Cylinder head retorque

[Louis-Alain Richard]

Peter,

What puzzle me is this : our other car, a Saab 9-5, had a small oil leak
from the back of the head. A Saab technical service bulletin was published
about retorque/bolt replacement. The instructions on that TSB were like this
: first try to retorque the bolts. If the spec is not attainable, replace
the bolts. 
When we first tried to retorque the bolts, it was clear that we wouldn't
reach the spec, as the bolt was almost impossible to torque with a standard
24 in. torque wrench. It would need a lot more force to add the 90 degrees
after the gentle initial torque. So, in my mind, a dead stretch bolt is
elongated to the point of being stretched to its limit and would break if
given more force. The problem on this engine was the bolts themselves : they
changed the surface finish to give the bolts more grip in the engine block.
See this image :
http://www.saabnet.com/tsn/members/gallery.html?memberID=112&do=show&id=746.
After the bolts replacement, the engine is fine.


On the urq, it was not the case. It was relatively easy to add 180 degrees
like in the initial operation. They acted exactly like if the bolts were
new. Or maybe these are not stretch bolts ?

To add to the confusion, in the Bentley book there is some notes, page 15.13
:

Caution : do not retorque cylinder head bolts at first service or again
after repairs.
Note : It is not necessary to replace polygon head bolts when making engine
repairs. Bolts can be reused. However, new bolts are to be used in sets
only.

So, maybe the holes in the block were not clean enough to keep the clamping
force ? However, the block was cleaned in a caustic tank and prepped by a
reputable shop. More, we installed the head and all exhaust plumbing on the
engine while it was in my shop. We then installed the complete drivetrain
from under the car in a single step.

Any ideas to prevent this loosening to happen on the upcoming rebuilt ? I'm
a bit tired of pulling the head on this car, since it is such a big task to
do it in situ with all the plumbing attached to it. 

Louis-Alain 





-----Message d'origine-----
De : Peter Golledge [mailto:petergolledge at gmail.com] 

AFAIK all I5 Audi head bolts are TTY "Torque to Yield" AKA "stretch" bolts.
I'd not expect them to have the same
torque + turn after heat cycling but I don't know if that would explain the
120 Deg difference?