Hey guys, I have been through this before. Rebuilt my last V12 that had a dropped valve seat. As I left the shop that did the valve job, seat replacement, I asked how I could avoid it ever happening again. He calmly said "don't ever start it". I have tried searching, but surely there is a way to do something to keep a v12 from doing this. My last dropped valve seat happened when I had run the engine up to the 7K range. This time, it wasn't dropped when I shut it off, but was when I started it. No overheating, no sign of a problem. I don't know any history on this car, but plan on keeping it. I am tearing it down now, plan on having back on the road as soon as I can.

 

As I understand it, it is heat cycling that leads to the problem. Constantly heating and cooling the valve seats causes a different degree of contraction/expansion in the aluminium / steel respectively leading them to drop over time. So damage may not present at the time of the actual overheating, it may occur sometime later.

 

Exactly my point. Say I fix this one, two weeks after getting it running, it does it again. There has to be a fix

 

There's a shop here in jacksonville that " peens" the seats in after they install them, some places call this "staking" to try to eliminate a dropped seat from happening again.

 

I thought I had seen something like that in a article, but couldn't find anything on it.

 

Still researching this issue. Came across a very interesting article that may describe a dropped valve seat during cool down.

Valve Seat Installation Procedures - Engine Builder Magazine

"When seats get too hot, microwelding occurs between the valves and seats. The valves are harder than the seats so microscopic particles of metal from the seats stick to the valves," says Emert. "When the engine cools, these particles are then washed into the exhaust. This causes rapid recession of the seats and is most common in dry fuel (LPG or natural gas) engines."

Could it be that this "microwelding" occurs, and actually pulls the seat out upon start up?

 

The biggest problem is the XJS has Aluminum heads and steel seats, the hot/cool cycling takes its toll, the seats are in with adhesive and the too different metals contraction/expansion rates during overheating or a lot of hot/cool processes takes its toll and viola, dropped seat. Iron head cars with steel seat don't suffer the calamity as much because their expansion/contraction rates are more in line with each other. At least that's my take on it.

 

very good explantion of seats, all of it is really the technition doing the actual work, his ability to be careful and precise on all seats, jag HEs should have all exhaust seats replaced , for your piece of mind!
inlet seat rarely loosen(not never).

 

I don't know if any other aluminum head engine suffers dropped valve seats as much as the Jag V12. And maybe, just because I have been around the Jag V12 more than any other aluminum head engine, is why I take so much notice. Yet, after reading this article, I have to wonder is it a combination of choice of materials, machining processes, both? I was very in tune with Corvettes for several years, many more thousands of aluminum head engines built than Jag, and yet, I've never heard of a aluminum head Chevrolet dropping a valve seat.

 

Ask them to use a different tolerance and make them "tighter" or get them pinned. It isn't so much an overheating post shutdown as a difference in cooling of different areas and metals. Do you know what spec they used? Because I believe the Jaguar spec is pretty loose, closer to what cast iron engines usually use instead of most aluminum specs.

I haven't had this issue, and we were recently discussing electric pumps, which I don't believe are necessary. After all I have overheating the hell out my mine several times and not had this problem. I suppose post shutdown cooling could solve your issue, and it would be less expensive then having the seats re-done a third time.

My only question is whether there is some other underlying issue here, do you know specific temps or any other data like that for your car? As I have said, I have driven my car in the top of the H and even blown a hose as a result before ( AND THEN DRIVEN IN ANOTHER MILE) and not dropped a valve. So ....what is the difference I wonder? A very interesting question I think.

JW, did you cruise in a bit after that? I suppose some issue could just a be shock temperature differential, that wouldn't be experience if you let the car cool down after a 7k run with 15 minutes or so of moderate to light driving.

I am mostly interested in why some cars do this and others don't though. For one, there may be some changes in machining or metals used that are never going to be information we have access to. What year is your engine?

 

My first seat problem occured in my 71 XKE V12. My machinest said that the seat was cracked. It never fell out, but sounded exactly like a dropped valve seat. Excessive valve clearance that wouldn't go away. I had the car about 6 years before it did that. It never ran hot, or even seemed to, in that time. After having it repaired, I asked the machinest what I could do to prevent it from happening again, and he said "don't ever start it again". He was very familiar with the Jag V12s, and said one time of running hot, and all of the seats were were at risk of falling out. From what I have read, peening the seats in, can cause hot spots, making the engine detonate.

On this current car, I bought it as a non-running project. I don't know the history of the car. It dropped a valve seat on my second outting. No running hot, no anything but running as it should. It just had that same sound as my XKE, as soon as I started it. I know the sound well.

