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, Sat, 29 Nov 2003 14:03:49
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The experienced mechanics say its OK to reuse the Saab headbolts, partly based on their experience and partly because they are very skilled in torquing bolts.
I wonder if they would still feel the same way if they knew what a stress/strain curve looked like for carbon steel. Steel can take a lot of stress up to the yield point (the point at which it starts to stretch). After the yield point, it can't take more than maybe 20% more stress before mechanical failure (bolt fracture). Obviously there are a lot of variables (friction is the biggest), but the point is the bolt is so close to failing after it has begun to stretch that you must be extremely careful about the amount of additional stress imposed.
When you reuse a bolt, it needs to be stretched even further than the first time (to be sure you reach the yield point), and that means riding the stress/strain curve right up to the limit. If the bolt doesn't reach the yield point, then you do not know what the stress is on the bolt. You can easily torque some of the bolts to lesser or greater stresses than other bolts, because there is an incredible level of friction involved in torquing the bolts. If the bolts are not evenly torqued to the same stress levels throughout, its not too hard to figure out what can result- head gasket leaks.
The bolts aren't inexpensive, they are downright cheap. I think you can get a complete set for under $15 from sponsors like Sobstory. To me, that is cheap insurance for a head gasket job... at least at my mechanical skill level.
One tip I would recommend regardless of what you do is to thoroughly clean out the bolt holes. Blow them out with air, spray in bit of oil, and blow it out again. The last thing you want are particles or sludge on the thread surfaces that will add friction while you are torquing the bolts.
posted by 66.44.60...
, Sat, 29 Nov 2003 14:03:49
, Sat, 29 Nov 2003 15:07:52
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