Engine knock after complete rebuild

[crm318]

I just rebuilt my gm goodwrench 350 and installed all new bearings and flat top hypereutectic pistons. It idles very smooth, smoother than any engine I have owned. It is also smooth in gear, but when I go for a cruise, it has a slight knock/clatter under acceleration. I can also rev it up in idle and you cant hear any knocking at all. What might this be? could it be the transmission?

[Todd Hepler]

Doesn't sound like a bearing, otherwise it would do it all of the time.
I would look at a couple of things first:

- Loose accessories on front of engine - Alt/Pwr steering/Fan/etc

- Valve train - are your valve adjusted correctly?

- Timing - is it pinging? what comp ratio ? what grade of gas are you using?

Good luck!

Todd

[68gmsee]

Yes, it may be "pinging" or "rattling." First check for vacuum leaks in all the hoses. Make sure they don't have any breaks or cracks. Check the vacuum advance/dist mechanical advance weights for proper operation and check your timing. Maybe advance it a few degrees.

Engine noises are hard to describe to others. In most cases every part of the engine has a distinct sound when it fails. An experienced mechanic can differentiate these sounds and usually can determine what's causing them.
If you can't locate the cause, you could have someone that knows engines drive it and see what may be causing the noise.

[1970 CST Short Wide]

If it’s an Automatic check your flex plate for cracks, it will sound just like a rod knock sometimes

[Ticker]

First step is to check for an exhaust leak, which will often show up only under load.

Second step is to ensure the timing is set correctly - especially the vacuum advance.

[RIA69]

could be a flat cam lobe if you didnt replace the cam during the rebuild I just went through this with the 307 I had In my truck 4 flat lobes make for a very unhappy running truck. just a thought.

[gkesseru]

Use a stethoscope to listen to each bearing and lifter area. A bad bearing cannot hide. A poor man's version is to use a long screwdriver and put one end on the engine and the other on the bone in front of your ear. I usually put my thumb between the two to protect my face.
However it sounds more like pinging from your description.

[big_al_71]

Besides what has already been mentioned I would check and re-adjust the rocker arms...

[crm318]

It has standard size flat top 4 valve relief pistons with the factory goodwrench 76 cc heads so it should be ok on 87 octane (9-9.3 comp). The valves were adjusted using that hot rodding magazine article proceedure. A mechanic friend of mine thinks its the torque converter ballooning. He says that I should try reinstalling the converter as sometimes it doesnt seat properly. Ill keep yall updated.

[piecesparts]

Check your timing, to much and you will detonate
Check your gas (octane level and what compression ratio you are running at.
Yes. it could be the flex plate.
Is there a chance that you have a piston clearance that is too large (skirt slapping)
Do you have floating wrist pins or pressed in?
Check the fuel pump actuating arm. It can sound like a rod knock