Knock/clatter from engine when reving

[Andy.B]

m20b25 with approx 120k miles. Had a bad HG and a worn cam lobe on cylinder one when I got it. I swapped in a rebuilt head (cleaned, machined, valve job, new eccentrics, etc) and replaced all the maintenance/wear parts.

The engine makes a clatter/knock noise that sounds like it's coming from the center of the engine.

Link to flickr video. My phone's microphone doesn't capture the sound well, but you can hear it best starting at around 10 seconds in the video. It's distinctly louder/sharper than the normal m20 sewing machine valve noise, and seems to be timed to one cylinder.

The specifics of the noise are:
- makes the sound both cold and warm (so probably not piston slap).
- comes on strong at about 2.5k rpm and stays pretty consistent through the remainder of the rev range.
- sounds like its coming from the cylinder 3/4 area.
- no change in sound with spark plugs disconnected (so possibly not rod knock?).
- tone changes slightly from revving the engine to constant rpm, and from constant rpm to returning to idle.
- sound sounds a little clearer through at stethoscope on the bottom end. not louder, just clearer, since it doesn't get drowned out by the valve train clatter.
- it actually sounds worse to the naked ear than through a stethoscope.
- it does sound like metal to metal. it is a sharp, crisp rattle/knock noise.
- no filings/metal visible in oil.
- has been making this sound for several thousand miles now.

It doesn't sound as solid as rod knock normally sounds. Its a lighter crisper sound. I keep thinking its coming from the valve train, but I have adjusted it multiples times, using different feeler gauge sets and the spring tool, trying both a slightly tighter and looser approach. Outside of this noise the engine drives fine. It pulls hard, and runs smoothly.

Still, the only things I can think of are rod knock or wrist pin bushing noise, or piston to valve contact. The problem is that the usual rod knock tests (pulling a spark plug) makes no change in the noise at all, and that it has been several thousand miles. It really sounds like it's coming from the top end, but I know that doesn't necessarily prove anything. If I had piston to valve contact (which seems unlikely on a stock head/cam which is properly timed and runs smooth) I suspect that by this point I would have bent valves, not just a constant clatter.

I have an m20b27 I can scavenge a bottom end from if need be, but I don't want to tear the thing apart without knowing whats wrong.

Also, I know about getrag rattle, and I have that too. I have seen other threads around the net discussing this issue, but I haven't found any where someone concluded what they actually found the sound to be. Just a lot of 'I have that noise too!', or 'Time to rebuild it' comments, without any closure to the thread.

[Andy.B]

Also, both dowel pins were in the block when I put the new head on, so the head is lined up properly.

I suspect it is rod knock, and I am simply in denial, since I already wrapped up a head swap and manual tranny swap. The only thing I HAVEN'T pulled out yet is the bottom end.

[Ether-D]

I have that noise too! Mine is very similar to pinging. More of a rattly clatter than a knock.

I also subconsciously think it's rod knock that I'm in denial about. No shavings in my oil either. Mine starts at 2750rpm and continues through 4000 or so, but only under super light throttle and when fully warm. I also have several thousand miles on the noise. No performance loss.

But foreal, I think it's a piston wrist pin bushing in my case. If it was rod knock, wouldn't it be rapidly getting worse? I had rod knock before, and it was absolutely unmistakeable. I heard it, and I was like "that's rod knock". I was right and put a yard motor in. Now I hear this noise and I'm like "is that rod knock?". So Idk. I'll be watching this thread to see what comes of it.

[Andy.B]

Wrist pin bushing sounds like a logical diagnosis to me too, except for the fact that eliminating combustion in the cylinders produces no effect on the noise (which also rules out pinging/detonation). I'm going to try thicker oil to see if that helps.

Also, and I can't imagine this is the case, but I am worried that I might have the slightest of valve contact with the piston. I may fit an adjustable cam gear and roll back the timing to see if that changes the noise. But I suspect that even the slightest of valve contact would have destroyed the engine by this point, several thousand miles later.

[redlightpete]

Mine starts at 2750rpm and continues through 4000 or so, but only under super light throttle and when fully warm. I also have several thousand miles on the noise.

The car I had with rod knock acted just like this. It was a vw so I could easily change the bearing on the bad rod journal every time I changed the oil. Never ended up failing. Those are tough motors, but so are m20s. With an m20b27 I'd say just run it.

[ahlem]

Take off the valve cover and inspect the rocker pads that hit on the cam. These are supposed to be convex. If they have worn to a concave, you get this rattle, especially if you leave too much space when you tighten your valves. On the heads I've seen with this, it was on the intake side. Maybe oil starvation was the cause.

[Andy.B]

Bump for another 500 miles, and the internal engine parts have remained internal. I haven't had a chance to pull the valve cover again to inspect the rockers, but I will do that over the weekend.