When a rod bearing is worn it causes the rod to slap against the rod
journal when the piston is changing direction. However, the loudest slap
will be on the combustion stroke (Initially the crank is dragging the
piston down so the bottom rod bearing is on the crank. As pressure in
the cylinder builds the piston is shoved down, slamming the top rod
bearing into the crank.
In a 4/4 stroke/cycle engine in a 4 cylinder car, one piston per
revolution is experiencing a combustion stroke... If only one rod has
bad bearings (and one will always be worse than the others, if only
slightly) then it is slapping once every 4 revolutions...
At 1000 RPM this means 250 slaps/minute or a little over 4 per second.
Voila!
Here's the puzzler. In my case, I have this noise ONLY on trailing
throttle. That is, if I blip my throttle up and close it I will have
this Tach/4 slaps per minute as the engine slows down. Note that I am
not under load at this time (engine is just driving itself and the clutch).
The noise is not evident on the road, though this could be a masking
effect. The noise started when I replaced my #1 piston... I did use new
bearings on that rod only. And finally, it does seem to be coming from
the front portion of the engine.
Your task: Analyze the "top" and "bottom" bearing loads and transitions
in idle, power under load, engine breaking, and trailing throttle under
load conditions. Compare and contrast the stress on the bearings under
those conditions.
I bet you'll be surprised... (I say engine braking is the most damaging
possible load condition because of the rapid change in DIRECTION of the
load).
As for oiling the camshaft... take off your oil filler cap and see if the
rockers are oily. If they are, oil is more than likely getting to the
cam. I say we would ALL have perfectly round cams if that wasn't true.
John M. Trindle | Williamsburg Town Square | jtrindle@tsquare.com
T-Square Company | 804-220-0533 login: guest | trindle@widomaker.com
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