As far as machining tolerance, from what I have read, too tight can make them fatigue and come out. Too loose, and welll..... Seems .005-.007 interference is the ticket. Reading a article by Joe Mondello, he says that MANY machine shops equipment isn't kept up to par, to be able to maintain that tight of a tolerance. He also recommends that the head be heated to approx 185-200 degrees, and the seats frozen, before installation.

Reading that article, gave me exactly the reason these engines will drop a valve seat not running....its not that they drop a seat not running. They drop the seat when started. Or should I say the seat is pulled out when started?

 

all what you guys say has merit!

i had a good friend who worked for Harley Davidson in WI, and he was in foundry castings, alumnium, he said when prices for used alumnium went up,

the buyers of scrap Alum. would use the lowest content of materials for heads and cases, and they did have problems with proper machining, galling, sticking, porus flaws, improper expansion and contraction rates.

i dont know if any of this applies to Jag alumnium,but it is possible because jag was in financial burden for years, could explain things.

when questioned about it ,answer was dont worry it will be fine!

 

You actual pose an interesting question. How many people discover only a dropped valve and how many have found a damaged valve that they assumed dropped? The question being do the v12 valves drop more often than they break?

 

in the 70's British Leyland had a horrid reputation here with people buying the next wonderful new thing the infamous triumph TR7 they would drop a seat as they were driven off the lot... after signing the papers... ( tow it back wait to have another engine placed in it under warranty ) only to have it do the same thing.... the TR8 did not have that problem... oddly if you got a good tr7 they just ran and ran....

 

I have tried to find all the information I can on this subject. From what I understand, the valve(s) have never been a problem. Yet, one thing I did find however, is that if the seat is installed out of absolute alignment with the guide, then the valve will hit the seat irregularly as it turns. (They do turn as the work??). After some time, I can imagine that the seat is actually driven loose by being pounded off concentric? Anyway, really makes me want to be 110% sure that I either do the job myself, or hire the right shop to do the job.

 

I asked the machinest what I could do to prevent it from happening again, and he said "don't ever start it again".

Wow! If that is the best your machinist has to say, you need a different machinist!

 

i have said this before, installing new valve seats in aluminum heads is close to doing art work, it is all up to the machinist, and his forthought thinking!

i have installed hundreds of seats and each job has to be taken carefully.

like i said i owned a race engine machine business for 30yrs.


and no, i dont know everything, just keep learning.

and something to think about, some valve face material can stick(when super hot), and some newer materials for exhaust valves dont stick to seat, we aint the only ones with this type problem!
you get what you pay for, i can buy new valves for $5.00 each, or i can buy some for $50.00 each, which would be a better quality valve?
and inlet valve are a different story.

 

My guess?

Because the Corvette heads don't have the coolant flow problem that the Jag V12 heads have.

Cheers
DD

 

Some have good coolant flow in the heads and others don't....and there is really no way for the owner to be 100% sure one way or the other. For an owner who is aware of the potential problem keeping the system in perfect condition and *properly bled* is the best most he can do, and is probably enough. An owner who is unaware of this potential problem is more likely to face trouble.

And, worse, a 'normal' reading on the temp gauge (or/and lack of obvious signs of overheating) does nothing to prove there is good flow thru all areas of the cylinder head.

There's always been a big focus on keeping coolant temps down, and that's fine and good. But that alone isn't the answer. Gotta have flow !




I'm sure many variables can and do play in....including perhaps original Jaguar materials and workmanship. But I'm pretty well convinced (as you might tell! ) that if you have good coolant flow in the heads you won't have dropped valve seats.

That's my story and I'm stickin' to it !


Cheers
DD

 

something i observed around 60yrs ago, how come Ford V8 flat heads had a water pump for each bank?

maybe old henry knew more than the bean counters did, when they wanted to put one pump splliting the flow to each side of the engine,(to save money) as most engines V engines are today.

not the best way, if you guys would examine the Jag V12 pump,its easy to see one side has better flow than the other, and without a good way to measure flow rates, we can only guess,one side VS other!

so if i were to do it over(not available in 1995), i like the Davies -Craig twin pump system, twin electric fans , all thermo controlled by a CCM.

along with an Aluminum radiator,(not available 1995).

i'm saying all heat problems would be cured.

along with EVANS waterless coolant, no boiling hot spots, no corrosion, or sludge!

cant even boil the coolant till 400*F, before you respond, it draws the heat away from the hottest areas, because it in constant contact of the metals.

unlike water based coolant that boils vapor bubbles and forces coolant away from metal contact!

anyway that my take on it,yup, i know not cheap!