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Old 05-17-2019, 06:36 PM
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Tapping Sound On My Rebuilt SBC

Info of the engine to get out of the way:
- GoodWrench 350 bottom end with Summit Racing brand flat top pistons + wrist pins.
-Summit mild cam with comp cams hydroulic flat tappet lifters
- Ported Vortec heads
Did everything I could myself like; ported the heads, cleaned the block, polish the crank, etc. I did have to take the pistons to be installed to a reputable machine shop in town to be pressed in.

After I got them back, i started piecing it together. I made sure the bearing clearances were good before torquing down all the main caps and rod caps for final assembly.
Put the engine in the car, ran it, broke her in, and everything was fine.... about a month later I finally hooked a transmission to it and drove it down the street ...

Okay... so the issue..
After I drove it a couple times.. I began to notice a faint knocking sound. And it seems like it gets louder as the engine gets warmer. The knock can only be heard when the engine is revved above1500-2000RPM.. when its idling you dont hear anything..
Heres a video: https://youtu.be/wWJJNAuI0sI

I tried using a long screwdriver and tried sticking it around the block to see where it was coming from. And it was mostly the loudest in the rear of the engine...

I had first assumed it might have been a flexplate issue or something to do with the transmission... so after I unbolted the converter and made sure the converter bolts were not coming undone, I started the engine again... and the noise is still there..

It has very good oil pressure btw.. oil isnt low .. I havent tried adjusting my rockers yet, but I honestly doubted that possibility since the sound sounds more like a knock..

Dont know if this has to do anything with it... but I did notice the sound start after i hooked up my gas tank lines to the carb. And used like 3 filters due to the gas looking pretty dirty. Before that I was using a small gas can with fuel line stuck in it ... I used 97 octane in the gas can... but I downgraded to 89 to use in my tank with some sta-bil ethenol treatment additive .. I thought maybe pre-ignition?
Noise doesnt go away when I play with timing though....


Does anyone know what it could possibly be ? Maybe bad wrist pin ?

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Old 05-17-2019, 09:59 PM
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Check distributor shaft end play in the housing, inspect rotor tip and cap for contact, check magnetic pick-up for contact with reluctor wheel. Too much end play allows shaft to climb the cam drive gear.
Just an idea of something to look for.

Does it have a mechanical fuel pump? Weak pump arm return spring can make a knock like that.

Engine blocks are great for transferring sound, many times a knock that sounds like it is coming from one end is actually travelling through the block first before ringing at an opposite end.

Could be valvetrain. Rocker arm slot to stud clearance, retainer to valveguide/seal clearance, pushrod bent, rocker out of adjustment, etc.

Did you check flexplate for cracks...especially at the crank to flexplate bolt pattern. Loose crankshaft to flexplate bolts??
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Old 05-17-2019, 11:07 PM
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Quote:
Originally Posted by ericnova72 View Post
Check distributor shaft end play in the housing, inspect rotor tip and cap for contact, check magnetic pick-up for contact with reluctor wheel. Too much end play allows shaft to climb the cam drive gear.
Just an idea of something to look for.

Does it have a mechanical fuel pump? Weak pump arm return spring can make a knock like that.

Engine blocks are great for transferring sound, many times a knock that sounds like it is coming from one end is actually travelling through the block first before ringing at an opposite end.

Could be valvetrain. Rocker arm slot to stud clearance, retainer to valveguide/seal clearance, pushrod bent, rocker out of adjustment, etc.

Did you check flexplate for cracks...especially at the crank to flexplate bolt pattern. Loose crankshaft to flexplate bolts??
Hmm, I was thinking bout checking the distributor since I bought it rebuilt years ago..

And I havent thought bout checking the fuel pump for noise, so I'll check that.

I'll check on everything you said tomorrow morning and I'll give an update.

Thanks man!
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Old 05-17-2019, 11:35 PM
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Maybe a crank counterweight is clipping the oil dipstick. Pull the stick and fire the motor.
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Old 05-18-2019, 10:13 AM
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Did you check EVERY rod bearing set for proper clearance?


I got caught recently on a set of King rebuild bearings. One half shell out of the entire set was .010 undersized (and it was marked as standard), had a nice knock that I traced to the number 8 hole. Replaced that set and all was good. Engine still running great.



I have built so many engines over the years ( a lot of small block chevys) that I had gotten in the habit on regular rebuilds to check 2-3 of the rod clearances and if good then put it together. Most of your engine shops do the same and unless you're paying extra to have it "blueprinted", they're in business to get the engine done and out of there and on to the next one.
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Old 05-18-2019, 10:59 AM
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Does the knock occur at crankshaft speed or camshaft speed. Hook up a timing light and you can get a visual reference to go with the sound. In sync with the light I would look at valvetrain, double the speed of the timing light, crank.

Did you use good flexplate bolts and tighten them properly?

May be time to drop the pan.

Good luck!
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Old 05-23-2019, 11:59 PM
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Thanks everyone for the help.
I dropped the oil pain and checked for any bearings or anything.

Turns out one nut from cyl #7 rod cap was loose.
Every single other one was torqued to spec... I couldve swore I torqued them all down atleast 3 times over...

I put it a back together and theres no noise at all.
Now I'm worried if i should have changed the nut..
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Old 05-24-2019, 08:12 PM
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Hate to say it but there's a good chance that nut is split. I would believe missing the torque on a rod nut maybe once as you're going through but missing it completely? - no way. I think you should open it back up and replace both nuts on the #7 - at worse you'll know it's good to go after that.
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Old 05-25-2019, 01:58 AM
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I agree....and I've seen the rod bolt head start to shear off too, so examine it for maybe that being the cause of a loose nut. I'd replace both bolt and nut on the side that was loose, and maybe just the nut on the other side of the rod.

Third cause could be bolt either has a shank flaw or was overtorqued at some point in it's life and has gone past its yield point....every time it is retightened after that it just stretches more and more until it breaks, either while torqueing it...or worse in operation at upper rpm and blows the whole thing up.

These bolt yielding the breaking scenario's are much more likely with stock rod bolts, that's why ARP built such a successful business selling better bolts and studs.

I'm actually amazed it was run and didn't flat-out spin that rod bearing , you got extremely lucky....go buy your lottery ticket.
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Old 05-25-2019, 10:47 AM
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Lottery indeed.

While your at it, if you have a cheap click type wrench, back off the adjuster and click a few times and reset it than check over the bolts again. Click 3 times on each nut. You don't want to tighten it more, just making sure the torque wrench is repeatable.
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Old 05-25-2019, 12:49 PM
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Stock GM rod bolts are weak grocery-getter bolts. Example, a new stock Pontiac 3/8”-24 rod bolt will stretch and break at 75-80 lb/ft torque. Stock Pontiac rod bolts should never be tightened more than 45 lb/ft torque which is slightly beyond yield strength.

Back in the seventies, I proved that by placing a new Pontiac GM rod bolt in a vise and checking it with a Snap On dial-back torque wrench. I have used ARP bolts since then when I had eight out of ten stock rods rebuilt. Two rods out of ten were usually beyond repair due to excessive twist. Stock rod bolts has always been a sore spot for Pontiac engines resulting in spun rod bearings. Spun bearings will usually get the crank too. Now, it is best to use new Scat rods.
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