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MG MGF Technical - Headbolt torque setting on a vvc

can anyone possibly tell me what it should be as I have a query.
TIA
Gareth
Ddraig

Gareth, I presume you mean the cylinder head stretch bolts?

These are torqued to an initial setting of 20 Nm, and then in sequence are turned 180 degrees and then through an additional 180 degrees. Therefore, there is no definitive final torque setting, as such.

Hope that helps!
Rob Bell

Ah. So what happens if they were torqued to 25Nm and then 2x180?
Ddraig

Nothing untoward will happen, they are operating in the plastic region so overtightening to a degree will not hurt, the clamping force will be similar. You may suffer some degree of work hardening on the bolts. Eventually they will neck and yield.

Dave
Dave Andrews

So would it be worth having it redone?
( I saw a note the mechanic had made stating 25Nm and 2x180)
Ddraig

>>So would it be worth having it redone?<<

I suspect not, unless you have reasons for suspecting a problem. Are these stretch bolts the engine's originals?

Dave is your man here, but I suspect that there isn't a definitive answer one way or the other.
Rob Bell

Leave well alone.

The difference in stretch between starting at 25nm instead of 20nm is minimal and probably less than the differential between one bolt and any other.

Dave
Dave Andrews

Please correct me, but I think:
25Nm or whatever is for sure nearly anyway and only to overcome probabble gaps between head, gasket and block and first friction in the thread. So put all this parts 'on block' together.

2 times 180 degrees in sequence is to put the pre-load on the bolts. This pre-load must be inside linear elastic stretch range.
This is one thread pitch height due to one complete turn of the bolt.

Any more will lengthen the bolt and make the stretch range circumference smaller. (smaller diameter means less pre-load)

BTW, what thread pitch have the K-Engine bolts ??
If any out there could supply ALL measures of such a bolt, I would be able to do some calculations (applied force per bolt etc.)

http://imperator.cip-iw1.uni-bremen.de/skripte/Schraubenverbindungen.pdf
Unfortunately german language only. See page 55 i.e)

Found this english language doc useful to explain. See page 22
http://www.gmrc.org/gmrc/techpaperspdf/2002/bolted2.pdf

Regards
Dieter
Dieter K.

Hi Dieter,

M10, 1.5 mm pitch? I'll try to blag a used bolt tomorrow.

Regards, Kes.
Kes

Kes, you're the greatest.

Measures required:

From top to bottom all lenghth (top down to shaft, stretch shaft height, etc ) from below the bolt head.
The affiliated diameters at each significant step (Best in resolution of better than 1/10 mm)(Or true measures inch)

In short. A correct drawing :)
The strech shaft diameter at a used bolt must be smaller than at new.

:o)
Dieter K.

The bolts are M9, not M10..

Sure the bolt circumference may be a little bit smaller when the bolts are not new, but deforming in the plastic region will work harden them which may actually increase the clamping force...

See the dialogue between a friend and a metalurgist on the subject of head bolts...

[Nig]Does this mean the additional loading is reduced relative to the additional elongation? In other words, taking the practical example of the head bolts, you can't overtighten them by an accidental extra 1/4
turn (or somesuch) because it won't crush the head gasket..?

[Chris]As long as you have not exceeded the maximum load capability of the bolt the clamping force will continue to rise with increased rotation BUT at a much reduced rate.

The Young's Modulus of 200GPa tells you the change in force per unit elongation in the ealstic region but once you yield the effective modulus becomes the slope of a secant drawn at a tangent to the amount of strain
in the bolt. ( A bit complex but I hope it is clear) i.e. the slope is much reduced after yielding.

In simple terms a lot more stretch for a relatively small load increase.

A 1/4 turn in the elastic region is a lot more force than a 1/4 turn in the plastic region which is exactly why they are more consistent.

[Nig]I see. So the reason why we shouldn't re-use the bolts is because we can't predict whether they've already entered this necking stage?

[Chris]Exactly, you don't really know how far along the locus of the tensile curve you have travelled and how much uniform elongation you have left. Working in the region of non-uniform elongation is really
dodgy.

[Nig]Although in practice we have been told that if they are within the tolerated Xmm length (when not in tension) then they can be re-used. I assume the thinking behind this is that while they remain short enough to be within this tolerance they will not have got to the maximum load stage?

[Chris]Sounds basically reasonable as the ratio of uniform elongation to overall length should be consistent to within 1-2% and providing sufficient saftey factor has been allowed all should be well and
failure/unloading shouldn't occur.

I think on a really expensive engine I may replace bolts anyway but that is just fear overuling science.

[Nig]But by considering what you say, they will get longer with each use. That's a fact..!!???

[Chris]Yes, If you re-use them and they don't get longer between use then you have converted them from "stretch" bolts to elastic bolts and lost the benefits. (Don't forget the definition of elastic means it returns to its original length when load is removed.)

[Nig]So the elongation that people report in their "free" bolts is owing to genuine stretch through being wound up past their yield then. Makes sense.

[Chris]Almost certainly, Creep is a three stage process, strangely called Primary Creep, Secondary or steady state creep and tertiary creep.

It is true to say that a small amount of primary creep may occur almost instantaneously, on initial loading but it is unlikely that any garage would have the laser interferometry system you would need to measure
elongations at this level. For example the primary creep of an aero enine turbine blade operating at 1100deg C needs proximity probe technology to
measure. If you have a micrometer and a steel bolt at 150 deg C operating you can forget it,day to day temperature changes would be more significant.

[Nig]They just say to torque each of them to 15Nm and then 2 x half turns.

[Chris]Bit Bloody crude. I think Ford do quite a bit better on Zetecs.

There is much more dialogue which confirms that overtightening in the plastic region wont overload the bolt and that unless the creep/elongation is well out side published figures the bolt wont 'neck' and that the work hardening isnt sufficient to be a problem unless the bolt has gone well beyond its yield point.

Dave
Dave Andrews

Dave, many thanks for explaining.
I think it's a good base to verify our later 'theoretical calculation results' which never can't be the whole truth due to inhomogeneous material, material itself, surface etc. but would help understanding the technology.
Engineers fun in any way.

BTW, I have worst memory regarding my own metalurgist practicing from the university 30 years ago. Was happy getting another job ;)

Regards
Dieter
Dieter K.

This thread was discussed between 21/03/2003 and 22/03/2003

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