Motor knocking

[78chevy]

A little help here guys, I have a stock '78 350 with all of the smog equipment removed. The truck has well over 100,000 miles and since i bought it last year has always had a small knock in it. I use it for a daily driver about 16 miles a day round trip.
A couple of days ago I was going down the freeway about 70mph which is about how fast I normally drive it on the freeway, and I noticed the knocking started getting louder.
I got it home and oil level was normal motor was not overheating. I haven't driven it since. it starts and idles fine except for the knocking.

Someone told me that I might have a spun bearing. how can I check this?

Can I replaces them from underneath without pulling the motor?

Is there anything else that could cause this knocking?

Thanks I advance for the help.

[Redneckchevy]

Fast knock? Slow knock? Hard knock or soft?

[Restrorob]

I've seen cracked flywheels cause a knock as well.....

[86 chevy silverado]

Several possibilities, A cracked flywheel someone else pointed out, can be visually inspected by taking the converter shield off. Main knock, a deep heavy metal knock, and a rod knock which is a little lighter sounding but distinct under load. All require more than a basic repair.
Do you have an oil gauge? If not borrow/buy one and hook it up where your sending unit in the back of the block is located and confirm your oil pressure (or lack of as the case may be).

[Fordeatinz71]

if it's a rod knock, you can isolate it by removing one spark plug at a time...when the knock gets quieter you have isolated the rod with the spun bearing.  take the plug wires off when the truck is OFF...or you'll feel GM's wonderful HEI system. 

does the knock get louder/quieter with the changing in RPM's?  if you pull all the plug wires and NOTHING changes the knock...then it's not a rod knock, and you need to look at outside possibilities...A/C compressors can knock too...

[78chevy]

Sounds like s deep heavy metallic knock from the passengers side of the oil pan. Knock speeds up when rpms go up. I have not had a chance to inspect the flywheel yet but I'm pretty sure it is the motor.

Thanks for the input.

[mike357]

The truck has well over 100,000 miles and since i bought it last year has always had a small knock in it. I use it for a daily driver about 16 miles a day round trip.
A couple of days ago I was going down the freeway about 70mph which is about how fast I normally drive it on the freeway

I don't think a rod would last over a year going 32 miles a day @ 70 mph. At least from my experience they never lasted longer than a few days.

[Fordeatinz71]

i dunno.  i had an '89 Camaro IROC-Z with a TPI 305 in my younger (and stupider) days and i got it with a slight tick which i knew was a rod knock (a bearing that had started to go but hadn't yet)...you couldn't even hear it unless you knew what you were listening to...

the plan was to repair the bearings before they had time to completely spin and destroy the crank/rod.  well, here's where the "stupider" part comes in...the car was a BLAST to drive, and seemingly ran well...so i just kept putting off what would have been a weekend-long job...finally, about 6-8 months later, coming back from a car show, the knock started getting louder...and louder...by the time i got home it sounded like a midget with a hammer was loose inside my engine...if i had only replaced those frikkin bearings that 305 would have gone a good bit longer...but i had to be young and stupid...

[78chevy]

Yeah, it sounds like I have that same midget in mine! It still starts up and idles great. I think I'm going to buy an oil gauge and check the oil pressure first like "86 chevy silverado" suggested, and then drop the pan and see what the bearings look like.

Or do you think I shouldn't even waste my time and plan on doing an overhaul?

[Fordeatinz71]

a general rule of thumb is, if you can audibly HEAR it knocking (loud knocking, not ticking), it's too late and you are gonna have to just rebuild...at least have the crank turned and install new bearings/rings/gaskets while you are in there...

[86 chevy silverado]

a general rule of thumb is, if you can audibly HEAR it knocking (loud knocking, not ticking), it's too late and you are gonna have to just rebuild...at least have the crank turned and install new bearings/rings/gaskets while you are in there...

Yup! There is no easy fix for a worn motor. If you have worn bearings to the point of noise you have to ask yourself; how is the rest of the engine components? Chances are they need replacing as well. Anything less than a rebuild should be considered a temporary fix.