[v12-engine] Plastic Parts in Engines

V12-Engine
1

This is a bit off-topic, but I believe it is pertinent to the
various threads about the plastic tensioner in the V-12.

Mercedes V-8’s, at least the pre-1990 ones, have a somewhat similar
issue. In case you are not familiar, they are an aluminum block,
twin overhead cam layout, like the Jag V-12, with twin roller
chain. They have a more durable tensioner, it is a spring-loaded
idler sprocket (all metal). But where they screwed up is in the
guide plates. The Jag has heavy gauge steel guide plates, but in
the Mercedes V-8 they are made of plastic. These things are the
achilles heel of the Mercedes V-8. The guide plates are there to
keep the chain from slapping around at high rev’s. They get brittle
with heat and age; the deterioration of the plastic has almost
nothing to do with mileage. If a piece breaks off at low speed and
falls down into the crankcase, no big deal. But if it breaks off
at 6000 RPM (more likely), the pieces can get sucked up into the
cam sprocket. The timing chain jumps teeth, pistons bang into
valves, and you are looking at a VERY expensive total engine
rebuild. Which could have been prevented by replacing fifty bucks
worth of plastic parts. This tragedy has happened to a lot of
owners of older Merc V-8’s. It’s crazy because the engine is
otherwise so rugged it can go 300K miles with the proper
maintenance.

Granted the Jag tensioner may not have a history of catastrophic
failure at high RPM (AFAIK), and replacement is more difficult and
expensive than the Mercedes guide plates. But I’m beginning to
think, ANY plastic part inside ANY engine is going to experience
accelerated aging, and therefore should be replaced at specific
intervals, not just mileage but years and/or mileage whichever
comes first. Some of you with engineering backgrounds might be
familiar with HALT and HASS testing. These are highly accelerated
life tests that are performed on products to find weaknesses, that
involve extreme temperature swings along with vibration. A plastic
part inside an engine is in an environment not unlike a HASS
environment. It is no wonder the plastic parts deteriorate so
quickly. In something so critical as the timing chain path, I would
look at it from a risk management perspective. IMO the cost of
doing a pre-emptive fix is a very good deal compared to the
potential cost of some sort of catastrophic failure. It’s like
buying insurance. I know, I’m preaching to the choir here. End of
soapbox.

KN–
Karl Nelson '86 HE Coupe
Monroe WA, United States
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2

At 19:49 2005-06-06 +0200, Karl Nelson wrote:

Mercedes V-8’s, at least the pre-1990 ones, have a somewhat similar
issue.

The stock timing sprocket on my Buick engine (old big block, not some dinky
new thing with a rubber timing belt) had NYLON TEETH. Presumably, this was
to dampen the sound, but as the part ages, the teeth get brittle, they
start to fracture (and appear in the oil sump), and once that happens, the
timing starts to get erratic. Very annoying to be checking the timing and
wondering why the mark is bouncing across a 6 or 8 degree range. Also, if
the teeth were to start failing rapidly, you could end up with a stripped
sprocket if you suddenly applied a lot of torque (which the engine in
question is seething with), which wouldn’t make your day. Fortunatley, I
changed my own oil and noted the plastic bits appearing in the sump and
investigated – I’m not sure Joe Bob at the quickielube would check or
bother to mention it…

Needless to say, when I replaced the sprocket, I went all-metal.

The natural progression here is to ask: has anyone produced an all metal
tensioner for the V12?

Seems to me that a heavy spring-loaded idler might have made a lot of
sense. Anyone who has dealt with replacing a serpentine belt on a newer
car (well ~ 10 years or so), knows what I’m talking about.

— '88 Jaguar XJ-SC 5.3L V12 (LHD) ‘Black Cat’
Sean Straw '85 Jaguar XJ-S 5.3L V12 (LHD) ‘Bad Kitty’
Sonoma County, California '91 Jaguar XJ40 4.0L (LHD) ‘Trevor’
http://jaguar.professional.org/ '69 Buick GranSport 455 V8

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3

Sean Straw wrote:

The stock timing sprocket on my Buick engine (old big block, not some
dinky new thing with a rubber timing belt) had NYLON TEETH.
Presumably, this was to dampen the sound, but as the part ages, the
teeth get brittle, they start to fracture (and appear in the oil
sump), and once that happens, the timing starts to get erratic. Very
annoying to be checking the timing and wondering why the mark is
bouncing across a 6 or 8 degree range.

My '66 Mustang 289 V8 “jumped time” on me once, so I had to go in
there and replace the timing chain and sprockets in a parking lot.
The crank sprocket was steel, but the cam sprocket was – gee, what’s
it called? – “micarta” or some such, fiber-reinforced hard plastic.
This was what’s called a “silent chain”; the chain and sprockets are
designed with gear teeth rather than rollers, which makes the drive
constant velocity. A roller chain actually is not constant, the
driven sprocket changes speed with each tooth, which introduces a
high-pitched vibration.

