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Engine knock on newly rebuilt 396

11K views 11 replies 7 participants last post by  rubes 
#1 ·
So last week I purchased my Caprice with a knock in the engine. The seller (member here) rebuilt the engine and discovered a knock after breaking it in and sold it. (long story) So I have been going through the basics before tearing it down. I’ve built quite a few engines before, both big block and small so I’ll trim the fat and highlight the oddities:

Rebuilt stock 396, bored .030”, forged pistons. Summit cam, 540 lift. The rest is all factory stuff as far as I know down to the Q-Jet carb and point based distributor. (prev owner installed an adjustable vac advance unit) Oil PSI is 60 at idle with no fluctuation.

Here are some odd things I haven’t figured out:


  • - Engine has a noticeable knock which seems to stem from the front upper half of the engine.
  • - Engine gets pretty hot after only 10 minutes of running. (= 200°)
  • - Timing appears to be way advanced though it isn’t hard to start. I threw a timing light on it and found the balancer mark nearly an inch+ from the timing tab. * If I try to retard the timing by turning the distributor the engine dies before I even get close to the timing tab. ???
  • - Engine runs a bit rough.
  • - You can feel the knock on the hard fuel line going from the pump to the carb.
Tried:
  • - Re-adjusting the lifters. No change. No noticeable difference in movement on any rockers.
  • - Checked rocker slots for stud galling. Nothing.
  • - Checked for bent pushrods. Nothing.
  • - Pulled plugs wires one at a time to see if the knock went away. No change. (usually a wrist pin or rod knock will subside a bit with no power in that cylinder)
  • - Ran a compression test on all 8 cylinders. # 5 has 15 more psi than the rest. (165 vs 150)
Right now I haven’t a clue what is causing the knock. My next attempt is to run it without the fuel pump.

Any suggestions to offer? I know it’s difficult to answer without hearing the knock, but 2 experienced big blocks guys are also scratching their heads after listening to it.

Worst case scenario is pulling the engine and going through it with a fine toothed comb.

-Scott
 
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#2 ·
Check piston to valve clearance after you have degreed the cam to verify it was properly installed. Sounds to be off on installation (my guess is it is too far advanced) so that the engine is dieseling (firing the cylinder as the piston is still approaching TDC). If you have access to a bore gauge (TV camera style is the best) look inside the cylinders via the spark plug holes for signs of impact on the pistons).

Big Dave
 
#5 ·
I had a very similar problem.

The problem was that I left a rag/sock stuck in the intake port, holding the valve open and it was lapping against the piston.

Pull the intake and check to see if there is any thing stuck in a port. Mine was stuck in port 2 (Front passenger side). Intake gaskets are cheap and generally not to hard to take off and put back.
 
#6 ·
Update

Well nothing really new to update. Have found a few things to help it run better but that doesn't make much difference when it has a knock. Tried many external options but the knock is still there. The engine is coming out next week. Sucks but I was prepared for this.

The engine and engine compartment is so pretty I hate the thought of yanking it but I guess so did the previous owner. :-/

http://planetofspeed.net/images/Caprice/PICT2936sml.jpg

http://planetofspeed.net/images/Caprice/PICT2994sml.jpg
 
#7 ·
Scott,
Please don't think I'm trying to insult anyone's intelligence,(gee,I hope I spelled that right!!) A friend had a similar noise on a 70 454 Monte Carlo.It turned out to be a bad fuel pump from the factory. At first one mech. thought it was a sm.blk. pump (shorter arm) knocking against the pump rod which transmitted noise up the fuel line. Also,did you pull #1 plug and bring piston up to TDC? It sounds like you could be off 2 teeth on the distributor,which Big Dave said could cause desieling and increase temps. The window on the Dist. should be parallel with the back of the carb. That would answer the timing being off at the crank. Once again I'm not trying to step on anyones D%&* here,BUT it would be a shame to pull the motor out at this time. If I can be of any assistance it doesn't look like I live too far away from you.
Gary
 
