SBC 350 diagnosis

[bikersmurf]

April 2016 the SBC motor started to have a knock about 2K rpm. Just recently replaced the stretched timing chain hoping that was the source of the noise... it sounded much better but still had a bit of noise at a low idle and about 2100 rpm. After running 15 minutes the noise started getting worse. The strange thing is, it still runs well... almost too well. No smoke or other weird things. Just this clacking noise in sync with valve movement.

So I'm asking for help because I'm running out of ideas. Ultimately I'll likely need to dig deeper, but while it still runs is there anything else to check?

1) the clacking/knocking noise is loudest at the intake

2) with the valve covers off, the pushrods all spin while it's running and rockers all appear to be working properly

3) I tried pushing the clutch, it works perfectly and makes no difference

4) compression is above 175 at all cylinders

5) pulled some plug wires... up to 4 at at time no difference. It even ran 1/2 way decent on only 2 cylinders

6) a friend suggested having the timing chain looked at. The Coyles True Roller chain was slightly wider than the OE one. To make it work required sanding a bit off the block casting. Could this be what's making the loud clacking?

 

[B y r o n]

With the valve covers off, did you bother to adjust the valves?

It's possible the ticking could be a loose valve or two.

 

[Tancruiser]

Could be a sticky lifter try one of the BG products.

 

[Youpicktokim]

Stethescope to pin point noise..go from there.
If it's ticking, may be valve adjustment issue or an exhaust leak.

 

[bikersmurf]

It isn't lifter tick. I've not tried adjusting the valves because it's not the right sound.

I had an experienced mechanic listen to it and they said it's not: rod bearing, wrist pin, lifter tick. They were suggesting that I didn't create enough clearance for the Cloyes True roller chain set and the sound of the chain is telegraphing through the block.

 

[POTATO LAUNCHER]

Could be a broken valve spring Smurf. Have you checked the rocker arms to make sure they are all pivoting correctly? If it is the chain it will wear itself in but then you have metal shavings in the timing cover. Doesn't sound extremely bad but I would dig a little further, maybe pull the intake to check your lifters. Im assuming it has hydraulic lifters.

 

[bikersmurf]

Stethescope to pin point noise..go from there.
If it's ticking, may be valve adjustment issue or an exhaust leak.

done that, It's loudest at intake and quiet under motor

 

[bikersmurf]

Could be a broken valve spring Smurf. Have you checked the rocker arms to make sure they are all pivoting correctly? If it is the chain it will wear itself in but then you have metal shavings in the timing cover. Doesn't sound extremely bad but I would dig a little further, maybe pull the intake to check your lifters. Im assuming it has hydraulic lifters.

Valve springs all look good. All the rocker arms pivot correctly and make no sound while running. Yes hydraulic lifters.

If you stick your head out the front door, I'll go start it up and you can likely hear it..

I've had two spun rod bearings over the years, and this is 5 times as loud... I'm surprised the neighbors haven't come running. But they're more likely the type to run when something sounds like it's going to blow up.

 

[whitey45]

Sbc will run all day long with a bad rod bearing! Pull the pan and check the rod bearings, almost guarantee it's a bad bearing! Also, pull each plug wire, one at a time, this takes the load off the bad cylinder, you will know when the knock goes away.

 

[POTATO LAUNCHER]

Tough call man. May have to dig a little deeper to find the cause. Manifold gaskets are cheap and not too much labor involved to pull the intake. Keep us posted with what you find.

 

[bikersmurf]

Sbc will run all day long with a bad rod bearing! Pull the pan and check the rod bearings, almost guarantee it's a bad bearing! Also, pull each plug wire, one at a time, this takes the load off the bad cylinder, you will know when the knock goes away.

Pulled them off and they made no difference. Even four off each bank and it sounded the same.

 

[Captn Conch]

Rod bearing quite possibly. How do you run your unit? Hot Rod, Mud grubber, Normal Cruisering with no foul abuse? Timing chain clearance?? If I remember right , and I think I do, I put the Cloyes Double roller t chain in my '76 SB GMC 350 back in '85. it is still a good running engine with no probs.

 

[whitey45]

Rod knock generally increases with rpm, if the sound doesn't change with rpm, it's likely something else.

 

[bikersmurf]

The rate increases with rpm, but doesn't get louder or change.

I'm good to my vehicles. I don't run them hard or abuse them. Just normal cruising. The points conversion has a rev limiter set at 4k. I've rarely hit it in the 21 years the SBC has been in the truck. The stock F motor died after 185k miles... this motor I've put about 45k miles on and it now has about 150k miles on it.

In 1997 I replaced the original cam. It was missing 5 lobes yet it still ran strong. With the RV cam it really woke it up. It has been an excellent motor since then. Other than the unhappy noise, it still runs strong.

When trying to diagnose the source of the noise, I found it'll run with only two of the plug wires connected... with 4 of the 8 connected, it started without any extra fuel and idled smoothly. Last time I checked it shouldn't run at all well. Compression was 175+ across the board and even between them. Last time I had it on the road, it'd idle up a gradual slope in 3rd gear high range at 650 rpm.

Lately it wasn't performing as well, but the 15-20 degrees of slop in the timing chain would likely have an impact.

 

[bikersmurf]

I've ruled out the timing chain,

Next up, pull the intake or the oil pan. I think I'll pull the intake first since that's where the noise is coming from and it'd be easier.

My theory is that if it's the bottom end... I'll want to know what the top end looks like to decide if it's worth it to Re & Re the bottom end. Of course that'll mean pulling the heads to get there.

Would there be a point to redoing all the bearings without the rings on a 150k mile motor? Or am I thinking correctly that it'll need rings and valves... possibly a bore & pistons.

 

[POTATO LAUNCHER]

If I were to get that deep into any engine I would go all the way with it. SBC parts are cheap and plentiful. With the compression numbers you have posted cylinders and pistons would just need cleaning to put back in service. Peace of mind down the road knowing that you repaired the issue.

 

[whitey45]

$1600 crate motor from summit, done! Have never had issues with these 290 hp engines.

 

[Dynosoar]

I bet you have a failing hydraulic lifter they act super weird when they leak down to fast.

Dyno

 

[want2wheel]

How did you rule out the timing chain? I have had torque converter bolts back out and cause an engine knocking sound. I bet that the timing chain gear is causing an issue somewhere since this all started after you installed it.

 

[bikersmurf]

How did you rule out the timing chain? I have had torque converter bolts back out and cause an engine knocking sound. I bet that the timing chain gear is causing an issue somewhere since this all started after you installed it.

Pulled the timing chain out, inspected it, and compared it to the old one. Not a single mark on it, and the alignment is the same as the old one (+/- 1 deg). From what I've read it's common with the Cloyes Double timing chains to have to smooth out a couple high points on the block. There has been no contact on the chain or the block