Everybody blames those silent chains and plastic sprockets for all
sorts of stuff, and in fact it’s common to replace them with roller
chain. I think the silent chains are actually a decent idea. The
biggest problem is the chain itself, which thanks to the multi-plate
design has a high wear rate. This results in accelerated chain
“stretch” (which isn’t really stretch, it’s wear), which results in
the thing eventually jumping time – the chain hops over the teeth.

Of course, those engines have no tensioner. As a result, the
ignition timing bounces around a bit when the timing chain is NEW,
and the jumping just gets worse as it wears. If it had a tensioner,
jumping wouldn’t be an issue.

Many years later I learned that it was possible to replace that chain
with a set of gears. That woulda been nice, but I dunno if it was
available at the time.

Now, my wife had a '68 Toyota with pushrods. In this car, the cam
was driven by gears, not a chain. The crank gear was steel, the cam
gear was micarta with a steel hub. On the way home one night
(dressed in white formal wear, just to make things interesting!) the
car just stopped. Come to find out the plastic gear looked just
dandy, but in fact had spun around on its steel hub. Took a while to
figure that out. A slightly more positive connection between gear
and hub, and that thing would have lasted forever.

On my parents’ ‘73 Volvo, the camshaft had case hardening problems
and wore one lobe completely down to nuttin’ so it was running on 3
cylinders. While replacing the cam, I couldn’t get the gear off and
ended up breaking the micarta gear. Obviously not an operational
problem, but I still dunno how I could have gotten it off without
breaking it. Got a replacement gear from a junkyard, and watched the
guy get an identical gear off a junkyard engine with a coupla whacks
with a hammer to split the nut and just slid the gear off with his
fingers.

Boy, there’s a pile of experience! All in all, though, I didn’t have
any problem with the use of micarta in these applications. The
problems I had were all incidental to the use of micarta, and could
have been avoided with design improvements. Switching to steel would
have been uncalled for IMHO.

Of course, all those examples are gears – which don’t flex. The
tensioner in the V12 flexes, which means it’s not micarta but rather
something like molybdenum-impregnated nylon. I still don’t think
that’s a bad idea. What I do think is a bad idea is that the
plastic of this arch is the flexible strutural member. The arch
should have been made of spring steel with the nylon a non-structural
facing riding on it.

Same kinda thing is probably true regarding those Mercedes damper
plates. Obviously it was a mistake to make them out of plastic. A
better idea would have been to make them out of steel with a plastic
facing.

The V12’s damper plates are steel without facing. I wonder if
there’s a functional difference there. For example, I wonder if the
Jaguar plates are intended to slightly miss the chain, and therefore
they don’t touch unless the chain starts whipping, whereas in the
Mercedes maybe the chain is actually supposed to slide along the face
of the damper plates during operation.

– Kirbert

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4

In reply to a message from Kirbert sent Mon 6 Jun 2005:

I was wondering about the spacing of the guide plates in the V-12
myself. After I got the cam covers and timing cover off, I was
sighting down the chain path and poking and prodding around.
Actually there is no spacing to speak of, at least on my engine.
The guide between the RH cam sprocket and the jackshaft actually
presses down on the chain a little. The guide between the LH cam
sprocket and the crank sprocket appears to be also set to zero
clearance, but not pressing in on the chain as tightly as the
former. It could be that on higher mileage engines, some space
might develop due to wear on the chain and the guide. Or maybe it
is just something the Jaguar engineers futzed around with over the
years.

On the Merc engine I am not sure if the guide presses on the chain
or has clearance. Haven’t looked at it yet. That is the next
project after Kitty gets back on her feet.

My only other overhead cam experience was with a 1980 Toyota Supra
straight 6. Great engine. The cam sprocket was steel but the teeth
had an overmolding of some sort of plastic, to make for a quiet
chain I guess. Eventually the plastic would fall away and you were
left with pointy steel teeth on the sprocket, which still kinda
worked for awhile until it eventually jumped teeth, and the engine
was toast. Come to think of it, I think that one was gear-shaped
teeth as opposed to rollers as well.

That’s always the problem with overhead cam engines, they need such
long timing chains and it is hard to control the resonances that
make the chain want to vibrate like a guitar string at certain
RPMs. I wonder why they don’t drive them with gears, but then I see
the guys at the local hot rod gathering with their souped-up Chevy
V8’s with the gear drives - the gears are so loud you can hear them
coming from 3 blocks away.

KN–
Karl Nelson '86 HE Coupe
Monroe WA, United States
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5

Karl Nelson wrote:

After I got the cam covers and timing cover off, I was
sighting down the chain path and poking and prodding around. Actually
there is no spacing to speak of, at least on my engine. The guide
between the RH cam sprocket and the jackshaft actually presses down on
the chain a little.

I’m betting that ain’t right. However, I’m not sure how to tell what
Jaguar’s original intentions were. The offical setting procedure
involves a special tool, but I’ve never had my paws on the tool.
It’d be handy if somebody actually used the tool on an actual engine
and reported on exactly how the damper plates sat relative to the
chain path.

It’s always been my guess that they are not supposed to touch the
chain as long as the chain remains on the straight and narrow path.
I don’t see why it needs to touch them when they’re not whipping, and
if it does it’ll just cause a lot of noise and unnecessary wear. But
that’s my guess, not the word from Jaguar engine designers.

The guide between the LH cam sprocket and the
crank sprocket appears to be also set to zero clearance, but not
pressing in on the chain as tightly as the former.

Now, y’see, the two sides being DIFFERENT definitely doesn’t sound
right to me.

It could be that on
higher mileage engines, some space might develop due to wear on the
chain and the guide.

The wear on the sprockets will be negligible; it’s not as though
they’ll get noticeably smaller in diameter. Wear in the cam journals
would seem to be likely to move the sprocket towards the crank, which
shouldn’t move the chain toward or away from the damper plates. Wear
in the crank bearings isn’t gonna move that sprocket more than a
coupla thou. Wear on the chain itself makes it a hair longer, but
the only effect that should have is in a change in the tensioner’s
position.

Or maybe it is just something the Jaguar
engineers futzed around with over the years.

Has anybody here ever heard of any problems with these damper plates?
Or any TSB’s related to them? For that matter, any significant
history of problems with the chain itself that might be explained by
the damper plates not doing their job? Just about the only issue we
ever have there is the tensioner itself, which appears more of a heat
and age issue than a damper issue.

That’s always the problem with overhead cam engines, they need such
long timing chains and it is hard to control the resonances that make
the chain want to vibrate like a guitar string at certain RPMs.

I like the Offenhauser idea better: a whole series of idler gears
from the crank up the front of the engine and off to the two banks.

I
wonder why they don’t drive them with gears, but then I see the guys
at the local hot rod gathering with their souped-up Chevy V8’s with
the gear drives - the gears are so loud you can hear them coming from
3 blocks away.

They probably don’t use helical gears, and they probably are all
steel. Make them helical and made of micarta and they should be very
quiet – as quiet as the chains.

Even so, your basic Chevy – regardless of cam drive scheme – would
benefit considerably by replacing the timing chain cover with a cast
aluminum cover, or even better a cover with some built-in sound
dampening.

My '73 Triumph GT6 has a roller timing chain with a sheet metal
cover, and it also had a really hokey tensioner – a simple arch of
spring steel that just leaned up against the slack side of the chain.
You could hear it a mile away. I eventually learned that the cover
was originally supposed to have some stuff bonded to its outside
surface to dampen all the racket, but at some time before I got the
car either it fell off or the PO scraped it off.

– Kirbert

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6

The V12’s damper plates are steel without facing. I wonder if
there’s a functional difference there. For example, I wonder if the
Jaguar plates are intended to slightly miss the chain, and therefore
they don’t touch unless the chain starts whipping, whereas in the
Mercedes maybe the chain is actually supposed to slide along the face
of the damper plates during operation.

– Kirbert-----------------------------------------------------------

The damper plates have a very hard black ( rubber ? ) wear track bonded to
them.
I set the damper plates on the pre HE so the chain was just kissing the
wear tracks.
Seems to work alright. So did the same with the HE engine.

Those wear tracks on the 2 engines I have stripped don’t appear to wear much.

Richard Dowling, Melbourne, Australia, 1979 XJ-S & 1985 XJ6.

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7

Richard Dowling wrote:

The damper plates have a very hard black ( rubber ? ) wear track
bonded to them.

No kiddin’! I never noticed that. Anybody else seen this hard
rubber facing? I wonder if it was missing on my '83 for some reason.

– Kirbert

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8

In reply to a message from Kirbert sent Tue 7 Jun 2005:

All the talk of plastic timing gears sure brings back the memories.
Back in the 70s, when I was mis-spending my youth, a buddy of mine,
and self decided to drive his old Volvo 142 out to colorado. We
left DC, drove up to Cleveland to visit relatives, and made the
rest of the trip on US50. We stopped for a swim in a man made lake
in western Kansas. When we continued, we decided to see how fast
the loaded Volvo would go.(straight roads, no traffic) It went 84.
After that the old b20 was tapping a bit. We figured that perhaps a
valve adjuster had loosened, and ‘‘No big deal’’ We stopped for gas
in Las Animas Colorado, and when we tried to leave the engine just
spun.After a bit of checking, we determined that the cam was not
turning. Oh OH. We went to the only auto part store in town. They
orderd the timing set from the dealer in Pueblo, and had them put
it on the next Grayhound that was coming through. They were kind
enough to lend us the giant socket for the cam nut, so we tore the
engine down in a conveniance store parking lot. We were able to fix
it without any more trouble. We broke down around 10 am, and were
back on the road by 1 am the next morning. Not too bad for a couple
of kids. I will be forever grateful to the folks who worked in that
store. Without their kind assistance we would really have been
screwed. I try to remember that whenever I am given the opportunity
to help a traveler. And I would still like to choke the engineer
that specified the plastic gear. Oh yeah, I need to do the chains
and tensioners on my 3.6 xjs. The car has 95k on it, and I am
surprised that they have failed so early. Perhaps I have plastic
tensioner blades.–
the Pro from Dover
washington d.c., United States
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