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Technical File - Engine Bolts |
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Tuesday, 01 February 2005 |
Lifting the Cylinder Head off an engine was considered a fairly normal
way for a home-mechanic to spend a Sunday afternoon in the 1950’s, 60’s
and 70’s. Do it yourself car owners would do it as often as once a year
to clean carbon deposits out of the combustion chambers.
It’s a much more involved job to take the Cylinder Head off a modern
engine. Lurking under the plastic covers is a trap for the domestic
mechanic – the torque-to-yield bolt. Neither skinned knuckles nor bad
language will get one of these properly back into place.
Modern engines are assembled to much finer tolerances than those of the
mid 20th century. To keep to these tolerances bolts must be precisely
fastened. Torque-to-yield bolts are designed to be tightened until they
reach the point at which they begin to stretch. This ensures maximum
and consistent tightness of fit – very important in an engine which is
stressed by heating and cooling as it starts and stops.
Torque-to-yield bolts are usually fitted by factory robots and can only
be used once. The process for manually replacing them is complex: First
they are fastened to a base torque, or tightness, using a torque
wrench, a tool which measures how much tightening effort is applied to
the bolt. Then they are tightened through a specific number of degrees
using a protractor wrench. This method is more accurate at high torque
loads than using a torque wrench.
The good news is that modern engines rarely need their Cylinder Head
lifted until several hundred thousand kilometres, if at all.
With Cylinder Head lifting both difficult and unnecessary, modern car
enthusiasts are free to carry out easier tasks, such as reprogramming
their engine computer chips, or fitting more powerful stereos.
Editors note: Reprinted from The Australian February 16th 2005
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Editors Report 
