Rod Knock

[C M Smith <cmsmith@domain.elided>]

My off digest message to Zach (who now apparently knows more about rod 
knock than he EVER wanted to know) follows in response to a recent request 
on digest.

There are two distinct types of rod knock. The gudgeon pin end or wrist pin 
or little end bearing (as you prefer) makes a light metallic clack. Often 
you can hear the double clack as the con rod reverses the piston direction.

The main connecting rod bearing or big end makes a loud deeper toned knock 
and is very bad news. Engines can run for a surprisingly long time with 
little end knock. Big end knock is usually rapidly terminal.

Disconnecting the spark plug wire (and carefully grounding it in the case 
of high energy ignition!) and then running the engine will cause almost all 
little end knocks to disappear. The main source of little end knock is the 
combustion event hammering the piston down onto the bearing taking up the 
excess clearance with a clack. If the wrist pin is really loose then 
disconnecting the spark plug can change a double clack to a single as the 
lightly loaded piston reverses direction.

Disconnecting the spark plug rarely makes big end knock disappear altogether.

The really troublesome thing about conrod problems is if it does let go you 
need a new block so the risk of continuing to run the engine is quite high. 
Always run an engine at low rpm, 1500 to 3000,  if you suspect rod knock of 
either type. Not generally known is the fact that inertia loads on the TDC 
reversal of the piston on the exhaust to intake stroke is when loads on the 
conrod peak due to no downward pressure on the piston face and high speed 
loading of the bearings.

The noise a conrod makes can be similar to detonation (the cause of the 
sound is similar, the cylinder rings with the banging of the piston on the 
rod bearings in the one case, and with the sound of the detonation wave 
hitting the piston face in the other) but a shot conrod should make noise 
at idle. Also, changing ignition timing, or increasing octane will cause 
pinging to go away, but a worn conrod will continue to knock!

Cheers


Michael Smith
Calgary, Alberta,Canada
91 Alfa 164L
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided