[v12-engine] Valve clearances too tight, compression test and reading plugs

V12-Engine
1

Fellow V12 drivers.
I am looking for some guidance from the more experienced.
In the attached table and photo I have documented the valve
clearance or tappet gaps, comp test as well as the color of
the 1000 miles old 12 spark plugs.
As you can see the valve gaps are almost all tighter than specs.
The compression test seems OK.
The plugs seem to indicate that the B bank runs leaner than
the A bank. Between the individual cylinders there are
differences in the spark plug color.
Now to my questions:
Should I re-shim the valves that are too tight ?
Is the compression test OK ?
Is there a connection between comp test, plug color and
valve clearance ?

Thanks … Ole

http://www.jag-lovers.org/snaps/snap_view.php3?id=1398076281&n4=--
Ole-XKE1974
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2

In reply to a message from Ole-XKE1974 sent Mon 21 Apr 2014:

we talked back a few weeks ago about exhaust temperatures of
the V12 , and it is assumed that they do run different, on
mine it runs cooler on the right side than on the left.

so that would change the color of the plugs, hot side lighter
than the cool side!

mines been that way for 20yrs,no problems, so I dont give it
any more thought, nothing is ever perfect, but sometimes,in
futile effort we try!–
The original message included these comments:

The plugs seem to indicate that the B bank runs leaner than
the A bank. Between the individual cylinders there are
differences in the spark plug color.
Is the compression test OK ?
Is there a connection between comp test, plug color and

–
Ronbros
daytona fl. / Austin TX., United States
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3

In reply to a message from Ronbros sent Mon 21 Apr 2014:

I followed that thread. It would be interesting/funny if
all V12 or maybe only the older ones with carbs ran this
way.
Any thoughts on the valve clearances ?–
Ole-XKE1974
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4

In reply to a message from Ole-XKE1974 sent Mon 21 Apr 2014:

They are a bit tight, meaning that the seats are receding. That is
normal with our modern gas. Your worst is only .004’’ off,
though…which I do not think is enough to make a difference with
the compression check.

Your compression check looks pretty normal too for an engine with
some miles on it. They seem to all be within 10% of each other.

You can either re-shim to get your clearances back up, or keep an
eye on the regularly. The concern is that, if they ever close up
completely, you will burn an exhaust valve. You are still a way
from that.–
John. '95 XJS 6.0L convertible. Southlake, TX
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5

In reply to a message from CJ95 sent Tue 22 Apr 2014:

Thanks for your thoughts John.
I didn’t think about the gap being smaller due to seat/valve
wear. Do you think that could be the case with my car, when
it only has 40K miles on it and my guess is that it hasn’t
done more than 1K miles in 14 years ? That was me driving
it last year and it was stood for an estimated 10 years
before I bought it in 2010.–
Ole-XKE1974
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6

In reply to a message from Ole-XKE1974 sent Tue 22 Apr 2014:

It’s really hard to predict seat wear. But, any time the
clearances close up, it has to be from the valve seating deeper.
Increasing clearances is from any other part in the valve train
wearing.

If I had to guess why your clearances closed up in so few
miles…and this is completely a guess on my part…is that perhaps
the steel seats get a bit of surface rust while the car is
sitting. When you winterize boats, you give the motor a shot of
fogger oil while you shut it off the last time for the season.
Maybe you could try that when you plan to let the car sit for a
month or so.–
John. '95 XJS 6.0L convertible. Southlake, TX
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7

In reply to a message from CJ95 sent Tue 22 Apr 2014:

Thanks for the explanation John,

what I have forgotten to mention is that some kind of valve
and or cam job has been performed on my car in the past.
At least that’s my thought since some of the cam nuts have
lock washer teeth marks on them, on the surface pointing
upwards.
I guess at any rate it seems like I have to re-shim the
cams which should be a Sunday’s worth of fun.
It’s given me a chance to by a new digital Micro meter, so
it’s like Christmas.

