Why do valve seats drop?

Hi Norman,

I have had two seats drop, one on each head. One aft and one fore. The first one 10yrs ago. It happened one night in LA. Temperature in the high 50s. I went out about 9pm and then got back to the Jag about 11pm and ‘clik’, the valve dropped at start up. No over heating occurred at all. Left bank.

This last one occurred one morning with a temperature of -15 deg F. Did a short fun to the dentist and an hour later starting the car, ‘clik’, dropped valve seat again. I thought it might have been a lifter and drove it for a while thinking the lifter would loosen up. It never did and did some damage so a new head altogether this time.

Both beginning runs were under a half hour and of course no over-heating occurred. Both instances were not in summer nor daytime, runs were short and no prior overheating.

Any ideas why the valves dropped? The machine shop guy said he has seen both drops due to cold or no heat and due to over-heating and in many different makes and manufacturers. Key to all is aluminum heads with steel valve seat inserts.

Paul

All dropped valve seats are a result of overheating, but not necessarily at the moment the seat drops. Prior overheating can cause the seat to move allowing the final movement to occur without a heat effect.
It is unusual for a front seat to drop as most overheating occurs in the rear of the head due to inadequate water flow at low speed, such as occurs in slow moving traffic on a hot day.
TO prevent over heating in the rear you need to modify the water rails as per my mod instructions. (See Lutz water rail mod).

Like Norman says, always due to overheating, but not necessarily recent overheating. And it’s not due to the coolant reaching 220F. It’s due to the coolant turning to steam inside the heads, allowing the local areas around the valve seats to reach temperatures hundreds of degrees hotter than they should see.

I do disagree with Norman slightly on the rears being the usual. I dropped one in 2B cylinder in my car.
While flow to the rear is no doubt an issue, there’s also a possibility that the steam will rise into the forward part of the head, I guess.

There’s an argument to be made that the B bank is more liable to dropped seats than the A bank due to the issue of the upper 1/3 of the radiator getting plugged up, but even that doesn’t hold all the time – people have dropped valve seats in the A bank as well.

Many, many, many have recommended “staking” the valve seats so they don’t fall out. Unless you’ve machined a chamfer on the upper OD of the seats to have something to peen the metal over, I can’t see how that would accomplish anything at all. Of course, most people who have dropped valve seats never drop them again – staked or not – which may mean that they have learned how to avoid the overheating issue or the local machine shop does a better job of installing those seats than the factory did.

More than 90% of dropped valves I have had experience with have been in the 2 rear cylinders, but unfortunately Quality Control after the Leyland “takeover” wasn’t the best so it is possible to get seats in other positions to drop.
I know my Head Guy makes them a much tighter fit (no staking needed).

Wait: they had such?

:persevere:

i have heard after Leyland takeover , save money, reduce quality control personal!
finances were tight around that era, also with Aston Martin, UK was in a major change!

My XJS has a plate that says Leyland!
ron

on the dropped seat thing, some say that when engine was last run and possibly HOT, a seat may have microwelded to the valve , so next time opens it takes the seat with it!(or loosens it and things go down from there).

but one thing is for SURE , the Jag V12s did drop more than most other Alloy head engines !
i collected 4 V12s in about 5 yrs!, all had some valve train problems! so i junked 3 of them,got $75. dollars for the bunch, saved the best for my car(full rebuild ++ some performance mods).
ron

.
notice 1st pic , oil gallery mods to cleanup some casting flash and smoothing oil flow in/out of block(cant hurt).
ron

Does anyone have any photos of a Jaguar V12 head cut open to show the coolant path?

Greetings All,

Valve seats drop because the area where the seat is either too big or the aluminum head expands quicker than the steel.

Once you have dropped a seal, its then up to the machinist to get the fit correct, hopefully the seat didn’t do too much damage, enlarging the recess.

There is only one size seat for the valve size. I have seen some knurl in an attempt to assure it doesn’t fall out if the fit is “questionable”. Not a fan. I’ve had oversize seats made and hardened, but there better be a good reason for the expense.

A V12 head cut…crap! Kirbert, I would have thought you of all people had one of those?