New engine is making a LOUD knocking noise, what could it be?

Question

we built a chevy small block 350 to go into a street rod, after trying to start it (minimal difficulties) she started, we turned it off and did it again a little while later. Only this time, while cranking it started to make a LOUD "knocking noise" (coming from the engine) about 4 times a second. The engine still started but continued/continues to make the noise. I havent gotten a chance (at the cabin for the last week) to tear into the motor yet and have been mulling the situation over. Any ideas as to what the noise may be/what may be causing the noise.
*One thought is that it a rocker could be too tight/the rod is down too far and is getting hit. But idk if that would be as loud as the sound actually is*

Answer 1

You prob. wiped out a rod bearing. Either incorrectly clearanced or not prelubed properly before start up.

Answer 2

You probley have no idea how long you were running 4 quarts low.If you check the spec.you may find that that is all it takes.The engine may have a bad rod from lack of lucubration witch means an engine replace. A few questions,is the engine leaking any were?The reason I ask if there are no leaks and that car is well maintained ( as it sounds like you do) it is not smoking or oil in the coolant .Did the station that did the last oil change forget the oil. Now that you have oil in the engine check that the filter is sealed and that drain plug on the oil pan is not dripping..You will see drops of oil under the car if leaking. Seems odd that a car lost 4 qt of oil in a short time..Check for service bulletins or recalls too. If you need an engine get a rebuilt.It is cheaper to install a rebuilt engine then to fix the one in the car, Good luck

Answer 3

4 times a second eh?

Well, It could be a few things, I am sure that you tightened everything to spec so I will skip things like loose piston rods but don't rule those out,

First thing to do is take a stethoscope (the thing doctors use) and start the engine. Then go around the engine listening for when the knock is loudest. Then inspect that spot.
Some spots to check are...

If you have a modified oil pan, are you sure the crank is clearing the pan? Also check the valve covers, exaust manifolds, intake manifold, timing chain cover, and any other area. Be careful not to burn yourself on the engine and you dont have to put the stethoscope on the engine just close. and don't hurt yourself on the fan.

Answer 4

only way to know for sure is to tear it down. I would start with dropping the oil pan and check there or even check to make sure all the flywheel bolts are tight. I had oh that knocked like that and it was a flywheel bolt that had backed out.You are right about a rocker, that would be more of a loud tap than a knock.

Answer 5

Could be a spun rod bearing. That would be very LOUD. When you assembled the engine, did you check the clearances w/ plastigauge? Did you torque the lower end to specs?

Answer 6

If it has aluminum full- roller rocker arms, one might have come loose. Dont blame it on a rod until you have checked everything else.

Answer 7

is the vehicle a automatic or manual trans? auto trans maybe torque convertor bolts are not tight or the flex plate bolts to the crankshaft . also the flex plate may have cracked near the bolt holes

Answer 8

listen to Eric p your rocker arm is a good place to start bad piston maybe basically your going to have to tear it down and just find it.

Answer 9

last time that happened to me... my engine blew... yikes don't drive it until you get it checked out via tow truck maybe...
seriously... I have had 3 engines go on me that way.
Make sure engine oil is topped up and get it in to the shop asap

Answer 10

IF IT IS IN THE CAR, CHECK THE FLY WHEEL OR MAKE SURE YOU DID NOT DROP ANYTHING DOWN THE CARB! GOOD LUCK,