It is a topic that comes up from time to time. An interesting note I thought I would share:
Note on Con Rod Stretching
By Ron Graham - Autumn 1984 issue 'MV News'
Yes, connecting rods do stretch (Modulus of Elasticity).
The amount of stretch depends on the RPM, physical dimensions and material used. Connecting rods never break under compression, only under tension (i.e. stretching). The compressing and tensioning of the con rod occurs four times per cycle (four strokes), at obvious parts in the cycle.
Now suppose an engine is revving at 9,000 RPM with the throttle fully open, then shut the throttle. The engine will still be doing, say, 8,000 RPM, but the tensioning of the con rod will be at its highest. This is due to the fact that, with the throttle closed, the induction is severely restricted, effectively reducing the compressive load on the piston. But at 8,000 RPM the piston is developing very substantial weight and tension on the con rod at near TDC, thus the con rod stretches.
If the clearance between piston and head is used up, they touch, the con rod breaks, or the con rod absorbs all the loads and recovers.
