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Afternoon all. I have just finished replacing the head gasket on my 1991 Mk2 1.3 Golf Ryder, torqued it up to the Haynes spec's. My question is this, do I need to re-torque after a few miles, or after it's first warm up and cool down? If so what torque does it need? Many thanks in advance. Take care.
Edited by Pugugly on 23/02/2009 at 14:02
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Usually after first warm up/cool down;then re-torque to spec.-I would not bother to drive it-warm it up on driveway till thermostat opens.
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Just the info I needed. Many thanks indeed, have a great rest of the day.
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>>torqued it up to the Haynes spec's
What were the specs?
If it's angle tightening, leave well alone - don't re-torque.
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First stage: 40nm
Second stage: 60nm
Then 180 degrees
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As the man says-if it's angle torquing,don't retorque.
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Many thanks fella's, I'll leave it be. All the best.
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As it is an angle tightening spec, I'll add a bit of background info;
Although tightening with a torque wrench seems like it should be accurate - after all, you're using an expensive gadget - it's actually hopeless as a method of controlling how tight a bolt is after installation. Yes, the torque is controlled accurately, but, it's actually how much load there is along the length of the bolt, the load which clamps the gasket, that's important, and a torque wrench can't measure this.
The source of the inaccuracy is the unknown level of friction between the threads, and between the head of the bolt and the washer (or washer and head if flanged) - the result is *huge* scatter in the preload of fasteners installed with just a torque wrench.
Angle tightening, on the other hand is much more accurate. It begins from a snug torque; which although using a torque wrench is at a much lower load, minimising the friction error described above. Then, by stipulating an angle, the exact level of thread and bolt head friction become irrelevant, and an accurate preload can be achieved.
Then, the final good thing about this method is that you can accurately (even with a cheap protractor) install the bolt so it is just on the verge of its yield point, and most cylinder head bolts do exactly this. When the engine starts and warms up, the bolt is stretched a little more, and yields. Although this sounds bad, it's actually good, because yield, for a given good quality bolt material is a very well defined load - the gasket is clamped with a calibrated and controlled load - this minimises leakage problems caused by poor bolt preloading.
So, in use, at operating temperature, the bolt is allways at or very close to its yield point, and its stress reduces a small amount fom there as the engine cools - this is making absolutely the best use of the bolt material.
However, the cost is that the bolt is plastically (i.e. non recoverably) stretched a little every time its installed, and this is a trick you can only do once or twice with any bolt before it goes PING! A reputable garage will always fit new bolts, but, manufacturers sometimes specify length limits - if the bolt is shorter than this, you can use it again.
This plastic deformation is the main reason why the bolts should not be re-torqued, the second reason being that it's simply unnecessary.
You did use new bolts, or checked their length against the spec didn't you?
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I re-used the original bolts, they were all the same length and threads in good condition, according to my visual inspection. They all torqued up nicely and gave the same feedback/resistance as they were tightened. By what you have written it sounds like I should've used new bolts? Darn it!!!
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>>By what you have written it sounds like I should've used new bolts? Darn it!!!
Possibly, it depends upon whether they were within the length spec (If VW give a length spec rather than just stipulating new bolts should be used)
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