Quote:
Originally Posted by fourWheelDrift
Good expalanation here http://www.gomog.com/allmorgan/stretchbolts.html and they are specifically designed to be tightened such that they go beyond their elastic limit which has a number of advantages explained in the article, particularly for automated assembly and reduced maintenance. |
Thanks fourWheelDrift.
This is due to the fact that when a fastener is tighten into plastic deformation, it will not change it's effective tensile strength, (and clamp force) by very much over a given amount of elongation, (stretch). So when the pneumatic machine tightens the bolts in your engine, it can vary up to 1/2 turn and still have the same clamp force... at least in theory. At least I know why they are used.
I have been substituting the eye wateringly expensive "low grade" stretch bolts with cheap high grade 8.8 Metric bolts for some time now with no ill effects; I do re-torque the bolts though! The spec for these bolts in the article is wrong they min proof load of 85000 psi.
Over time the head gasket in such situation will deteriorate to the point where a leak is visible and over-heating occurs. Regardless of how competent the installer is, if the head bolts have inadequate clamp force, the problem will re-occur. The solution then is to install better quality fasteners with adequate strength to meet the demands of the application.
Thanks FWD, much appreciated explanation into why the lousy things came into being, and confirmation they are in fact inferior low grade bolts!