On valves, what exactly does wear?

(Mitch in Tac) #1

I’m trying to understand what happens with my engine and it’s valves.

When we have to re-shim our valves, it seems we are going to thinner shims, so something is wearing, and the valve stem reaches higher toward the cam lobe.
Are the valves themselves grinding away (slowly), or is the area of the head contacted by the valve what is being ground?

(Paul Saltwick) #2

Both. The exhaust valve is the hottest part in the engine. Valve clearence closes up with normal wear and it is nonsense that noisy valves just need an adjustment.

(Robert Wilkinson) #3

Nice to see a post from you, Paul! I don’t understand your inference, though. I understand that normal wear shouldn’t cause noisy valves, but if valves are noisy (presumably for some other reason) why do they not require adjustment?

Mitch–the phenomenon is called valve recession and there is much written about it.

(Paul Saltwick) #4

Hi Bob,

I died in a cycling accident a year ago and am slowly climbing back. My experence is “noisy valves” often need to be openned up and the noise is somewhere else, often the guides or tappets.

(Scott) #5

The valves pounding the valve seats causes valve/ seat recession hence tighter clearences.

(Phil.Dobson) #6

That’s sounds serious Paul. best wishes for a full recovery.

(Nick 53 XK120 OTS) #7

what PS said…both seats and valves wear.: .the XK engine does have :“hard” valve seats, but still wear is at both the seats, and the valve edges. The better the “tune’”, the less valve edge wear will be. Stems, tappets, shims, cams all are hardened metal and do not wear much.but will a little. zddp in oil is for metal coating that protects those moving parts that bear, and have oil to them. Use oil that has it. Xk DOHC engines have valve “clatter”. Jaguar tried various means to reduce it due to customer complaints…but it is normal…if it is the normal clatter…not some other noise, all is fine. A tick tick or knock knock is not normal. Valve edge or seat wear will show up as loss of power, sometimes spit or backfire…tested by compression check which if low can be valves/seats or the piston rings, or both. …a wet test will help tell how much is rings.
Valve clearance can be easily checked, but not so easily adjusted,but should be set to be exact…and exactly same on all…so if using say, .006 intake…all intake should be .006 exactly.
Similarly for exhaust, if say, .008. That is what takes some time, and re-do. Jaguar revised the 006/008 to 004/006, likely to try reduced clatter. I choose to use the 006/008 because that is what the engineers established, not the customer service reps later. (IMHO)
Paul S has a super article on valve adj in the Saloon archives.

(PeterCrespin) #8

Very sad to hear of your accident, welcome back Paul.

My experience with many many hemi engines Nick (not MOPAR) is that valve stem wear (= one main source of clatter) is what causes me to replace valves. Seats can be recut on valve or in head, but stem wear is the killer. The decision is easy too - any ridge near the top that can be felt with a fingernail means replacement, unless its veeery faint and a micrometer shows it is just about serviceable.

The thing that puzzles me is how come the tilting stem wear is the same on an inverted-bucket XK valve train as in rocker engines (OHV or OHC). You can inagine side loads from the rocker tip wiping across the end of the valve, resulting in side thrust, but under a bucket tappet?