Knocking noise help.

munsonbw · 12-02-2007, 03:45 PM

Guys, I have been reading some old threads here on "knocking" noises and wanted to get some early info. I just got my 81 SC Row back together after a head stud job. Since it was low miles, I inspected things for wear and put it all back together pretty much as is. The heads, cams and rockers never came apart. Pistons stayed in their bores, except for one that slipped out on me. When trying to get the rings back in I snapped an oil ring, which was replaced. The exhaust system is new, as are carrera tensioners.

Here is the problem: I have a knocking or clunking sound coming from the right bank. I have used a long screw driver to try and isolate it, but everything sounds normal. The noise is not transmitted through the stethoscope. It does change with rpm and seems to get quieter as it warms up. I first thought piston slap, but wouldn't I hear this by listening to the cylinder? Also, the very first time I started it after the teardown, I don't remember hearing it (and I was super paranoid about noises). I am thinking, given these circumstances, that it may be an exhaust leak. Just sounds too clunky for that though. Only appears to happen once a revolution.

Any ideas? Could I not hear a rod knocking with the stethoscope? Maybe the intermediate gears as Henry Schmidt suggested in another thread? How can I start isolating sources without a teardown?

Thanks,
Ben

ZOA NOM · ZOA NOM's Garage

Drain the oil through a strainer and look for shiny flakes from a bearing. Sounds like a rod bearing may have gone south.

Also, did you put the same size cylinder base gaskets back on? The cylinder-to-head clearance may have changed if you didn't.

munsonbw · 04:29 PM

Just saw in another post that you can help to find suspect cylinder by pulling plug wires. Looks like it is number 4. When the plug wire is pulled the noise ceases. Also to note, appears to get less severe are rpm increases. I will try the easy thing tomorrow and try and tighten the exhaust flange on #4.

More food for thought: it didn't do this before the tear down and the oil pressure is fine (about 1 bar per 1000 rpm hot).

Zoanas, I did put the factory size base gaskets back on. Oil check will come next. What kind of strainer do you use? I am thinking a paint strainer may be too fine?

Thanks,
Ben

ZOA NOM · ZOA NOM's Garage

I just used a flat frying pan spatter cover:

munsonbw · 12-03-2007, 03:15 PM

Further diving into the archives I found advice for a different problem recommending that I use a piece of hose to pinpoint sounds. Wow, what things you can here with that! I began listening around to #4, since the noise stopped when that plug wire was pulled. To my amazement, I heard a faint whisper. A sound so sweet it could have been from the Sirens... it said "I am not a rod bearing, but an exhaust leak." Hallelujah! In all seriousness, I used the hose on the other exhaust ports and no noise. Moved all around the flange and nothing until I hit a certain spot, then loud and clear. I could even feel a faint puffing of air. Seems pretty definitive to me.

So, next question: I used all new gaskets and tightened the nuts pretty tight. A friend recommended some Permatex copper seal (?). Anybody use this or similar? Please don't suggest any sealant that can only be found here at Pelican unless it is the only thing that will work... I hate to pay $10 in shipping for $5 in sealant. And, thanks to everyone on the board who offer their help. It is really an invaluable resource.

ben

Grady Clay · 12-03-2007, 04:19 PM

You might consider it is a broken inner valve spring. Pelican kobaltblau had one on his ’82 911SC with similar symptoms. We changed the spring in the car. Well ... Andy did the work and I consulted and supervised.

Best,
Grady

munsonbw · 12-03-2007, 05:11 PM

Grady,

How would I diagnose this without taking it apart (the spring that is)? I would never have suspected that. Seems rather odd that the noise would be isolated to one side of the exhaust flange if it were a broke spring. I'm not trying to argue, only get a better understanding of how that kind of problem manifests itself symptoms wise. I'll search for posts by kobaltblau and see what turns up.

Thanks,
Ben

munsonbw · 12-05-2007, 02:58 PM

Update for the group:
After the results from the tube listening device pointed to the #4 exhaust flange, I took of the HEs for a look. Sure enough, I didn't notice before I put them on that one of the flanges has some pitting that was obviously ground on a little and made the flange tilted relative to the head. Some more judicious grinding and afterwards verification with a straight edge has yielded positive results. I would have swore that the noise was more metal on metal contact sounding than an exhaust leak, but mine was really bad so I guess I was hearing a pretty good bang. Thanks for the suggestions. Now it is time to drive!