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From my experience, the two can make very similar sounds. But:
1) Rod knock - more prevalent at higher rpms, especially on the overrun (rev it up, close the throttle - the rod will tend to knock worse as the compression slows the engine down). Often at idle they won't make much noise at all Recently, I was able to tell my PV's motor had a bad rod by pulling the plug wires (whoich would have no effect on a bad timing gear). 1,3,4 made no difference, pulling #2 would make the knock appear. When disassembled, yep - spund bearing on #2.
2) Timing gear woes of various sorts - more prevalent at idle and low rpms, and often fading completely away at higher rpms. At low speeds the oil pump is placing very little load on the cam, and once the lobes go past 'TDC' the lifter pushes the cam forward until the next lobe hits. This back and forth torque on the cam exacerbates any slack in the gear or thrust washer which creates the clatter. When revved up, however, the oil pump and lessened rebound tension on the springs places the cam under a constant pull, so the slack isn't slopping back and forth, and it gets quiet. On my PV's original B18 I had the hub come loose from the fiber gear. It made a *horrible* noise at idle, like an old diesel engine just started up. Rev it up past 2000 or so and it got completely quiet, sounded 100% normal. When I took the timing cover off the fiber portion fell right off.
Also - I was certain I had a rod knock on my 1800E once, it turned out to be the heavy cast iron double pulley on the front of the engine coming loose. Sounded and acted just like a rod knock. I was actually going to pull the timing cover off just to see what the timing gears looked like when I found the bolt on the pulley loose. Tightened it back up - no more nasty knocking.
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I'm JohnMc, and I approved this message.
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