Thread: Cast Iron heads for 300TDi?

Originally Posted by Bush65

Low carbon steel (AKA mild steel) has a definite yield point, high strength alloy steels do not.

I don't have pics with me, but will attempt an explanation of what happens in a tensile test, starting with low carbon steel.

As tensile load increases, the elongation (stretch) increases proportionaly until the proportional limit is reached - the slope of this straight line corresponds with the elastic modulus (E = stress/strain where stress = load/area and strain = elongation/original length so E = (L x l)/(A x e) ) curve. Near limit of proportionality the elastic limit is reached - where original length will be achieved if load is released. Just after the elongation of the test piece will start to increase with no increase in load - this is yielding and the curve goes horizontal. As further elongation occurs the load starts to drop (because cross sectional area has become smaller - waisting) and the curve dips down. The material begins to work harden and the load starts to rise again to increase elongation. Load starts to fall again because waisting becomes severe. During the last 2 stages the curve resembles an arch. Finally the test piece breaks.

With high strength alloy steel, the curve starts the same and the slope until the limit of proportionality is reached is the same because with steels E is much the same (small differences do occur but are usually neglected except for stainless) regardless of alloy or heat treatment - the value of load at proportional or elastic limit is higher but slope is same. Then as waisting occurs the curve has a similar arch shape until the break occurs. There is no yield point where elongation increases for no increase in load.

Because yield strength changes (material property) we commonly base design strength = 0.nn x yield strength. We avoid yield because it will result in permanent deformation (also formation of a plastic hinge which can turn a ridgid structure into a mechanism (resulting in collapse).

As the high strength steels have no yield point, we create an approximate value on the load/elongation curve by drawing a line parallel to the linear slope, but offset by 0.2% elongation. Where the 0.2% offset line cuts the curve from the tensile test, we have a value to use with our much loved equation (design strength = 0.nn x 0.2% offset strength).

For bolted joints in steel structures, it has become normal and more economical to use high strength structural bolts these are different to common high tensile bolts which are called hexagon precision bolts in the relavent Australian Standard (AS1110 from memory). HSS bolts are designed to be tensioned to the prescribed proof load (near as damb the yield or more correctly 0.2% offset) and the bolt proportions (particularly the head) were developed after much research by the international committee - the Aus Standard is identical to the ISO standard.

The Steel Structures Code (AS4100) and it's commentry have a section on tightening bolts. For bolted joints designed as fully tensioned (tension to the proof load) one of the acceptable tightening methods is part turn (snug tighten then further angle as given in table for bolt length). The other acceptable methods involve tension measurement and torque control (e.g. tension wrench) is generally not permitted.

It is well documented that the part turn method often results in tension exceeding the proof load, but it is not an issue. The standard allows bolts that have been fully tensioned to be re-used once, but only if they are used in the same bolt hole as they were removed from - because permanent deformation of threads, etc. can prevent proper tension being achieved if used in a different location.

With mechanical equipment, where parts are required to be pulled apart and re-assembled more than once, the bolted joints are usually designed for bolts tightened to approx 65% proof load and tables give tightening torque for achieving this tension.

When bolts are tightened to the proof load, the friction forces become so large that torque control results in errors of resulting bolt tension something like 25% - this error has been found in many tests making it unacceptable.

To avoid fatigue failure of bolts subjected to cyclic loads, the best practice is to use bolts (quantity and diameter) so that when tightened their pre-tension is at least 2 times (up to 5 times) the external applied load (in the bolt). Together with this tension, the joint must be designed so that it is considerably stiffer than the bolt. Then during pre-tensioning the bolt elongation is much more than the elongation of the joint. During the load cycles the variation in the tension in the bolt will be small and the variation in the compression of the joint will be large (in proportion to the relative stiffeness). The sum of change in tension of bolt and change in compression of joint equals the external applied load. So fluctuation in the bolt tension (the most important factor for fatigue strength) is much less than the fluctuation in applied load.

Note: it is not a requirement to tighten bolts to yield to achieve ar pre-tension 2 to 5 times the external applied load (we normally use 65% proof load, but where it is not possible to use more bolts or larger dia bolts then we use greater pre-tension, but achieving the pre-tension becomes more difficult.

I think we are talking at cross-purposes here as bolting together structural steel bears no resemblance whatsoever to bolting down an aluminium cylinder head that gets hot. Back in my school days in Manchester there was a Physics Practical that had to be done. It would be about fifty years ago now but I can still remember doing it. The experiment was titled "Youngs Modulus of a Steel Wire". Anyhow the test rig involved two lengths of piano wire each about six feet long. One wire had a constant weight applied. The other wire was loaded up with ever increasing weights and between the two wires there was a spirit level with a micrometer screw. A graph of load against extension was plotted and for the most part the graph was linear. Once the yield point was reached however small increases in load produced large extensions. The wire was of course micrometred in several places, an average diameter was then calculated and the maths was done. Amazingly the calculated figure tallied with that in an Engineering Handbook to four figure accuracy, amazing! (and no there was no cheating) Many many years later I worked for a while at British Leyland in Birmingham. As to bolts and bolting the consensus of opinion was that an under-tightened bolt represented money wasted because the same clamping force could have been obtained with a smaller, lighter, cheaper bolt. There were even multi spindle computer controlled nut wrenches that would tighten four or five wheel nuts at one go. The crafty computer would plot the Hookes Law graph in its "brain" and get those nuts really tight! In the case of aluminium cylinder heads there is the perennial problem that the aluminium expands about three times faster than steel. In this case having inch thick bolts made from Kryptonite would not be a good idea. The reason is that the expanding aluminium would crush the gasket and once the engine cooled slightly, the gasket would blow. The ideal bolts are ones that are strong enough to hold the head down but also sufficiently elastic to stretch very slightly when the head gets hot. High class engines have a separate head for each cylinder so warping is not such a problem but we can't all afford those!

If one has an iron engine with iron heads, gasket problems are much less likely. One example of this was a Ford Granada V6 (can't remember whether it was the Cologne 2.8 or the Essex 3.0 litre) After a trip to the shops the car was blowing steam like a steam locomotive as a twig had gone through the radiator. I switched off and left it until the next day. In such cases never lift the bonnet or add water as this can cause trouble, slow natural cooling is best. Once the hole in the radiator was soldered-up the car ran as good as new.

On the 300TDi there are some little vent pipes that go via a plastic widget. These can easily get blocked with rust or radiator sealer and all kinds of air locks will then occur. Very expen$ive!

Personally...

tdi300 headbolts can be reused 3 times

I recommend replacing them if you don't know or if you've already reused them once so that IF you have to reuse them in the field you have one more shot left on them.

I've only ever snapped bolts on the tdi that have been done more than 3 times already (known by good service histories and reciepts/owner knowledge from doing their own work.)