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Discussion Starter #1
Hey everybody.


I've got a strange light knock.. sounds a lot like a rod knock, but it goes away when I'm over around 1500 RPM or so.


I checked my oil, and cut open the filter and found bearing babbit.. so my first thought is it was a rod knock, so about 2 months ago (after I found bearing babbit in the filter) I pulled the pan and pulled all the rod caps and main caps and inspected the bearings, but they were all damn near brand new looking (as they should be, motor is less than a year since rebuild). So I put it all back together and went about my business.


Fast forward a bit and I'm doing a cam swap.. I discover that my cam bearings were wearing out around the 5 to 7 o clock position. Measured it with a snap gauge and the clearances at this position were about .006" to .007" (as opposed to the .002" - .003" around the rest of the circumference of the bearings). I noticed that the machinist really didn't clock the oil holes around the 3 to 4 o clock position as recommended, so I bought new Clevite AL-3 aluminum bearings and got them put in (properly clocked of course). Oil pressure increased by about 10 - 15 PSI.. so that's where my babbit was coming from I presume, considering I checked all the other main and rod bearings and they were fine.


Anyway Oil pressure is between 25 - 40 psi at idle depending on how hot it is, and between about 50 - 65 psi at my cruise RPM of 3000 also depending on how hot the motor is (this is with the new bearings in).


However the mysterious engine knock is still there at Idle.


So I researched it and I found people say it could be a cracked flexplate or loose torque converter bolts. So I check my converter bolts and they were all tight, then I removed the flexplate and inspected for cracks... none to be found.


I also read that it could be loose bellhousing bolts, and guess what? They were all tight too.


I highly doubt it's piston slap, I'm running hypereutectic pistons with .002" clearance at the bottom of the bore and taper to .0015" at the top of the bore (block plate honed when it was machined).


One thing I did notice is it only seems to be when the engine is hot, and I did notice that my right side header was glowing a little bit at idle.


Could this just be a noise in the exhaust from running too rich at idle, causing excess fuel to burn off in the header and make it glow/knock from the fuel burning in the headers?? I have a wideband AFR gauge (I know it isn't recommended to read off that gauge at idle, but it reads about 13:1 at idle which is rich).


My valves are all lashed to .022" hot (solid flat tappet cam).


On a side note, I did try loosening my lash up some to get a little more midrange out of the cam (240/240° @ .050") from the recommended .018" hot to .026" hot and I noticed it made a knocking noise when I did that, but when I'd tighten it back up to around .018" it would lessen the noise... just to clarify it is a knock and not a tap or tick like you'd hear from a dead lifter or loose valve lash.


I haven't done a compression check yet, but I'm considering it.


Would a faulty plug wire cause this as well? It was acting like the transfer slots were out of adjustment below 2000 RPM, but I checked them and they were all about .030". I know in the past when I've had a plug wire arcing off the headers it would only act up at low RPM.


Any ideas or input would really help as I'm very confused here.


Thanks.
 

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Would we be safe to assume that you have roller rockers? If so, then I've found them to be noisy at low rpm. And considering you have solid lifters, then I'd be leaning towards that or piston slap (but you said you don't belive that's the case.)

You didn't mention what the build is, but keep in mind that the shorter the pistons, then the more likely there is a possibility of piston slap at low rpms. When I say short pistons I'm describing a 383 style piston for a 6" rod that is a bunch shorter than a stock 350 piston.

Of course these are my guesses. I'm sure someone else with a tad bit more knowledge and experience than I will chime in. Personally, I'd be checking the oil filter every oil change if there were any concerns, and it is a GREAT practice on a freshly built engine or a performance engine.

Best of luck - Jim.
 

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Discussion Starter #3
Oh and it's a SBC 355 10.1:1 SCR. Standard 350 stroke and .030" over bore on the block. Quench is .040" with a .025" deck height and .015" Fel-Pro shim gasket. Gross lift is only .490" intake and exhaust with the 1.6 ratio rockers and .022" hot lash. Advertised duration is 278°. Max lift allowed on the valve springs is .540" (after the .060" is subtracted from the .600" coil bind height).
 

