Grant,
I have NO IDEA how you know all this stuff, but am most thankful you do!
I drove it today in traffic and on the highway. Got it up to 140kmh @ 2,500rpm to test it.
The clacking has gone. Its replaced by a tappety sound most obvious when backing off the gas at highway speeds.
When I drove it previously it sounded like a 1980's diesel to the extent that I didn't want to drive it, it sounded so bad.
Now, I couldn't tell you in all honesty if it was or wasn't making this same sound last Fall, its that much quieter (I only drove it a handful of times before winter layup). My mechanic tells me it wasn't making any tappety sounds when he drove it back then though.

Current plan is to drive it over the summer and see if the noise returns as bad as it was before. If it does, then we are no furher ahead or behind than where things are at right now, and major surgery will be happening.
All the best,

Quentin

 

It cant be a dropped valve insert, you would have wrecked the head by now! Have you checked for an inlet manifold leak?

 

The inlet manifold has been on and off more times than I have had hot dinners!
I exaggerate slightly, but it has been on and off probably three times as they have dived back in to investigate further (new gaskets).
All I can say is they are very thorough, with three expereinced mechanics poking at it and coming up with possible diagnoses.
A leaking exhaust manifold has been ruled out. The noice was traced to somewhere in the region of Cylinder #4 on the exhaust valve side.
Coincidently the injector in that cylinder was playing up and has been replaced (new plug was showing lean burn).
The plugs are new and I did enquire as to whether maybe one was not seated 100%. Don't think that's it.
I did listen to the noise of two dropped valve seats on a V12 online. Sounded similar to the clacking on mine, but was evident at idle. On mine only evident above ~1,500rpm, before the clacking disappeared to be replaced by tappety sound.
Short of taking the head apart, don't think a definitive diagnosis is possible right now.

 

As you say the head will probably have to come off but a compression and leak down test would be interesting beforehand. Slightly clutching at straws now but if it has overheated could a loose valve guide be the culprit, over to you Grant!

 

Compression is okay in all 12 cylinders, although I gather compression may be fine even with a partially dropped valve seat.
Valve lash has been measured and all okay.
Loose valve guide........ thats a new possibility to keep me awake at 3am

 

Forget the loose valve guide. Nothing to lose by driving it and seeing how it goes. Could be a bit of metal dropped into the cylinder, this is very easily done when chnaging pluigs, manifolds on and off.
Have you thought of using a borescope down the plughole?

 

I am with Greg.

A valve guide coming loose on any engine will shatter in seconds, and then the valve will do obscene things to the piston.

Since the tappet clearances are deemed OK, its not valve train to me.

We had one, an loooong time ago, and a PreHE.

Weird rattling noise SOMETIMES.
Did the exhaust to head gaskets. Nope
Did the exhaust flange donuts, including the 2 alongside the transmission. Nope
Went into the dizzy, looking for a broken spring.Nope.
Drove the sucker, and the Redline got more than kissed, and no change.

I made a comment about a snapped Chain Boomerang, which cost me a slab of beer for the "silly" comment. " It sounds deeper than that, and up top" was the banter, but I maintained my comment, that It sounds more chain related to me on that day.

3 weeks later, timing cover off, and the Boomerang was in 3 pieces, and the chain was clattering around.

Maybe?????
Drive it, as said by many, the chain will NOT snap. If it starts running rough SHUT IT DOWN.

 

Quentn,
Thats simple, they are SIMPLE engines, and the systems that work them are the same simple stuff, until Marelli arrived, oh boy.

Remember they are a Double Six in reality. The firing order dictates that.

OK, they have their issues, fact, but overthinking does little, then there is the PO (AKA Freddie Fiddler) that usually has messed too many things up its a wonder it runs at all.
Working very methodically is the key, its NOT Rocket Science.

As I said, and others also, DRIVE it, use it as designed, see what happens, and then decide.

 

Greg,
The first thing they did was scope all 12 cylinders and the intakes to see if they had left anything behind - nothing.
If they had an articulating borescope they could have peaked at the valve seats, but sadly not something they have.

Quentin

 

Gents:
Agreed - going to drive it over the summer and monitor the noise.
As ever, thank you for sharing your collective wisdom; much appreciated.

Quentin

 

Quick update!
This morning tappety noise gone entirely. Stationary at 3000rpm using a stethoscope just a V12 purr!
Champagne time I think!

 

GOODO.

Something was in that cylinder, SMALL, but there.
Have a look at the OLD spark plugs, maybe a tiny piece of the porcelin insulation ois missing, and that would be all it takes. Had that on the Pretend Jag, it also burpe it out, those V6 "things" scare the hell out of me, a V12every day.

The beast burped it out, as they do.

Road trip, Vegas and back, only way to go.

 

Road trip to Vegas?
Now there's a grand idea!