So much good input, so little time..........
Aluminum survival in combustion temperatures - air is ~20% oxygen 80% nitrogen, for these intents and purposes - no inner-cylinder surfaces come in contact with the burning fuel - the combustion-heated nitrogen expands faster than the expanding flame-front, shielding the piston crown and cylinder head from melting - oil-spray into the piston underside, coolant passages in the head, and incoming fresh-air on each intake stroke keep exposed surfaces well below aluminum melting point
Normally, no Diesel fuel is burning when the exhaust valve opens - ever noticed how, at a birthday party, someone is poised to blow out the merrily-burning candles, when whooff!, the flame(s) is extinguished - too much oxygen for too little fuel - pilot injection allows fuel injection as far down the power-stroke as there is available oxygen for combustion, then injection stops, flame goes out = maximum torque, minimum wasted fuel - exh valve opens, piston shoves combusted exhaust gases out - exh valve still open, intake valve opens, BOOST blows any residual exh gasses out, cylinder fills with fresh 20%oxygen\80%nitrogen charge - inject fuel, repeat as required...........
Heat-shielding - since inception all Chev small-blocks and big blocks have utilized heat-shields to shield the spark-plugs from exhaust manifold radiation - very effective, even tho there is nothing combustible about a spark-plug - the starter solenoid, also non-combustible, is also heat-shielded - all oems placed a heat-shield on\above the catalytic converter, shielding the passenger floor area - engine exhaust heat radiates into ambient temps, cooling rapidly - however, if another heat-source is radiating close within the radiation area, the resultant temps are averaged: the turbine scroll could be wrapped\shielded to effect more efficient radiation\cooling of exhaust-manifold, allowing the head to be exposed to lower temperatures - added benefit would be higher exh gas energy into the turbine for quicker spool-up, which we do not really need, having VVT technology to ensure quick spool-up - shielding would likely not be entirely effective
Number of head-bolts per head - not as important as number of head-bolts directly surrounding the cylinders for maximum clamping force - VM CRD has 18 total, but eight (count'em: '8) are ~2-3" from the cylinders they are supposed to be directly clamping - not even good for aluminum head in turbocharged service - cylinder bore affects clamping force distribution - the VM CRD is 2.8L, one of the largest-displacement 4-cyls available, not to mention the fact that it is also a Diesel engine - a 1.9L engine would require less bolts per cylinder because the bolts would be closer together around the smaller bore - cast-iron head would increase the effect
Head metallurgy - aluminum absorbs heat and divests that heat (expands\contracts) much more rapidly than cast-iron, making it thermally unstable, even cast-aluminum - aluminum requires more directly-distributed clamping force than cast-iron = more bolts per cylinder - cast-iron is more rigid than aluminum, more thermally stable - still, most turbo-charged Diesel engines have at least 5-bolts closely adjacent the cylinder, some have 6 per cylinder - IIRC, some of the big-bore larger-displacement Diesel engines have seven per cylinder
Cutting head\block for circular groove to utilize copper o'ring gasket - would be an excellent solution butcept for two major concerns - aluminum is softer than copper - which will "crush" first? Would a steel shim washer between the head and the copper ring help? - the 2.8L engine has an aluminum head with only 4 widely-spaced bolts per cylinder = uneven "crush", what we already got
Studs vs TTY bolts, or even standard bolts for tension\compression service - studs say it all: block\stud threads are fully engaged at beginning\end of torquing sequence - no twisting forces in block during torquing
I would be very interested in results from checking the bolt-holes across the head with a depth-micrometer, noting any where the dimension between the bolt-head surface and the deck surface was less than the others, indicating heat-stressed crush\compression of the aluminum from the head-bolt surface side - this would give same symptoms as loose or heat-stretched bolts, with reduced torque required for removal
Also, it might be a good idea to post the ARP stud part number(s) in this relevant thread
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