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Discussion Starter #4
Would we be safe to assume that you have roller rockers? If so, then I've found them to be noisy at low rpm. And considering you have solid lifters, then I'd be leaning towards that or piston slap (but you said you don't belive that's the case.)

You didn't mention what the build is, but keep in mind that the shorter the pistons, then the more likely there is a possibility of piston slap at low rpms. When I say short pistons I'm describing a 383 style piston for a 6" rod that is a bunch shorter than a stock 350 piston.

Of course these are my guesses. I'm sure someone else with a tad bit more knowledge and experience than I will chime in. Personally, I'd be checking the oil filter every oil change if there were any concerns, and it is a GREAT practice on a freshly built engine or a performance engine.

Best of luck - Jim.


Yeah the compression height on the piston is 1.550" on a 5.7 rod.. so they're not the shorties. I'm not completely throwing out he possibility of piston slap, I mean it definitely is a possibility. I just want to try and boil down what else it could be (things I can actively fix without doing a complete engine overhaul).


And yes Jim, they are in fact roller rockers. The noise just sounds deeper than one you'd normally associate with the valvetrain.


I plan on cutting this filter open too.. once I get around to changing the oil again. It's very fresh, I changed it after the cam swap, it only has about 300 miles on it.
 

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Discussion Starter #5
One thing I did notice is the bolt holes in the mounting flange of the converter are quite large and easily fit a 7/16 bolt. Even the mount holes on the flexplate fit a 7/16 bolt. I was using a 3/8 bolt, but I think I'll go up to 7/16 to see if maybe that helps.
 

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I once had a low speed knock like that in a 396. Turned out to be detonation. Too much timing coming in too fast. Sounded like a rock in a coffee can.
What's your timing set up like?
 

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Discussion Starter #7
I once had a low speed knock like that in a 396. Turned out to be detonation. Too much timing coming in too fast. Sounded like a rock in a coffee can.
What's your timing set up like?
Hmmmm.. that's a thought. My timing is at 18° initial and 34° total, all in by 3000 rpm. Vacuum advance is ported, not manifold, and brings total timing to 51° at max advance.

But even with my old cam (comp 282s) which had a little less duration at .050" and a tighter LSA, I had timing at 16° initial and 35° total and it still had a knock at idle. I wouldn't think 16° is too much initial, even 18° isn't bad compared to what I've read from other people and their combos... unless if I need to run colder plugs?? I'm running NGK BKR6E's.
 

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Just remember, your ported vac becomes manifold vac as soon as the throttle cracks open.
Easy test. Plug it off. If it gets better, pull the timing back a bit.
Costs nothing but a little time.
 

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One thing I did notice is the bolt holes in the mounting flange of the converter are quite large and easily fit a 7/16 bolt. Even the mount holes on the flexplate fit a 7/16 bolt. I was using a 3/8 bolt, but I think I'll go up to 7/16 to see if maybe that helps.
This I don't understandthe 7/16th versus 3/8ths bolt, the converter will accept one or the other. While either will probably pass the flex plate hole, maybe even the 10mm bolt, but the converter will dictate which dia and thread gets used. Yeah, I hate the flexplate with the one size hole fits all 3 common bolt sizes but that is what we get.

A header tube that glows would indicate a lean mixture on the cylinder or retarded timing, amybe a sticking valve. Rich mixtures burn cool as heat is removed to vaporize the fuel. Lean mixtures burn hot, late firing also burns hot as the energy of the fire is going out unused with the exhaust rather than having been put to work moving the piston which removes heat from the fire.

I agree with 64nailhead, roller rockers and I think especially 1.6 rollers make a lot of noise but that sounds more like loose lash on everything rather than a single repeating tick. Since youre running solids youre probably more familiar with that noise than you ever wanted to be.

