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Hardened valve seats

Hardened Valve Seats


Just put my 140DHC on the road and immediately realized the need for gas
with lead. Should I use an additive or drive to the local airport for

Walter Hickman
57 XK-140DHC

Hi Walter H. & all -- the story I have, and have applied to my original Mk
IX, 792817 BW, is that ALL XK motors from October '48 had hardened valve
seats -- so unleaded doesn't affect them. My car has eng. # NC8338-9 (9:1
head) so I've always used Shell or Chevron super unleaded because of the
9:1 head -- no problems whatsoever, runs fine with timing set to book at 5
degrees BTDC. Have run this way since acquisition in Feb. '89 -- 83,009 to
117,240 miles today, 34,231 miles so far with no problem. In fact, in Sept.
'92, swallowed a valve -- no machine shop at home so head went to John's
Automotive in Sacramento, and the old-man owner confirmed that seats were
hardened, so no changes because seats were still good. He followed my
instructions and "made it right" for $1100 -- cheap for this kind of work
nowadays! Even re-shot head in original metallic blue (colour for the "B"

In early '96, letter from Chevron about new "oxygenated" gas, which could
damage early fuel systems -- JAGUAR JOURNAL's Mike Cook researched and came
up with STA-BIL, an additive which solves the problem -- 2 ounces per tank
(twin tanks) or 10 gallons on fill-up -- have used since Mike's notice --
to date, no problems whatsoever. STA-BIL is available at Ace Hardware and
at Walmart.
Hope this info helps
-- Larry Martz **********

Hi David Drenzek & all -- if it's an XK150, you should know that ALL XK
engines from 10/48 (introduction in first XK120) came with hardened valve
seats -- in other words, seats in ANY XK engine are OK for unleaded. On
yours, possibly bodged, difficult to tell by simple examination, and you
have to have the head off and valves out to see them. How long ago was work
done? How many miles since? I suggest a compression check on all cylinders,
looking for at least 130 PSI (-10% is still OK) -- if OK, just run unleaded
and it should do well. If more than 10% below 130 PSI on any cylinder, you
may have a valve/piston/ring problem. When the exhaust cam broke on my
original Mk IX, 792817 BW four years ago, I had the head off & done by a
totally compentent shop; seats were fine, and they told me no reason to
change for unleaded in this engine (NC9339-9) so I didn't and have always
run supreme unleaded (because of 9:1 head), as I did before cam broke -- no
Hope this helps
-- Larry Martz

Don't know how to best test the valve seats without putting part of one to
the grinding wheel and examining the color and pattern of the sparks (have
a manual somewheres that tells how to interpret pattern) - check with a
local machine shop - portable Brinnell hardness tester may help. Cheap
solution - run only Amoco / American 'white' gas ("Ultimate" high octane).
If you're really worried, have the seats replaced, and don't forget about
the valves, too! Alternate - contact a farm supply store (Agway) and buy
tractor fuel (pay road tax with federal income tax form - right!).

Hope that helped!

Good luck!

Larry Schear
Twin Cam, Inc.

What made you realize the need for lead?

I'm sure this is in the archives, but the consensus was at one time, these
engines don't need lead. The Aluminum heads cool the valve seats quite
well. Add whatever makes you feel good, most of us don't worry about it. I
concur and I don't bother with any lead additives for any of my 50s' and
60s' British cars. I could easily come to the conclusion that adding lead
was a conspiracy by the lead industry to sell lots of their product into
our environment lower our collective I.Q. (a benefit of long term lead
ingestion.) to sell us more wonderful products. I will state once again,
lead=badness, short sparkplug life, valve deposits, oogy stuff in the oil
pan as a combustion byproduct.

$0.02 :-)

Ken Boetzer

I'm not an authority on the effects of lead but I remember using Amoco
lead-free gas in my cars in the late 50's and early 60's for many reasons
that were consistent with the Amoco promotions at the time.

I do use an additive in the tank for almost purely nostalgic reasons. I
found a case of Castrol R last year and I have been using about a cup in
each tank of gas. I have heard that it is a good upper cylinder lubricant
but the real reason I use it is it gives the exhaust an aroma that I
remember from the pits at Lime Rock, Bridgehampton, Watkins Glen, etc. when
I was a mechanic in a foreign car shop in Salt Point, New York. My boss
(Bob Purdy) owned and raced an MGTD - one of the last of the breed who
drove it to the races and (hopefully) drove it home afterward. Castrol
doesn't use the oil from castor beans any more so my enjoyment of this
scent of the past will run out some day. I have already used four of the
twelve quarts and, since we drive the 120 about 6,000 per season, the rest
will be gone in a couple of years. In the mean time, it does bring back
Bruce Cunningham
'53 OTS

I too, as Larry Martz noted, thought that all XK motors from October 1948
had hardened valve seats. I discovered this was not true on a recent head
rebuild. My machine shop friend was reworking a Mk II, 3.8 litre, "B" head
for me. During the process of removing the valve seats, he called to advise
that the seats he was removing were of soft material, unlike the hardened,
unleaded-type seats he routinely installs in Jaguar cylinder heads. Visibly
you could not tell the difference. But when he was cutting the seat
material he noted the softness.

While one can speculate that the seats were changed over the years, I don't
believe this to be the case. I bought the engine in 1968 from a friend who
junked his Mk II sedan. The speedo showed 62K. Before beginning the rework,
we examined the head thoroughly and could only see evidence of grinding the
seats, not replacement. So the machine shop was surprised when he cut into
soft seats.

Recognizing that there are many approaches to the issue, I tend to be
conservative and feel I am protecting my engines by using a lead additive
and an octane booster in my 120 and 150 cars. And I might add that I enjoy
the extra tinkering and attention that I give these fine mechanical pieces
as I pour the contents of the bottles. The cost is nominal to the peace of
mind I get. To each his own.

Bob Oates

For what it's worth, Jaguar stated a few years ago in the UK that all
engines after 1976 were fitted with hardened valve seats - which applied to
my Series II XJ6, which is why I remembered it. It implies there may have
been a time when the seats were soft.

One of the UK suppliers recommends brass valve guides for unleaded petrol.
It's interesting an unleaded user mentions a dropped valve problem; I've
run unleaded on my 1981 XJ6 Series III for several years; after 70,000
miles I had a dropped exhaust valve guide that smashed a piston and damaged
the head. I wonder if this is the result of not having the lead lubrication
on the valve stem;
my guides were standard steel. The engine was a non-Jaguar reconditioned
unit, so there's a chance of substandard parts, of course.

Paul Gover
1981 XJ6-III 4.2

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