#8 ·
No problem Gary I'm all for suggestions. But I've tried everything. I pulled the fuel pump, jumped a tooth on the distributor, tried another distributor, re-adjusted the lifters, did compression tests, pulled plug wires, and pulled the belts off for the chance it might be a water pump or clutch fan bearing. In the process I found a closed electrode on # 8 plug, found 5 and 7 wires crossed and ramped up the idle mixture adjustments, all of which did get it running better, but still knocking. Speaking with the previous owner, he told me he also tried all the same things before me. The last thing I will do is pull off the front end and look at the cam gear timing marks. At that point I won't be able to move the car around on my own any more so that is the last thing to look at before pulling it. Before actually pulling the oil pan I'm going to inspect the flex plate as well.

Talking with the prev owner, the advanced timing issue is most likely the wrong springs installed on the advance weights as it starts easy but then sends the timing up as soon as it runs. Bringing # 1 to TDC is right on the money on the rotor/cap position. ;) The noise isn't deiseling. I know how that sounds. I also know what forged piston slap sounds like when cold. Not it. It's a repetetive knock in time with the engine. Deep, thick sound. Not loud, but always there, cold or hot.

Yanking the engine and going through it is no biggie. I'm on vacation next week. I'm so sick of trying to guess what it is. ...just wasting time. :yes:

I'll keep your offer in mind! I'm in Lombard.

Thanks,
Scott
 
#10 ·
Lucky for Scott, an idiot put that motor together...LOL that idiot would be me. We all have a laps of concentration now and again, and assembling a motor over a few years time is never a good thing to do. Never the less, Scott found the cam bolt loose, so the wobbly cam gear was rubbing the timing cover. Supposedly the big blocks didnt come with the bolt locking device, and all I can think of is that I was planning on getting one, then forgot to check the bolts in one of my spurts of productivity on that project.

So I should probably be seeing it around town this weekend eh Scott?? LOL
 
#11 ·
She lives!

Ha! Just got back from vacation and came to follow up. Rubes I was gonna spare you the flame job but you jumped into the pool before I could stop you! :p Like you said we all have our lapses in memory and I'm sure anyone can relate. Like the story I told you of leaving the tape on the bottom of my blower when rebuilding the engine in my Camaro. Or the wrong length intake manifold bolts that shredded my race push rods. :eek:

Knocking Noise was the cam gear rubbing on the timing cover. I didn't have to pull the engine after all! Looking at the factory indentations on the timing cover, I wondered if that was a design to prevent the gear from coming off completely in case of situtations like this-? I was able to get it all buttoned back up without dropping the oil pan and so far no leaks. It was cool to start it up and not hear a knock! :hurray:

Heat issue was simply an air pocket in the drivers side head caused by a sticky thermostat. Probably corrosion from sitting. Removed that sucker and she's flowing coolant well. I'll either try another or go with a restrictor.

Timing issue was a stretched spring on one weight allowing the weights to flop around and give erratic timing. Installed some Crane springs for the suggested 'common' setting, reset the points and was able to get the timing at 12° BTDC where it likes it. Also discovered the adjustable vacuum advance unit isn't working right and sending the timing way up when putting the vacuum hose back on so for now it's disconnected. I may just replace the whole distrib with an MSD unit and new coil and be done with all the old school hassles. I think the coil is weak as well.

Still working out little bugs but she purrs like a kitten now. Actually, it has a nice sound with the cam you put in Rubes. With tail pipes it will sound a little sleeper-ish for a 4 door. :cool: Carb needs some work and I want to try some other plugs to experiement with heat ranges. A new coil and MSD distributor may negate the need for that.

Got some new KYB gas shocks on her and getting the front end lined up and snugged. Have to go through the brakes and some other little things but it should be on the road soon. Fun stuff! ;)

-Scott
 
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