Cosworth 205 cracked block
06-11-2017, 05:59 PM
#82
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That's why I don't think stretch bolts and long studs are a good combo as one allows the head to lift and the other doesn't. Also I think longstudding is better as the threads are at the bottom of the engine where there is less stress rather than at the top of the combustion chamber.
in a std installation the stock bolts will heat up and cool down at approx the same rate as the aluminium of the head. however, the head, being aluminium, will want to expand about twice as much as the bolts for every degree of extra heat so if you put NON-stretch bolts (bearing in mind that ALL bolts stretch as thats how they clamp things!) they will only want to expand half the amount that the head does.
*something has to give*.
either the gasket will compress, the bolts will be forced to stretch or the threads will pull. if you have a steel gasket (non-compressible) and short 'normal' (non-strech) style HT bolts you will be trying to defy the laws of physics!
using long studs (which are NON-stretch) means that the double length of them compensates for the 50% co-efficient of expansion of steel v ally or, equally, using bolts designed to be more elastic i.e. 'stretch bolts' does the same. if you don't consider this issue e.g. by fitting 6 long non-stretch and 4 short non- stretch bolts i think you will stress the threads.
the stock stretchy bolts are designed to be fitted so that they are in tensile stress when fitted (like all bolts) but when they get hot (or rather when the head gets hot) they expand within their elastic limit and, critically, they are willing to expand more than the threads in the deck of the block are trying to defy them
06-11-2017, 06:02 PM
#83
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long studs also mean there is less distortion caused on the deck and in the 'corners' of the bores so they will stay round when the head is clamped down