| for about 10 rotations of the crankshaft upon initial startup of a completely dry VTC mechanism. The VTC piston helix does not 100% seal to the outer body or the cam helix and as soon as the valve at the back of the cams is closed, the oil pressure is going to force any and all air out of the piston helix "cylinder" - the area between the face of the piston and the front of the main sprocket body. Consider it like a leakdown test in a cylinder. As soon as you release the pressure on the leakdown tester, all of that air is going to bleed past the piston rings from 100psi to 0psi within 10-20 seconds (and there are TWO ring seals in a cylinder). The oil pressure in the VTC mechanism is going to purge any air out in very very short time - like within a few seconds of VTC advancement... Air will find even the smallest of leaks and with just a little pressure behind it, it will make its way through. The early model VTCs are even less susceptible to holding an air pocket for the fact that the inner cam helix does not have a seal on it to seal to the piston helix. There is a few thousandths of running tolerance there which any and all air will be forced through virtually immediately following a VTC advancement event. Even if there were air in front of the piston, it isn't going to make it more or less prone to rattle - the piston has nothing to do with the noise to begin with.
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