Out of balance flywheel can cause loud banging??

[Hallen]

After I got my engine back in and I started it, I heard a very loud banging sound. The banging didn't happen when I just was turning the motor over with the starter. But after the engine started up, that's when it happened. It was quite loud. It was nothing like rod knock or anything muffled like that.

I took the engine back out and I found that I didn't align the pressure plate with the flywheel properly. In other words, they were balanced as a pair and I didn't put them back together the same way.

I had the flywheel balance after I had the clutch done and I had a bad vibration. They balanced it and the vibration diminished a lot, but I don't think it was totally gone. It looks to me like he welded a bunch of metal into place to balance this stuff.

Would that out-of-balance condition cause some banging? Transmission to transmission well? Driveshaft against shifter? ( I never moved the car. I started it three times to check with the clutch in, and out, but the I never spun the drives shaft.) Yes, I know anything is possible, I just want to know if anybody has some experience with this. I don't want to put the engine back in to find that the banging is still there.

Pictures below:

Weight added to the pressure plate to balance the assembly.


Weight removed from the flywheel and the two dimples for aligning the pressure plate.


And the two marks on the pressure plate that I was supposed to align with the flywheel.

[xLibelle]

What the hell is going on??
Who balances the flywheel and pressure plate as an assembly?!!

And even if the flywheel were out of balance, it would be obvious there are other problems because of banging.

Can you see if there are any metal on metal contact points in the bell housing?

Does the sound appear as soon as the engine is idling or does it come with some reving?

[Devil Inc]

I've never heard of one having to have the pressure plate "balanced", usually that process just involves the flywheel.

[jlevie]

That is not the cause of the banging. What else was done to the engine and what propted its removal from the car?

[Hallen]

I comes on as soon as the engine fires. I didn't even touch the gas pedal.

There is no obvious signs of contact on the bell housing or the tunnel. I also checked the exhaust but couldn't see anything there. But, I only let it idle for a second each time before I shut it off. So there was a total of about 3 seconds of run time.

The pressure plate and flywheel were done about a year ago when I had the clutch and the throw out bearing done. I drove the car a little bit, half a track day and a few hours of freeway time after that. Then, since I was going through a quart of oil for every twenty minutes on the track, I pulled the engine to rebuild it. I did a standard rebuild replacing rings, bearings and seals. I had the head checked and had the valve seals replaced. I had the engine honed and check out. That's it.

I have a line on a different flywheel (used) and I can get a new pressure plate. They are not that expensive.

Obviously, from your reaction, that big blob of weld on the pressure plate isn't normal. It looked odd to me too, but I don't know anything about this kind of stuff.

The banging could have been the exhaust. I honestly don't now about the vibrations in the car, I was so startled by the sound that I just shut it off.

Would getting the flywheel balanced by itself and then putting on a new pressure plate be a good way to go? Should you check balance of the whole assembly or is that totally unnecessary?

[Liam]

Seems a little crazy.

[Hallen]

Seems a little crazy.

lol, yeah, it is. I could have totally hosed something in the rebuild I suppose.

The banging sound is pretty rapid and it isn't a misfire type of bang. It is more like when you take piece of sheet metal and bend it back and then let it slam back down onto another metal surface. Maybe more of a whap than a bang...

I am not saying the flywheel IS the problem. I'm just wondering if it could be the problem.

[xLibelle]

youre gonna have to get a video/sound clip of this because your sound descriptions are not adequate. plus it would help if you could locate the sound not to mention the volume of it. its speed of percussion would be nice as well, if you arent providing a clip.
forget about the flywheel for now. as i hinted earlier, if it were unbalanced, you wouldnt have this knocking unless something else were seriously wrong. for all we know, someone didnt tighten a rod bearing and its raping your internals.

[jlevie]

Unless the flywheel was loose on the crank shaft, it don't see it being the cause of the noise. And I certainly don't see the balancing being, or lack of balance as being the problem. I suspect that something went wrong in the engine rebuild.

[NC325iC]

ive had the assembly balanced by a machine shop, AFTER they balanced teh flywheel alone

[Hallen]

I can't get a recording for you since the engine is back out already. That's how I got the pictures of how I assembled it wrong :)

I'll pull off the oil pan in the next day or so and see what I see. I think I will get a new pressure plate and see about getting the flywheel balance checked. Even if it isn't the problem, it should be done anyway. That big blob of weld on the pressure plate bothers me.

I am sure the timing is right and I checked the valve clearances before I put the engine back in. I think I would have felt a valve hitting the piston as I turned the engine through while checking the clearances. It's pretty easy to line up TDC so I don't think I screwed that up. Plus, I started it three times, you would think if I was smashing valves, it wouldn't have started again.

What I hate is not knowing... no smoking gun as it were. If I don't see anything when I pull the pan off, I'll be at a total loss.

[NC325iC]

when i had an unbalanced flywheel on my car it didnt make any weird sounds it just shook the car, alot.

[Hallen]

Thanks for the input.

I have AKG urethane mounts all around so it's pretty solid. We'll see how it goes...

[jlevie]

I'll pull off the oil pan in the next day or so and see what I see. I think I will get a new pressure plate and see about getting the flywheel balance checked. Even if it isn't the problem, it should be done anyway. That big blob of weld on the pressure plate bothers me.

There is nothing wrong with adding weight to a pressure plate to balance it. It is either that or removing material from the other side of the pressure plate. The flywheel should be balanced first and then the complete assembly balanced and any weight adjustment then done to just the pressure plate. It would seem that was the process used here.