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Re: interference fit
> Conventional wisdom is that there are two classes of engine;
> interference and non-interference.
> I expand that into three classes; non-interference, dynamic interference
> and static interference.
> Dynamic interference is the dangerous type - the engine is a
> non-interfernce type on paper (i.e., in the books) and might even be
> a non-interference type when within 'new' manufacturing tolerances.
> But if the timing belt goes at speed, and on a worn engine - the
> engine is toast.
Dynamic interference - nice term.
Now, why does it happen?
Quite simply, the acceleration of the piston is at its greatest at TDC
and BDT. To accelerate anything, you have to supply a force. At TDC,
that force will be downwards and has the effect of _STRETCHING_
the rods! Hence an engine which can safely be turned over by hand
may well be interferance at high RPM. Simililarly, at TDC, the
force will be upwards and tends to compress the rods. Rods are much
stronger under compressive stresses, so we don't much worry about this.
Within reason, this makes turbo-charged cars slightly kinder to rods since
the extra pressure in the cylinder resists the stretching effect...
at least that's what the books say. Whether this is significant,
I don't know.