IML: Valve recession

Robert Benzinger, a Chevrolet engine engineer in the 1960's era once recalled "Years back, Studebaker had a Chief Engineer by the name of Stan Sparrow who was quite a character and quite a comedian. He once said that the automotive poppet valve was a metallurgical triumph over a mechanical monstrosity."

My work used to take me to a lot of automotive machine shops and for the most part they did very poor work on valves and seats. The shortcuts used to repair valve guides and highly worn wobbly equipment most use for grinding/cutting doesnt do a very good job. The people charged with doing the work are rarely aware of the effects that result from different flaws in the seat interface.

I havent got an Imperial shop manual handy but Cadil*ac gets very detailed about the precision needed for head work in thier manuals, things like smooth idle and overall longevity are really dependent on perfect head work. Turbocharged engines and trucks/rv's that operate at high rpm and high temperature will simply not stand still for any sort of defect. I think that you have detected some recession across the board tends to indicate your workmanship wasnt the critical issue however-

The lead doesnt contribute much if any protection, and has a lot of deleterious effects elsewhere. The actual lead content in fuel varied considerably between different fuels and over the years, the post 1972 era has very low lead content, and some premium fuel blends in the 1960's were essentially lead free. As I mentioned, if theres any oil consumption at all, it will give many times the protection lead ever could have provided in any event. Lead used to cause all sorts of huge problems, like contributing to corrosive compounds in the crankcase, helping stick piston rings into the pistons, and plugging crankshaft oil journals, as well as contaminating the oil and essentially cutting oil life in half. The old Cadil*ac 429 engines were very susceptible to stuck piston rings from hi-lead fuels in particular, the 1964 to 1966 versions of that engine often would have enormous oil consumption issues at very low miles, ~15,000-45,000 or so if driven gently.

The California Clean Air carbs are ultra lean, and really will get exhaust gas temperatures way up. The reduced energy in the modern fuels will exacerbate the situation. This will not be reflected in water temperature much if at all but will increase piston and valve temperatures. I would imagine as a general practice it would be worthwhile rejetting any 1966-to-pre catalyst carb somewhat richer in compensation for modern conditions. People forget carbs will operate much leaner than Fuel Injected vehicles, typical cruise mixtures in old cars are in the vicinity of 17:1. Until fairly recently most modern feedback-loop cars had to operate very close to 14.7;1 due to the limitations of the O2 sensors, which had a very narrow range of operation. Newer oxygen sensors can read a wider range of mixtures, and allow restoration of ultra lean cruise mixtures under favourable circumstances.

Valve recession is mostly caused from material being 'welded' off the seat to the valve face, where its pounded off with valve operation and blown out as the valve spins. The valve is rarely effected much by this but the seat will hit a point of no return fairly rapidly and start to deteriorate fast after a while. If the circumstances where the seat has been picked at are rare and short duration, usually the pounding of the valve will sort of peen the damage over and self-heal mild flaws. Usually damage will start with a nick, scratch or air bubble in the head casting, often in tandem with valve that wobbles a trifle in its guide allowing the flame to soften and heat the seat enough in a small area to allow material transfer, as the exhaust blows thru. To remove enough material to literally drop the valve into the head, the seat needs to be pretty hot indeed and operating at high enough speed to pick away a lot of material. The lead is presumed to stop the valve from being able to stick to the seat and prevents these micro-welds but in practice the circumstances that will permit seat recession are so severe that damage would be inevitable and there will be no way to tell from looking at the results whether lead free fuels were being used. Cars needed valve jobs in 1965 too, after all.

Theres a myth that something special was done after the 1970 era to improve the durability of seats for unleaded fuels, since 1972 models, the EPA has required all cars to meet thier emissions control obligations on 87 octane unleaded fuel. This remains the case. Detroit engines did not recieve any material changes to operate on unleaded fuels. Different engine configurations have always recieved different materials like stellite inserts and processes like induction hardening when the manufacturers felt they were warranted since the 1950's era. The 1960's heads are pretty comparable material and hardness to similar later model engines, basically. Cars with 'air injection reactor' (smog pump) often get better exhaust valves because the system operates pretty similarly to a blowtorch on the exhaust valves, they get VERY hot, exhaust temperatures can sometimes double from that system compared to non-AIR. Chrysler avoided the AIR system in the sixties to avoid some of these complications.

Not a lot of this will make much difference to you at this point Dick, you're doing the appropriate repair on your car, but people with the 1966-1974 cars and to a lesser degree other postwar non FI Imperial owners probably can benefit from your experiences. In a nutshell, its very important to undertake very careful valve work and most cars need to be re-jetted when theres a convenient opportunity, and the neccesity for this is dramatized by higher speeds and loads, like over 75 mph cruising and trailer towing etc. Rejetting 3-5 thou richer will be appropriate for most people and will not adversely effect emissions. Having ignition timing advanced and a well matched vacuum advance unit will also make a large impact on gas temperatures. These changes are very inexpensive and fairly easy for the home mechanic to accomplish.