Thanks all … Ole–
Ole-XKE1974
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8

In reply to a message from Ole-XKE1974 sent Wed 23 Apr 2014:

good idea , and run all clearences looser than specs!

like 3-4 thousandths loose, engine runs smoother, and has
a nice cam tappet sound, more torque.

and it gives some space for any future clearence tighting–
The original message included these comments:

I guess at any rate it seems like I have to re-shim the
cams which should be a Sunday’s worth of fun.
it’s like Christmas.

–
Ronbros
daytona fl. / Austin TX., United States
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9

In reply to a message from Ronbros sent Wed 23 Apr 2014:

Thanks Ronbros … Ole–
The original message included these comments:

good idea , and run all clearences looser than specs!
like 3-4 thousandths loose, engine runs smoother, and has
a nice cam tappet sound, more torque.
and it gives some space for any future clearence tighting

–
Ole-XKE1974
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10

like 3-4 thousandths loose, engine runs smoother, and has
a nice cam tappet sound, more torque.

The fact that it has more torque is one good reason to buckle down
and do it. It has to do with the “quieting ramps” that are built
into the cam profiles. Basically, the cam profile tries to gradually
close up the valve lash BEFORE it starts the actual valve opening.
As a result, clearances on the low side tend to cause the valves to
open – just a hair – long before they’re supposed to, and stay open
– just a hair – long after they’re supposed to have closed.
Besides causing a considerable reduction in low-end torque, this
can’t be good for valve life.

Those same quieting ramps also can cause confusion when measuring the
clearances. You’ve gotta be very sure to be all the way around on
the heel of each cam lobe. If you try to measure anywhere near the
actual lobe, you may actually be on a quieting ramp and get smaller
clearance measurements than you should.

Also remember that running clearances aren’t the same as cold
clearances, which are what you can measure. The whole point of the
clearance spec is that it won’t totally close up under any condition,
including shortly after startup when the valves themselves are hot
but the head is still cold.

I dunno if 3-4 thou is the best advice. IIRC, it was different on
exhaust than on intake. The exhaust should be 3-4 larger than spec,
the intake should be right around the wide end of spec, .012" I
think.

Doing this job involves removing the cams, which in turn involves
fiddling with the timing chain tensioner. Be sure you understand
what’s involved there. I think most of us who have done this job
would recommend NOT trying to fully retract the tensioner to its
latch. Rather, just go ahead and take the sprockets off and set them
on their supports, adjust the valves, and then “trip” the latch on
the tensioner while putting the sprockets back in place.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 23 Apr 2014 at 6:32, Ronbros wrote:

11

In reply to a message from Kirbert sent Wed 23 Apr 2014:

good advice Kirby;

all, including mounting the sprockets on the mount support
bars.

THX–
The original message included these comments:

The fact that it has more torque is one good reason to buckle down
and do it. It has to do with the ‘‘quieting ramps’’ that are built
Doing this job involves removing the cams, which in turn involves
fiddling with the timing chain tensioner. Be sure you understand
what’s involved there. I think most of us who have done this job
would recommend NOT trying to fully retract the tensioner to its
latch. Rather, just go ahead and take the sprockets off and set them
on their supports, adjust the valves, and then ‘‘trip’’ the latch on
the tensioner while putting the sprockets back in place.

–
Ronbros
daytona fl. / Austin TX., United States
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12

In reply to a message from Ronbros sent Wed 23 Apr 2014:

Thanks for advice to both Kirby and Ronbros … Ole–
Ole-XKE1974
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13

In reply to a message from Kirbert sent Wed 23 Apr 2014:

Just try not to go overboard with increasing valve clearances. If
the cam lobe ramps weren’t needed, they wouldn’t put them there.
Increasing the clearance allows the lobe to slam the lifter,
shocking the entire valve train. Springs, valve seats, keepers,
and timing chain all have to absorb the hits.

These engines are pretty mild…but on a real performance engine,
the quickest way to trash the motor is to spin it fast with too
much valve clearance.