Cam bearing wear is odd even with the missplaced oil hole, but high spring pressure can do that. A solid roller cam seems short on lift at .490 this sounds like a .5 lift rule dirt racer cam which can use some pretty stout spring pressures. If this is true I'd suspect your oil pressure is too low for street operations at 40cold-25hot pounds. The problem is extended low RPM, thus low(er) oil pressure with long duration force moments on the cam from high pressure valve springs. This puts a high and fairly slow load into the cam, not a lot unlike what happens to main and rod bearings when an engine is lugged a lot. If this is a track cam then bear in mind that normal track use doesn't include much idling at traffic lights, this can really work the cam with big spring pressures as it snaps back and forth as each lobe oens and then closes each valve. A lesson Harley got beat up on with the Twin Cam engine, admittedly fewer lobes so there was a lot less compensation where a lobe is opening a valve as another is closing but nonetheless a risky situation even when 16 lobes on a common shaft are involved and the SBC cam ain't the stiffest metal stick around something we tend to forget that the SBC cam moves like a worm with high spring pressures which is why the unrelieved cam was developed for the SBC and the LS engines substantially increased the cam core diameter.

Bogie
 

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Discussion Starter #10
Just remember, your ported vac becomes manifold vac as soon as the throttle cracks open.
Easy test. Plug it off. If it gets better, pull the timing back a bit.
Costs nothing but a little time.
Funny thing, I actually just started running vacuum advance because it ran like **** at low rpm part throttle. It would miss and feel sluggish, then I hooked up vacuum advance and now it's smooth and crisp. Plugs burn nice and clean too.
 

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Funny thing, I actually just started running vacuum advance because it ran like **** at low rpm part throttle. It would miss and feel sluggish, then I hooked up vacuum advance and now it's smooth and crisp. Plugs burn nice and clean too.
I've never been a fan of the ported vacuum setups. Running full manifold vacuum at idle allows you to close the throttle plates a bunch at idle (more economy) and once you're moving with your foot into it (even at partial throttle) it acts just the same.

My opinion - take it for what it's worth and I DO NOT want to start the ported vs manifold vacuum debate. To each his own preferences.

Good luck:)
 

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Discussion Starter #12
This I don't understandthe 7/16th versus 3/8ths bolt, the converter will accept one or the other. While either will probably pass the flex plate hole, maybe even the 10mm bolt, but the converter will dictate which dia and thread gets used. Yeah, I hate the flexplate with the one size hole fits all 3 common bolt sizes but that is what we get.

A header tube that glows would indicate a lean mixture on the cylinder or retarded timing, amybe a sticking valve. Rich mixtures burn cool as heat is removed to vaporize the fuel. Lean mixtures burn hot, late firing also burns hot as the energy of the fire is going out unused with the exhaust rather than having been put to work moving the piston which removes heat from the fire.

I agree with 64nailhead, roller rockers and I think especially 1.6 rollers make a lot of noise but that sounds more like loose lash on everything rather than a single repeating tick. Since youre running solids youre probably more familiar with that noise than you ever wanted to be.

Cam bearing wear is odd even with the missplaced oil hole, but high spring pressure can do that. A solid roller cam seems short on lift at .490 this sounds like a .5 lift rule dirt racer cam which can use some pretty stout spring pressures. If this is true I'd suspect your oil pressure is too low for street operations at 40cold-25hot pounds. The problem is extended low RPM, thus low(er) oil pressure with long duration force moments on the cam from high pressure valve springs. This puts a high and fairly slow load into the cam, not a lot unlike what happens to main and rod bearings when an engine is lugged a lot. If this is a track cam then bear in mind that normal track use doesn't include much idling at traffic lights, this can really work the cam with big spring pressures as it snaps back and forth as each lobe oens and then closes each valve. A lesson Harley got beat up on with the Twin Cam engine, admittedly fewer lobes so there was a lot less compensation where a lobe is opening a valve as another is closing but nonetheless a risky situation even when 16 lobes on a common shaft are involved and the SBC cam ain't the stiffest metal stick around something we tend to forget that the SBC cam moves like a worm with high spring pressures which is why the unrelieved cam was developed for the SBC and the LS engines substantially increased the cam core diameter.

