 valve setups. Many are now using Chevy stainless valves in their flathead motors that are about 0.086" longer than the Ford valves. Not only is the overall stem longer but the dimension from the center of the valve seat face to the loc or keeper groove is also longer. As a result shims are required to obtain the correct installed height. The base dimension from the valve seat center to the top of the horsehoe guide retainer is nominally 0.213" measured on a block by Vergil on the HAMB, on a cut-away block he has with replaceable seats , and myself on a late model block I have with no replaceble intake or exhaust seats. This dimension sets the top of the valve spring seating surface 0.2290" below this dimension on the guide shoulder. I tested a Isky 185G spring and found the following: Installed height 2.120"=50 #'s(very close to the Lincoln-Zepher springs @ 0.213" installed height) Installed height 2.100"=55 #'s Installed height 2.060"=60 #'s Installed height 2.030"-65 *'s Installed height 2.020"=70 #'s Installed height 2.000"=75 #'s As you can see a few thousands of an inch has a great impact on seat presure. To me it says it's very critical. On higher lift cams the spring pressure increases dramatically when the valve is wide open creating greater load on the cam lobes promoting faster wear. Too little seat pressure promotes valve float at lower RPM's than higher spring pressure. Correct installed spring height is a trade off between these two factors. Thanks again for your feedback. Jim .
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