If you want more low end…a much better way to get it is to
advance the cam about 3 degrees. The difference in low end torque
is much more noticeable than 3-4 thousandths of increased clearance.–
John. '95 XJS 6.0L convertible. Southlake, TX
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14

In reply to a message from CJ95 sent Wed 23 Apr 2014:

All changes to V12 cam forms and tappets have been in
pursuit of refinement. There were a number of detail
changes in the early 1970s ending with C42176/7 which
lasted until the 6 litre when further refinements were
introduced. The dealer information bulletin about the 6
litre improvements mentions the changes to the cam profile
and inlet valve but does not mention any change to the
tappets. It may therefore be that the change to a lighter
tappet could have been a running change introduced at some
other time, possibly as a result of similar changes
introduced earlier on the AJ6 engine.

Opening the clearances from 0.012-0.014� range to
0.014-0.016� made measurable improvements to torque,
emissions (particulary HC) and specific fuel consumption.
In fact, at one point V12s for the US market were assembled
to the wider setting range in order to pass FTP-72
emissions. A higher level of tappet noise had to be
accepted for these engines.

The C42176/7 cam form has constant velocity ramps extending
to 0.020� lift so any clearance within that range will be
collected by the ramp at the same rate of lift.

Because the exhaust clearance closes up with running
temperatures it can tolerate wider settings than the inlet
without becoming noisy. For most people setting inlets to
0.014� and exhausts at 0.016� would be the best solution.–
Roger Bywater / AJ6 Engineering
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15

In reply to a message from RogerBywater sent Thu 24 Apr 2014:

THX ROG, your info should clear this topic up!

some times we over think the knowledge we have accumulated,
without any actual experience. hehe.–
The original message included these comments:

Opening the clearances from 0.012-0.014� range to
0.014-0.016� made measurable improvements to torque,
emissions (particulary HC) and specific fuel consumption.
In fact, at one point V12s for the US market were assembled
to the wider setting range in order to pass FTP-72
emissions. A higher level of tappet noise had to be
accepted for these engines.
to 0.020� lift so any clearance within that range will be
collected by the ramp at the same rate of lift.

–
Ronbros
daytona fl. / Austin TX., United States
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16

In reply to a message from Ronbros sent Thu 24 Apr 2014:

Thanks guys.
I’ll set the gaps as per Roger’s recommendation.–
Ole-XKE1974
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17

In reply to a message from RogerBywater sent Thu 24 Apr 2014:

.020’'! That is the largest ramp I have heard of. It
sounds like Jag knew the valve clearance wouldn’t be
checked very often in service.–
The original message included these comments:

The C42176/7 cam form has constant velocity ramps extending
to 0.020� lift so any clearance within that range will be
collected by the ramp at the same rate of lift.

–
John. '95 XJS 6.0L convertible. Southlake, TX
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18

In reply to a message from CJ95 sent Fri 25 Apr 2014:

0.020’’ (0.5 mm) ramp is not unusual for solid tappets. Any
clearance up to 0.020’’ will be caught by the ramp at the
same constant velocity.

Cams intended for hydraulic lifters generally have smaller
ramps.

More recent cams tend to have more complex ramp profiles
with less change of lift in the active area to improve
refinement.–
The original message included these comments:

.020’'! That is the largest ramp I have heard of. It

–
Roger Bywater / AJ6 Engineering
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19

In reply to a message from RogerBywater sent Sat 26 Apr 2014:

A couple of weeks ago I took the plunge and fitted the new
shims to achieve the recommended .014’’ and .016’’ clearances
for the intake and exhaust valves respectively. What I
didn’t have in my calculations were that the previous
owner/mechanic/idiot probably didn’t apply any sealant
between the head and the cam block. How else can it be that
all my clearances ended up .002’’ bigger than expected. I
decided to leave them as they are and will drive with about
0.016 intake and 0.018 exhaust valve clearances. Hope to
get the engine with 4SP & OD in and started this weekend.–
Ole-XKE1974
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20

What I
didn’t have in my calculations were that the previous
owner/mechanic/idiot probably didn’t apply any sealant
between the head and the cam block.

Sounds like you used the wrong sealant. You should be using a
sealant that squeezes out as the bolts are tightened so it makes no
difference to the valve clearances. I think Loctite 518 fits that
bill, although not entirely certain. There are many sealants, such
as Permatex #2, that most definitely do NOT fit the bill, they’ll
result in wider clearances for sure.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 15 Jul 2014 at 9:54, Ole-XKE1974 wrote: