Stretch Bolts - Why?

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Stretch Bolts - Why?

Below is a link to a great video that explains why we use stretch bolts for cylinder heads, and why they need a low torque setting before the angle tightening sequence.
Enjoyable, and informative.
https://www.youtube.com/watch?v=-NKsNtHKpmE&feature=em-uploademail
I just came across this and want to say thanks PB. What an interesting and entertaining video. At the risk of sounding big headed I'm going to say that very little in it was news to me but the presentation was entertaining and easy to understand. I remember being puzzled by angle tightening when it first came in but I'm a total convert to this procedure now. It should also leave anyone viewing it in no doubt that stretch to yield bolts are always single use and must be replaced even if just loosened, let alone removed.

The one bit I hadn't considered was that stretch bolts are often used to gain space. He details how a bolt of this type can achieve the same clamping force as a larger conventional bolt. Very clever, this reasoning hadn't occurred to me.

How things have changed. In my youth it was standard practice, where a head gasket had been replaced, to have the customer return after 500, to 1,000 miles for the head bolts to be retorqued and valve clearances checked and reset. This would involve slackening each bolt/nut by about half a turn and using the torque wrench to retighten to the recommended figure. With torque to yield you just can't do this, but then modern gaskets don't have anything like the crush of the old copper asbestos composite ones we used.

A far more interesting and entertaining read than another thread running concurrently which I'm avoiding as I just don't know what's going on with it and I'm finding it a little upsetting to read.
 
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