Stretch bolts

[colinxm]

Can anyone explain to me why the norm now seems to be for "stretch" cylinder head bolts that you first torque down to a low value and then tighten by degrees - e.g 260deg.

How is this better than tightening in stages with the good-old torque wrench to a given value ?

Colin.P

[Dean]

The problem with using a torque wrench to try and evenly tighten down something like a cylinderhead is that one dirty or oily thread and you have one bolt too tight on not tight enough, this is bad news for your soft alloy cylinderhead, with stretch bolts if one bolt is too tight it will stretch to even out the pressure evenly between all fixings giving a more even clamping force, it is the way forward i believe

D

[colinxm]

Yup, that makes sense

Cheers Dean

Colin.P

[Dieselman]

Angle tightening is a more accurate method of applying a given clamping force. The initial torque tension is there just to apply a low but even preload, the angle tightening is what applies the clamping force.
Stretch bolts are used to keep the bolt within it's elastic range of tension, thus, as Dean said, they apply a more uniform clamping force.

People tend to forget how a bolt works, it stretches in length a given amount for each revolution. On some large engines the head nuts are not torqued down at all. The stud is stretched using hydraulic jacks and the nut is wound down hand tight only, then the jacks are released.

[citroenxm]

Possibly why the gasket blew on my 2.1

When I was undooing the Head Bolts, they were tight in different tightness ... um.. some were slacker then others..

When I assembled them I tightend down to second torque, then made a reference on the car and then tightened them by the angle Id made..

See what happens now - so far 650 miles ok! No water loss or power loss..

Paul

[Dieselman]

Paul.
Was the new laminated gasket noticably different in thickness to the regular sintered one? I've spoken to a reconditioners today and they said unless skimming the head just use a regular gasket.

I'm confused now, but might use a regular one as the original one has been on nearly 20 years anyway.

[Dean]

not paul but...................i wouldnt worry about the thickness of the gasket, you will never notice it at all and niether will the engine, even if you have to fit a head saver and the top end has not been skimed you would not notice any difference.
ive come to the conclusion that mls are fine for cast blocks or dry liner engines but are a bit iffy on wet/damp liner engines especialy if the liners have sunk a fraction.
The more i look the more mls failures i see so they are by no means indestructable, i have a feeling they were spec'd for vehicles that were renowned for blowing the fire seal out of the H/G or where rigidity problems were experienced with the blocks and thermal cycling caused failure of other more fragile gaskets, i have a feeling they have just cought on as the next new thing, they are certainly a fussy gasket so far as mating surface condition is concerned.

D

[Dieselman]

i have a feeling they were spec'd for vehicles that were renowned for blowing the fire seal out of the H/G or where rigidity problems were experienced with the blocks and thermal cycling caused failure of other more fragile gaskets,

Agreed, specifically for open deck engines like Rover K series and Honda petrols.

i have a feeling they have just cought on as the next new thing, they are certainly a fussy gasket so far as mating surface condition is concerned.

D

Reminds me of fitting a genuine gasket to an old Ruston engine, it always leaked compression afterwards due to the copper facing.

I might just go for a cheap standard gasket, after all, I'm sure it's only failing as a result of corrosion rather than compression or other cause.

[Dean]

I might just go for a cheap standard gasket, after all, I'm sure it's only failing as a result of corrosion rather than compression or other cause.

Yes, in the case of the XU's i think corrosion is the main cause of failure, After seeing pictures of Pauls gasket and what he has said above about his experience of the head bolts this could just be a bit of a red herring, my guess is improper installation helped that one on its way.

D

[Peter.N.]

I fitted a laminated steel gasket because it looked stronger, its certainly much heavier, my original one had rotted away, I couldnt see this steel one doing it. When I bought mine Citroen said they only do one thickness described as a 'repair gasket'. The compression seemed a bit low whan I first fitted it, slow starting, but it soon perked up after a few thousand miles.

Peter