Spring heights

Hello

How come the spring heights weren't shimmed with these heads? Genuine question, is this only necessary in certain applications or should we do it for all installs of new springs?

Not being familiar with the specific build in question, I can only make a general observation.

It depends - in theory one should check the installed height for each valve-spring assembly, and check that against a spring tester. Same with the compressed height where one should check coil clearances and for any guide/seal interference.

In practice, especially for mild engines and camshafts, they will be either designed to work well within the nominal limitations of the OEM springs, or of the provided springs.

On that, there are some "youtubers" who turn very high numbers with turbo' engines and who seem to rely on just dropping in "race" valve springs and leaving them at that, without actually checking anything. I recall one instance where one somehow escaped a total engine failure, finding one of the collets/keepers/stem locks had actually escaped and the only thing preventing the valve dropping was the remaining half being jammed in place.

The ONLY way that could ever have happened is if the inertial loadings from seat bounce released the force locking the collet far enough to pop it out.

Even without that, the primary reason for valve related failures is suspected to be poor control of the valve, and valve-train, allowing the valve to 'bounce' on the seat, fatiguing them, rather than being 'placed' on the seats.

Oh, just to avoid confusion, there are spring shims, used to set the correct spring installed height and/or seat force, and the spring seats as found on alloy heads (some iron, too, but it's uncommon) to protect the head against wear and fretting from the springs and/or locate the spring on the head. There is, sometimes, some overlap.