When the crush is changed on the main cap or big end because of a different torque setting on an aftermarket bolt and or even more so with a larger bolt, the perfect circle is compromised. When I say compromised, it is no longer round, without doubt, it’s physically impossible for the exact circular property of the tunnel to be maintained. Furthermore, the crank tunnel has to be perfectly in line 1 thou twist for eg will cause premature bearing wear and engine failure.
Tunnel boring is a process whereby the main caps are machined slightly ie 1 or 2 thou to allow enough material to be available to bore perfectly round tunnel through the main bearing journals.
Here are some examples of situations where you would need a tunnel bore
Main bolts upgraded in any way, ARP Main studs for example
Higher than the standard tightening torque applied to the main bolt
Main tunnel not dead straight (use a straight edge as a guide)
Its a fact that bores pull out if round when you torque your head down onto your block,
more so if you run aftermarket bolts with more than OEM tension, you should be using a
torque plate for boring AND honing, you should run the same bolt you wish to use in your build
with the same tension, and the head gasket in between.
Once you finish machining you can pull the plate off and you may notice your bores are out of round,
especially noticable if you put a piston ring in it while setting ring gaps.
Once you build your engine and torque your head, its all perfectly shaped and you WILL notice less
premature piston and bore wear and of course less blow by.
Torque plating before boring and honing is
The idea is to replicate the engine in a built and running state.