Getting a clunk from the rods, need help

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jrlegacy23

jrlegacy23

68 Barracuda
Joined
Jun 27, 2006
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Laurens, NY
I have an 86 1 ton with a 360 in it. Completely rebuilt the engine. Bored .030 over, new comp cam, 9.5:1 pistons, HV pump, crank turned .010 and .010. We started the engine and it runs strong and revs nice and quick. Everything was broke in properly with no issues.
The problem I am having is that at a low idle I can here a bottom end knock that goes away when revved up or driving. I drained the oil and pulled the oil pan. Everything looks good and clean, but if I grab a hold of the rods, I can move them front to back making a similar clunking noise. All rods are tightened to torque specs and not loose at all. I get no up/down movement. Just the front to back. Anyone know what could be the cause of this? Is it possible that it is a machining issue?
Thanks for any help
 
The rods SHOULD move. There is a "spec" in the shop manual for rod side play, but I'd say as much as .050--.020" won't give trouble. This is a guess.

Don't know what to tell you. I'd guess first step is inspect rod and main bearings, and get some plastiguage and check clearances.
 
Side clearance, built into crank. Check exhaust gasket first. I went as far as buying a new piston for a motor and it was an un- noticed exhaust leak, worst at idle. Went away at speed.
 
Oklacarcollecto said:
Any chance the noise is coming from the transmission dust cover?
Click to expand...


^^ seen this before-thought for sure my friend's Ramcharger had a bottom end knock-sounded so similar. Turned out to be loose converter bolts.

I don't suppose you have some video clips you could post do ya?
 
Measure the side play in each of the rods; there is a spec. If it is much above that spec, then oil can flow out of the rod journals too quickly, leaving the rod bearings with insufficient oil. It could indeed have been machined out too wide. This can be checked with feeler guages with reasonable accuracy with the rods still assembled on the crank. (Always check this during assembly.)

And as suggested, check the thrust bearing clearance/crank movement.
 
Don't forget to check the torque converter bolts. Loose converter bolts can cause this also. Flex plate to converter...
 
Oklacarcollecto said:
Any chance the noise is coming from the transmission dust cover?
Click to expand...

txstang84 said:
^^ seen this before-thought for sure my friend's Ramcharger had a bottom end knock-sounded so similar. Turned out to be loose converter bolts.

I don't suppose you have some video clips you could post do ya?
Click to expand...

krazykuda said:
Don't forget to check the torque converter bolts. Loose converter bolts can cause this also. Flex plate to converter...
Click to expand...

I was hoping this would get brought up before a perfectly good engine got torn into. :D
 
TrailBeast said:
I was hoping this would get brought up before a perfectly good engine got torn into. :D
Click to expand...

Yup.... We've seen that a few times here.... :banghead:



It's better to learn from someone else's mistake than your own.... :sign3:
 
Aw snap, forgot about those! And i just had this loose convertor bolt happen 2 months ago on the mommyvan!
 
Just so everybody can sleep at night, the culprit was loose torque converter bolts (which I did not put in myself). I put some locktite on them and retorqued them. Gotta love a strong small block (even in a 1 tone dump truck).
 
jrlegacy23 said:
Just so everybody can sleep at night, the culprit was loose torque converter bolts (which I did not put in myself). I put some locktite on them and retorqued them. Gotta love a strong small block (even in a 1 tone dump truck).
Click to expand...


Hey, I called it!

Glad you got it fixed! :burnout:
 
pishta said:
Aw snap, forgot about those! And i just had this loose convertor bolt happen 2 months ago on the mommyvan!
Click to expand...

That's one of the cases that stuck in my mind. :D


Like I said, it's better to learn from someone else's mistake than your own. (It doesn't cost you anything...). Luckily this fix didn't cost him any money for parts. :cheers:
 
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