Cylinder head - Torque down and head gasket problems
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11 July 2001
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A regular visitor to the message board, and something I
am asked frequently about in my other incarnations for other technical media.
Generally when something has gone badly wrong! Many folks have just as many
ways of dealing with the following subjects - all can and will claim their way
has worked for them for the proverbial millennia - so a few words as a basic
reference and guide as I see them; we all know there are always exceptions to
the rules. This is to help those floundering in an unknown sea of techno-babble
and 'magical' cures…
First a brief word on head gaskets. If it is not a
genuine Rover or Payen gasket, don’t use it; at least
half the problems presented to me are caused by cheapo head gaskets. If it’s a
copper-composite gasket, make sure it’s got a slate-grey coloured ‘filling’ and
a silver spray-paint-like coloured underside. If it’s a shiny tin/steel colour
(looks like clean bare metal), forget it. If it's got a copper top surface and
a black under-side (looks like painted metal), forget it. If it’s a dark
charcoal-coloured-looking thing, chances are it’s
genuine Rover. And the small-bore, strange, silver-grey-coloured things that
look more like exhaust manifold gasket material than head gasket material
supplied in Rover/Unipart head gasket kits are OK for standard motors. Anything
else - leave well alone!
OK. Basic brief on head gaskets over
– head studs and torque settings. Now, I've heard all SORTS of weird
stuff about how folk torque their head studs down. VERY
weird. I don’t understand any of this as it’s plainly laid out in black
and white print in a million manuals. All you have to do is torque the standard
head studs down to 42-45lb ft (a generous lee-way there I feel), DRY. That’s no
lubrication on nuts, washers, or studs. ALL torque settings in the manuals are
quoted with the relevant threads DRY. And that's the way they're done at the
factory.
‘Ah-ha’, many have said (or are saying now),’but I’ve
got a set of super-fandangly-mega-maglorious head
studs in my soopa-sonicly modified
thirteen-eighty-fourteen-thirty-fourteen-hundred-offset-over-wangled mota!’ - assuming something more brutal in the way of
torque-settings will be needed to contain the rampant mega-horsepower this
incredible unit will no doubt produce. Or why the standard set-up is finding it
impossible to deal with all that 'energy'. Well, the vendor should have issued
you with the relevant instructions on how to use them. Or perhaps it's a case
of ‘I know how to fit these!’ Believe me when I say - once I have enlightened
these folks, the faces said it all.
When using almost any engine cylinder head stud/nut
combination – be it standard or an up-graded after-market type – there are
certain points that must be remembered.
1 – DO NOT use any sort of thread lock compound. Yes,
some folk did. No, I couldn’t believe it either. I have to say it’s a new one
on me. Big-end bolts/nuts? Yep. Heard
that one. Mains bolts/studs/nuts? Yep. Heard that one too. But head studs/nuts? Nope. Never. The excuses (sorry, reasons) were many – but the
‘well I couldn’t afford new ones, and I thought the old ones might have worn
threads’ was the most common. Unless you use the specific thread-locking
compound that is made just for the application you are using it for - it will
detract markedly from the actual torque setting - some by as much as 30%. This
is because the compound starts to 'set' with the friction generated when doing
a nut/bolt up. Plus it can get under the head of the nut/bolt, set when torquing up, but will 'powder' from vibration/pressure when
the engine is used. A small gap is then caused, and the torque setting/clamping
pressure is relaxed/reduced. Not good.
2 – If standard spec studs/nuts are used, do them up
with NO lubrication to 45lb ft. If you must oil the threads, do them up to 42lb
ft, or if using a moly-based oil, 40lb ft.
3 – If using either Mini Spares up-graded stud/nut set
OR ARP ones, use a lubricant – and preferably a moly-based one – and torque ‘em
up to 45lb ft.
Why? The main reason (there are several) is because
over-torquing will almost certainly distort the gasket
to the point where failure is likely to occur. The most likely areas are the
fire-rings that seal the head face (chambers) to block face (bores) off.
Particularly between the bores where the steel fillet inserts are in
non-copper/composite gaskets, and where modified heads have been a little
over-zealously ‘relieved’ around the gasket fire-ring line. Especially between
the centre two exhaust valves.
What happens here is the
excessive torque clamps the fire-ring down so hard it forms a sharp edge that
either splits, or gets burnt away by combustion chamber temperatures. Or in the
case of the steel-fillet insert type; it twists the fillets out of place. In
either case this then blows combustion pressure passed the gasket, pressurising
the water jacket. The engine then over-heats or forces water out the front
joint between head and block. Uh-hu?
Yeah? Makes sense? Happened to you did it? Now you
know why. The other scenario is the block and/or head is burnt away between the
middle two bores/chambers. Expensive to sort, that.
Alternatively, particularly on standard specifications
studs and nuts, increasing the torque value stated will deform the threads on
the nut-end of the studs - inverting the thread rather than pulling the head
down. This weakens the thread engagement capability, slackening off further
when the engine gets hot.
And then there's the problem of pulling the stud
thread out of the block. A particular problem when up-rated studs are used, the
user believing these super-strong studs will allow higher torque-down values to
be used to clamp that head down. Problem is there is not sufficient thread
engagement in the block (only a coarse thread remember going into relatively
'soft' cast iron) in a vast majority of cases for the thread engagement to withstand
grotesque torque values. 60lb ft is really pushing the holding capability of
the standard thread and thread depth. A great deal more thread engagement is
needed to cope. But then you just go back to the aforementioned head gasket
problems.
The point of using up-rated studs/nuts is to achieve
maximum clamping force through quality thread production (maximises engagement
area and reduces thread deformation) and reduced material stretch. So the
lion's share of the torque applied goes in to clamping the head down, not
stretching/deforming the stud.
As a closing note - an example of 'other folk do it
differently'…
A very good friend of mine, whom I raced against and
have known for many years surprised me a couple of
years ago. We were discussing things technical to do with engines. Can't
remember how the subject came up - but head nuts torque settings came to the
fore. Cutting a long story short - he stated he had only ever used 40lb ft on
his engines - and he's been messing about with race A-series motors for longer
than I have. He was (is - sorry Steve) a very quick competitor who is
definitely always at the sharp end on the field and very, very rarely suffered
mechanical failures. None were ever head gasket orientated…makes you think, eh?
Useful part numbers:
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C-AHT280 |
Mini Spares
up-rated head stud kit (11 stud suitable) |
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C-AHT288 |
'ST'
disc-spring washer set |
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CAM150 |
A+ long head
stud |
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CAM151 |
A+ short head
stud |
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TAM1617 |
A+ short head stud
with extra threaded section for coil Mounting |
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CAM4545 |
A+ head nut
with built-in flange/washer |
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51K1193 |
Original head
nut - more thread engagement than CAM4545 |
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GUG702560 |
HG Latest
non-asbestos, steel reinforced, composite head gasket, large-bore
engines |
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TAM1521B |
'Silver-paint'
coloured steel reinforced composite head gasket - forerunner to above |
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GEG300 |
Payen
copper-composite head gasket, large-bore |
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C-AHT188 |
Payen competition
copper-composite head gasket, large-bore - has reinforced fire-rings and
larger forward head stud holes to prevent gasket distortion by head stud
'pull-up' |
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C-STR1057 |
Turbo competition
steel-reinforced composite head gasket, large-bore. Only suitable for bores
up to +0.040", and requires special block machine-work to fit |
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GUG702506HG |
Payen
copper-composite head gasket, small-bore |