Bolt preload. A torque wrench controls torque, not clamp load. T = K F d with nut factor K (about 0.15 oiled, 0.20 dry). Preload F = T / (K d); stress sigma = F / A_t with the ISO thread stress area A_t = pi/4 (d - 0.9382 P)^2. Spec torques are chosen for about 55 to 65 % of the proof strength; a dry thread at the oiled torque loses a quarter of its clamp load, an oiled thread at the dry torque gets near yield.
Sequence. Head bolts are tightened centre-out and loosened outside-in so the head is pulled down flat. Out-of-order loosening can warp an aluminium head by some hundredths of a millimetre; this simulation adds 0.011 mm per mistake (an illustrative figure).
Clearances. A feeler gauge slides in a gap if its thickness is not more than the gap, a micrometer reads when the anvil just touches. Flatness limit, journal and spring limits are typical values for a small petrol engine, not a specific make.
Slider-crank. Pin height y = r cos(th) + sqrt(l^2 - r^2 sin^2 th), swept volume V_s = pi/4 B^2 S, compression ratio r = (V_s + V_c) / V_c (B 84 mm, S 90 mm, l 130 mm).
Compression test. Cranking pressure p = p0 (V(IVC)/V_c)^n with the effective ratio fixed by when the inlet valve closes, so a timing error of one chain tooth (20 crank degrees) changes it. Leaks from a poorly clamped head, an unlucky ring-gap stagger or a worn bore reduce it.
Run model. Otto efficiency 1 - r^(1-gamma) scaled by 0.70 for heat loss, volumetric efficiency curve, friction mean pressure rising with speed; torque T = bmep V_d / (4 pi). Oil pressure falls with the cube of bearing clearance (laminar leakage). Valve float speed scales with the square root of spring rate.
Simplifications: no cylinder-by-cylinder gas dynamics, no knock, no thermal expansion of the clearances; all prices, times and wear distributions are illustrative game values. Not affiliated with any engine manufacturer; the engine is generic.