[xk] Cylinder Head Corrosion Welding

XK
1

Bob Oates’ comment reminded me that I never got around to telling about
my experience with my XK120 cylinder head during last winter’s rebuild.

It was corroded around all the water passages going to the block, but
none going to the intake manifold. I suppose it must be something to do
with galvanic reaction between the head and head gasket, which for some
reason did not also occur with the intake manifold gasket.

This is a standard A-type studless head made by William Mills foundry. I
wonder if they have a higher or lower incidence of corrosion than the
West Yorkshire heads? My 140 C-type head is a West Yorkie and is not
corroded at the block face, can’t see the intake manifold face yet.

I cleaned the head thoroughly with grease solvent, then sandblasted it
to get a good look at the corroded ports. They were all bad, up to about
1/2 inch wider and longer than originally at the face, and irregularly
tapered down to the uncorroded portion deeper into the head.

There was a picture in the Apr '01 Jag World showing a shop repairing a
head for the same problem by MIG welding.

Now I’ve designed a lot of things over the years that involve aluminum
welding and I’ve watched my guys doing it for years, and I have gotten
pretty good with my MIG with steel. So I thought I’d try it. I bought an
Argon cylinder and some ER4043 Aluminum wire and gave it a shot. Total
failure. My lumps of bead would not join with the parent head material.
I also bought a tungsten tip and tried it as a TIG, still no luck. But
at least I didn’t make it any worse.

So I took it to the experts. Now I don’t want to say which particle
accelerator laboratory this is, let’s just say I’m grateful to the
taxpayers of a certain large country for voting for the politicians that
voted to fund this place where there are welders who do a lot of
aluminum welding for ultra high vacuum and they have to have the best
equipment. And fortunately there are times when these guys are not
particularly busy.

The welder used a Miller Dynasty Aerowave machine, set on DC squarewave
with an AC biased wave, mostly DC electrode negative, with an AC topper
cleanup, a 2 percent thoriated tungsten tip sharpened to a point, and
helium shielding gas. This makes a very concentrated arc so it doesn’t
heat up the whole head and warp it. I copied all that down exactly as he
told me; hope it makes sense to the welders among us.

Anyway, the first thing he did was take a burring tool and scarf out the
corroded areas even bigger than they already were, so he could get down
to new clean metal.

Then he laid down a beautiful bead all around each one, and built it up
with several passes so there was enough metal for me to remove and make
it the shape it should be.

A funny thing happened while my head was in the machine shop, they told
me about later. The welding booth is way back in a corner where the
machine shop superintendent never goes, and my head was only in there
for three hours one morning, but that was the one morning he decided to
take a tour. He saw the head and casually asked the welders who it
belonged to. If it had been one of the guys in the shop that would have
been ok and he would have ignored it. But they said it was for a staff
member. Immediately he got a black look and said “It better not be…”
and he mentioned a certain physicist who has given the shop a lot of
trouble. The foreman said, “It’s ok, its somebody who has brought a lot
of regular work into this shop.” Not satisfied. “I want to know who it
is!!!” So he had to tell my name, and the super brightened up and said
“Ok he’s a nice guy, take care of him.” So I found out in a practical
way the value of building a good reputation with the people who do
skilled work for you.

Naturally these guys can’t take cash or tips or gifts, but there is an
unwritten agreement. My cost for the welding work was donuts and bagels
for the whole shop.

I milled the welds myself, first opening up the ports to the right size,
and in one little spot where he didn’t give me enough metal, I tried my
MIG again and it worked, my bead melted in with his new bead. I milled
the welds down to .001" above the deck, then finished off with a hand
file and grind stone, so each port is flush with its surroundings.
Flatness over the whole deck is about .005", certainly within the
ability of a head gasket to seal.

Incidentally, I don’t know if you have ever noticed there are four flat
spots outside the gasket contact area that are up like .025" above the
deck. These are left over from the initial rough cut, and are where the
clamps held the head down on the mill when the factory did the final
finish cut.

Rob Reilly

2

OK, Rob. You’ve motivated me to write so more. Since I am not an engineer
as you and certainly don’t come close to the same level of knowledge and
understanding of MIG/TIG welding as you, I think I can add some observations
that the non-technical of the List might enjoy.

I’ll talk about two of the items you mentioned. First, the possibility of
the head plane changing (warping) during any welding. On this, I earlier
referred to a local individual whose shop does NASCAR engine building.
Well, the shop has all kinds of machine shop equipment for fabricating and
repairing any aspect of the engine, i.e., mills, lathes, drills, etc. (This
description doesn’t do any justice to all the stuff that is available.)

Anyhow appreciating the effect of heat distortion on XK heads, I was
fascinated when the shop owner recently gave me an explanation of a
custom-made piece of equipment, and the process, he uses to manage heat from
corrosion welding when he works on customer’s heads sub-contracted to him by
my mechanic friend. I can’t remember all the details, of course, but the
equipment had a flat base, waist high, like a large table to receive the
head. Then there was a heavy gauge metal frame work (bar stock, don’t
recall the size) which was placed on top of the head. The whole contraption
then sandwiched the head under compression. He used huge threaded rods to
apply a load to the head and base of the device. In effect, it was a giant
vice squeezing the head flat to maintain trueness.

Adjacent to this piece of equipment was a huge temperature-controlled oven
for heating the heads.

I don’t remember the sequence of a typical head repair process, but I recall
him saying during weldup he constantly assesses for warpage and through his
experience he knows when to stop welding and he knows enough of the
metallurgy of the head to usually know if the warpage will level upon cool
down. If he has warpage after all the welding, he pops the head into the
oven for a temperature and time controlled period and then places the head
onto the leveling table under compression. Viola! Another XK head has been
renewed through the process!

My mechanic friend has educated the shop owner to only removing the
bare-bones minimum of metal during the final skim of the gasket joining
surface. (In the mid '80s, my mechanic friend built concours XK engines for
customer’s of Bill Welsh. I recall the grief when a machine shop “junked” a
140 C head by removing too much metal to get head trueness. Of course, with
Welsh’s extensive inventory an original was quickly available to replace the
terrible foul-up.)
Fortunately, the shop owner appreciates the XK head and such a mistake
should not occur for him.

Concerning your other point about the four rectangular flat spots outside
the gasket area, I was taught to use these as a way of knowing if the head
had ever been skimmed. These are the points that will soon be lost about
these cars since most of the XK heads will now have less metal in this area
due to successive rebuilding. And, if you haven’t seen an unmolested head
with these flats you have no point of reference. After the first cut is
when it can get dicey and you can trash a head if the shop lacks
appreciation for this.

While most Listers may not give a tinker’s damn about knowing how to “read”
a cylinder head or ever have the opportunity or need, I’ve found that
assessing the “flat spots” can provide another means to evaluate the quality
of the head. Like most mechanical things, XK heads have a useful life
which can be shortened or eliminated by unknowledgeable and injudicious
metal removal.

Bob O

3

Bob:
Very enlightening. Learned a lot!
I have an old 16 stud head (a spare so to speak) that has never been “planned
down” but it has a crack in it between an intake and exhaust port as I
remember the location?
I would of course like to save the head as these 16 stud heads are
becoming a rare breed. Where do I go and what sort of a dollar figure are we
looking at assuming there is just this one crack?
I will be the one who determines whether it’s worth it.
Mad Ludwig