Getting a knocking sound all of the sudden

[WidowMaker68]

Went out to start my 68 Tempest this morning and after a few minutes of running it started to have a slap/ knock noise. Turned it off. Checked rockers and everything seemed fine. Rotated the engine by hand and didn't feel anything exceptional. No real resistance, scraping noises nothing. Fired it again slap slap slap. Engine is newly rebuilt and hasn't even been on the road yet. Any ideas?

[bmbrzmn101]

Connecting rod bearing is what it sounds like. If you already ruled out lifters and rockers, next would be con rod. Probably spun it. Did you pre lube the engine before its first start? If not that's when the most damage occurs. I hope I am wrong, but your description sounds like con rod bearing.

Chris

[WidowMaker68]

I pre-lubed everything. I have run it a few times. One time just to check if it would fire. Second time to break it in in the driveway. Ran it for about fifteen minutes at 2200 rpm and it ran fine. Don't have rims or tires( long story) so it hasn't seen the road yet. Put in an aftermarket dipstick and have a pretty good oil leak. That's what I was checking this morning. I had rtv'd it because I did a hack job installing it. Needless to say ill be doing that job over. So that's when this noise began. I'm wondering if it getting low on oil because of that leak would cause that. It is about a quart low. Any thoughts. I really hope I don't have to tear it down again. I just put the whole front clip on, except the bumper. Would be a bummer to do all over but if I must I shall

[moreHP]

Do you have an oil pressure gauge on the car? If you do, Has the reading changed from when you first started the engine? 1 quart low isnt going to ruin the engine provided the dipstick is giving you an accurate reading.

[WidowMaker68]

I do have a gauge. Mechanical but haven't hooked it up because didn't like the idea of having oil in the compartment

[WidowMaker68]

So I guess that would be a no. Just a dummy light.

[Rod]

what break-in oil did you use?(never use a synthetic oil for break-in), also hook up the gauge for oil pressure

[WidowMaker68]

Sae 5-30 w lunati break in additive. Was running great. Timing was good. 1 stroke of the key and it fires up. Don't know what to do to diagnose the bearing issue. Besides tearing it down. Unfortunately I did the whole rebuild myself. First timer and if anyone is at fault it would be me. That vein said I'm praying for simple fixes!

[WidowMaker68]

So I'm going basic and replacing spark plugs( fouled). So a/f is off at the carb. Adding oil and well see. If it doesn't stop the knock then Yankee Doodle Doo

[SSLance]

Automatic trans? Check to make sure the flex plate bolts are tight.

Sure it's not an exhaust leak?

[WidowMaker68]

Auto trans. Flex plate ill check. Exhaust leak would cause this?

[Grease Monkey 23]

Does the slap/knock get faster w more engine speed? If so i wld say this:
Connecting rod bearing is what it sounds like. If you already ruled out lifters and rockers, next would be con rod. Probably spun it. Did you pre lube the engine before its first start? If not that's when the most damage occurs. I hope I am wrong, but your description sounds like con rod bearing.

[WidowMaker68]

No actually goes away with revs. Still scary. I'm thinking oil and accounting for the faulty dipstick installation I hope low oil/oil psi is to blame. Might have wiped a bearing but well see

[Red67Mustang]

Not having heard the sound first hand, i assume you've ruled out simple spark knock (too much timing). Before tearing the engine down to look for a bearing failure, it'd be worthwhile to pull the exhaust manifolds/headers and look for a blown out exhaust gasket. Sometimes this condition can sound like a bad knock. If everything there is fine, then noodle around with a mechanics stethoscope, but I agree the next likely culprit would be a bearing.

[benno505]

put your oil pressure gauge on, it will tell you alot, I am 90% certain you will be stripping this engine, sorry for the bad news. as it sounds like a conrod problem.

[ccm399]

Agreed, sounds like connecting rod issue to me too.

First things first, put a gauge on it, mount it outside if you don't want oil inside. Then try this, take the revs up to 1500-1800 and hold then release the throttle, the increased vacuum normally will increase noise. You might have to warm the engine a little before trying this though as the oil being thick can mask the sound.

I once had a guy that heard a slight knocking and we could not get the sound to re-produce until the car was fully warmed up. Then the raised, steady RPM then quick throttle closed test showed it was indeed a rod issue. A later teardown confirmed. We caught this one in time though, re-sized the rods and polished the crank and all was OK. We suspected dry start (IE: oil pump not primed) in this case.

Check the converter and exhaust leak deal too though. I would suspect either of those to get worse with RPM though...

Hope this helps,

Chris

[WidowMaker68]

Thanks for the help everyone. I've enjoyed this forum a lot. Had hoped to not have to air these types of problems for myself but that's life. Looks like ill be pulling it apart. Not sure when but ill keep you posted.

[sccacuda]

I'm assuming your carb'd? Before tearing it down, I'd start by pulling the plug wires one at a time to see if you can isolate which cylinder your dealing with. I'd want to rev the engine as well with the plug wire off.

[WidowMaker68]

Considered that. Still haven't bothered with I since yesterday. Would be nice to know which cylinder is the problem. However if it is a rod knock(wiped bearing) I'm gonna have to get it all apart anyway. Not too excited about that but I'd rather not blow the rod and potentially ruin the engine. Still gotta try adding oil, check exhaust, flex plate etc. In perspective it would allow me the opportunity to do some engine compartment work that I overlooked or just dismissed as aesthetics that I would like to do. It's a project car so no big hurry. Plus I could start a build thread and post the progress here. Looking at the bright side

[WidowMaker68]

So drained my oil and filter. No signs of metal. Got a mechanic friend coming by to check it out. May just be taking the car to his shop and doing the rebuild again. Excited though because his skills are better than mine

[WidowMaker68]

FIxed. And I feel dumb. Ended up being a rocker arm that vibrated loose again! Gonna upgrade to some arp locking nuts because I think this cam just pushes them right off. Set the timing up and now I runs like a dream. I thought there was no way it could have been that. I just reset the valves prior to the previous startup

[Ron Sutton]

That's great. Better to have a small problem & learn ... than to have to rebuild the engine.

Good luck with your project.

.

[bmbrzmn101]

Glad to hear it was nothing more than that. Good luck with rest of your project.

Chris

[moreHP]

Good to hear!

[Grease Monkey 23]

Glad it was minor and ya figured it out!! Gd luck!!

[WidowMaker68]

Agreed. Way better than what I thought it might be. Live and learn.

[MonzaRacer]

big thing with a Pontiac engine is you need hardened washers under nuts(with bottle neck studs) then just use stock Chevy style nuts, which ever version you need for said rockers.
\ALSO good you found it but alas i had a weird one, rebuilt engine, driven less than a year,,,,,,,loose bellhousing bolts! Oddest intermittent knocking that bugged me too no end.