Bogie


It isn't the threaded type recepticle on the converter flange, it's just a hole, like on the flexplate. The diameter of the hole is large enough to fit a 7/16 bolt, and so is the flexplate. So i'm wondering if going from the 3/8 bolt I had to a 7/16 will help? (Theres no signs of contact or wear on the flexplate bolt holes though so this is a long shot). I know I'm not running lean, in fact i'm probably a tad rich, but what makes me wonder is why is it ONLY one side header glowing? And by the way it's not glowing extremely bright, just a bit, and it's only around the collector (they're the shorty headers with the collector next to the oil pan).. but they are a cheapie chinese e-bay header, so maybe it's just thin tubing.


Oh and it's not a roller Bogie, it's a flat tappet. It's the old school isky Z-25 grind. My spring pressures are only 132lbs seat and 292lbs open. Nothing crazy.


I just don't know why my oil pressure would be "low" anyway, my rod clearances are .002", mains are .0025", and the cam bearings are pretty tight, around .002" - .003" also. It's got a brand new oil pump in there too.


I never really thought 25-40psi at idle and averaging 50-60psi at cruise was low? But then again I really don't know too much so my thought process is not quite as credible as yours haha.


I'm just concerned there may be a mechanical interference somewhere and I really have no clue where to check...


Anyway I appreciate the input Bogie. I'll probably drop the pan AGAIN and check over everything AGAIN because I'm paranoid like that.



Watch, it's going to end up being something stupid and simple too.. Like a loose valve messing with the harmonics of the valve operation and causing a disruption of some sort in the exhaust cycle. Who knows... but then again it never hurts to go through everything and check so you know for a fact everything is ship shape.

 

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Discussion Starter #14
https://www.youtube.com/watch?v=AC3RWzAZ4_M&feature=youtu.be


Here's a link to a video I took of the motor a few months ago (with the old Comp 282s cam and the old cam bearings) and before I checked all the main and rod bearings.. but the noise it's making is the same one it's making in this video.


You can hear a faint knock/tap kind of noise in the background of the exhaust.
 

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https://www.youtube.com/watch?v=AC3RWzAZ4_M&feature=youtu.be


Here's a link to a video I took of the motor a few months ago (with the old Comp 282s cam and the old cam bearings) and before I checked all the main and rod bearings.. but the noise it's making is the same one it's making in this video.


You can hear a faint knock/tap kind of noise in the background of the exhaust.
I couldn't hear anything other than some exhaust noise that I would say is due to the headers. Heard that noise before with long tubes at low idle, but maybe I'm just noit hearing what you're hearing. Like I mentioned previously, I'd check the oil filter(s) at every oil change and run it.

Best of luck - Jim
 

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Have you tried pulling each plug wire off one at a time to isolate the source?
I'm guessing piston pin, or maybe cracked piston
 

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While mine is a SB Ford, I had something similar. Lots of work for a 'phantom' over 2-3 years.

- I did the timing 'thing' and even tried a different distributor. No change
- I did the roller rocker check including installing better ones with new hardened push rods. No change
- I checked the Stewart water pump bearing. Still no change
- I checked the fuel pump against the eccentric - Noise is still there
- Pulled the motor mounts - one was touching the header when the engine rocked - clearanced it - Same ole thing, still a noise

- Jan 2014 pulled the engine and checked all of the bearings, dimensions and clearances - perfect but changed the bearings anyhow. I also replaced the oil pump as it had a strange bottom plate wear pattern - THEN, just because I could, spun the torque converter in place on the transmission. OH @#$%. New Hughes converter, PA flex plate and - no knock. A note, the PA flex plate is about 50% thicker then the flimsy TCI and the Hughes converter is a new one, not a revised OEM/TCI core.

Dave W
 

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Discussion Starter #20 (Edited)
I couldn't hear anything other than some exhaust noise that I would say is due to the headers. Heard that noise before with long tubes at low idle, but maybe I'm just noit hearing what you're hearing. Like I mentioned previously, I'd check the oil filter(s) at every oil change and run it.

Best of luck - Jim


You know Jim, that's what I was thinking too.. but I was driving through a grocery store parking lot the other day and a guy mentioned to me it sounded like I had a rod knock. So I got paranoid.


RWENUTS: Yes the fuel pump pushrod is out.
 
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