... to normal bolts? I presume they stretch when you tighten them. Do they stay stetched, and this is why you can't use them again in case they break?
Also, why do car manufacturers use them? Surely 'normal' bolts work just as well, and had done for year!
Cheers
Mike Farrow
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Head bolts are normally stretch bolts, they stretch as you tighten them and stay under tension like springs, phenominal tension, they don't 'relax' after getting hot and cold a few times like normal bolts would.
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The best example of stretch bolts I have come across is on the crankshaft of the BristolHercules radial aero engine.The crankshaft has to able to be dismantled at major services in order that the master conrod of each cylinder bank which in turn carries the conrods for the seven cylinders in each bank can be accessed.
The joint between journal and crank web is not splined as one might think but simply clamped together using what the makers term maneton bolts.The method of assembly is to ensure parts are clean and grease free,then assemble line up and tighten enough to maintain alignment then measure the overall length of each bolt with an outside micrometer before tightening each bolt to a laid down amount of stretch using special spanners one of which I remember was 7Ft.long,I never heard of any failure due slipping of these joints,amazing as the later marks of this engine developed 2000BHP
Not strictly motoring I know but fits subject.
ndbw
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Re my post above,the maneton bolts refered to are of course made from high tensile steel.
ndbw
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Normal bolts cannot take the torque applied to stretch bolts.if you remember the old type head bolt.they were tightned to around 75/80 lbs/ft.stretch bolts will go far in excess of this.and yes they do stay stretched untill released.which is one reason a given measurement is made as to maximum length a bolt can be ie if stretched beyond given length they must be replaced.I expect some will correct me but think its to compress head gasket to help reduce any chance of it leaking.thats a guess.so may be wrong?would like to hear if anyone knows
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Was mech1
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That was the main idea, I think. It also avoided the need for the practice of checking the cylinder head bolts at the first (1000 mile) service, and was one of the things that eliminated the need to service the vehicle for a year
Peugeot use the things quite extensively for things like bearing caps and the dreaded bottom pulley (that's what makes it such a pig to get off)
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All bolts stretch when they are tightened, but usually they stay within their 'elastic limit'. So, when they are loosened, they return elastically to their original length.
The normal tightening method is by torque wrench. The tightening torque is calculated to apply the correct stretch, and so put the correct load on the joint. It's not very accurate, and is affected particularly by thread condition, lubricant on the thread etc. Even using a bolt a second time when the thread is polished gives a different load on the bolt.
Stretch bolts try to eliminate some of these problems. A steel bolt will extend steadily in proportion to load up to its elastic limit. At that point it yields, and takes on permanent deformation. For a short while the material continues to extend steadily for very small increases in load (the load/extension curve being virtualy horizontal), and after that it takes only very small loads to keep stretching it to the point of failure. That is the load/extension curve drops rapidly. Stretch bolts are made of steel which has a relativly long horizontal section of the load/ extension curve past the yield point. In use they are tightened by stretching them a specific amount, by measuring the rotation after the joint is nipped. This is calculated to take them a short way past their yield point, to the point where the load extension curve is horizontal. The load in the bolt and hence the load on the joint is accurately known, as opposed to being at some ill-defined point on the elastic part of the load extension curve.
Because the deformation is permanent, and further extension would actually reduce the load applied, it is not possible to check the tightness of such bolts after a period of service. It is also unwise to reuse them, as they would not apply the correct load to the joint.
Regards
JS
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Thanks for the replies everyone, particularly John S, really useful info!
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John's answer is spot on - almost word for word what I was going to say!
However, I will add that tightening them up is really odd - you can actually feel them begin to stretch as you keep turning but it doesn't get any tighter - makes you think they're going to shear!
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RichardW
Is it illogical? It must be Citroen....
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What alloy do they use with such a long plastic part of its curve?
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