Found the manifold leak, but....
 

The knocking noise seems too loud - it does go away once the engine is warmed up except it comes back when I accelerate. I checked the compression (starting with the cylinder closest to the radiator) and got the following (numbers are psi):

#1 105
#2 110
#3 115
#4 120
#5 110
#6 115

YouTube video here:
https://youtu.be/4IIO-C9vnhs

Re: Found the manifold leak, but....
 

Those pressures are all within a few pounds of each other, I would not worry about those.

My untrained ears cannot hear a knock however, it does sound like your exhaust is leaking.

Bill

Re: Found the manifold leak, but....
 

Sounds like you could possibly have a bad piston wrist pin,try removing a spark plug wire, one at a time and see if the noise changes,if and when it does you just found your problem cylinder.

Re: Found the manifold leak, but....
 

Ok, when I pull the plug wire off cylinder 3 the knocking stops...wouldn’t a bad wrist pin have a double-click rather than a single knock?

And, the exhaust leak is on the opposite side of cylinder 3....

Re: Found the manifold leak, but....
 

Wrist pin sounds similar to a rod or main bearing knock but tends to be noisiest when cold and as motor warms up and expands the noise usually will go away or be less noticable.

Re: Found the manifold leak, but....
 

I agree and when you pull a plug and the noise changes from a low frequescy thus to two high frequency clicks, that may also sound like a wrist pin.

Have you ever used a piece of heater hose or old garden hose about 4 feet long, use it like a stethiscope to seek out and localise a funny, (or not so funny) noise,

rod noises or main noises will come from around the bottom of the block. wrist pin noises under load and unloaded are different and can come from lower on the side of the block and up at the top. .lifter noises also come from the side of the block but are more frequent.

descriptions are abstract and subjective. get a a hose and listen to all the different points on several cylinders where moving parts are are doing just that. You will quickly and easily distinguish what parts make what noises, and if one makes a different noise than the other five your heading in the right direction.

your job is easier still because its a streight six rather than a v configuration.

Just A Thought
 

It's a fairly simple thing to remove the cylinder head and drop the oil pan, send the head off for new guides (always, get ride of the crappy original cast iron valve guides) and hardened seats, undo the rods and push the pistons out, replace at least the rings, have the rods rebuilt and balanced to zero grams, replace the bearing shells and unless they look perfect, replace all six pistons with good quality ones, button it all back up and motor off happily for the rest of your life .

You didn't complain about poor running, just noise so now is the time to do a light overhaul on that obviously worn out engine .

Just a thought from a 6 cylinder lover who learned how to keep them running from GM training plus decades of working on them back when i6 engines were the standard and mainstay of work rigs .

Re: Found the manifold leak, but....
 

My grandfather (who was a John Deere diesel mechanic by trade) rebuilt the 250 in his Biscayne from underneath. He dropped the pain and the rods and pistons out the bottom. Head came off the top to ridge ream. Ball hone, new bearings, ran for another 80,000 miles.

Point is, hood never came off. So it's amazing what's possible!

Re: Found the manifold leak, but....
 

I did that once on a '56 Chevy pickup. What a nasty job that was. I was covered in dirty oil by the time I was done. I was 16, working for an independent mechanic on the weekends on a fleet of service trucks.

'A Nasty Job'
 

LOL ! it's called being a Mechanic .

Re: Found the manifold leak, but....
 

It's a dirty job but somebody's got to do it, right? ;)
I suppose we could be called automotive surgeons?

Re: Found the manifold leak, but....
 

I’m more of an automotive quack.... :hh:

Re: 'A Nasty Job'
 

:lol: True enough. The last time I got that greasy/oily was when I put a rack in my '83 Celebrity. That thing was leaking all of its bodily fluids by the time I bought it ($300, in '91) and ran like crap. Had over 120k on it and the original owner had never put a cap or rotor in it. Did that and it ran like a top. It took me a lot of money- like $11 in quarters- to clean the thing enough to find the bolts before I started on the rack. I drove it for 4 years and sold it for $1100.

Re: Just A Thought
 

That’s some fine advice, and I’m gonna do it!

Thanks to all for the advice and entertainment!

Re: Found the manifold leak, but....
 

Just do your best, these are simple machines and are designed to respond to simple basic repairs on Farms, in the field and so on .

Re: Found the manifold leak, but....
 

So...I take the truck to a mechanic that has experience with these wonderful trucks, and he declares that the manifold leak is causing the knock. I take the truck home, spray the manifold bolts and studs with Aerokroil over the next three days, remove the bolts and nuts without breaking any of them, replace the old gasket (pic below), put it all back together - the knock is gone! The truck runs fine! I’m a very happy guy!

VWNate1 - I have no doubt that I’ll need to do the engine work you suggested at some point, but since I bought the truck from the original owner and it had believably 113,000 original miles and he was an 84 year old retired anal retentive engineer that kept meticulous maintenance records (the guy GREASED that truck on schedule for crying out loud!) AND kept it in a garage while he owned it, I figure that it’s never been run low on oil - the guy believably said he never drove it past 60 mph - so I probably have several tens of thousand miles left before I need to rebuild that engine. Plus, and more importantly, the wife and I just moved to another house and she’s wanting to move all our crap to the new house and I need a truck to do that, so....

Thanks to all for the advice!

Re: Found the manifold leak, but....
 

Mission accomplished! ;)

Found the manifold leak !
 

So ;

? Is the knocking gone ? .

One of the great things about these sturdy, well designed work horses is : they'll run just fine with badly worn out everything, for decades, as long as you keep it sharply tuned and full of clean oil .

If you don't mind the blue smoke just set the dwell to 33 degrees then the idle timing to 10 degrees BTDC @ 700 or less RPM's and no vacuum to the dizzy .

If it begins to foul spark plugs, just get an original Delco HEI distributor, install it and open the plugs to .050" and it should keep on running just fine for years .

Re: Found the manifold leak !
 

The knocking is gone! The plugs look fine! I’ve thought about dropping in an HEI dizzy - now that the carb is right the truck idles fine and doesn’t spit, sputter and/or die when I give it gas and let out on the clutch. No (noticeable) smoke from the exhaust. Points/condenser ignition systems work fine if one is willing to piddle with them regularly...and I’ve self-diagnosed me as a piddler. :D

! Excellent !
 

Just so Sir ;

If it's good to go don't worry just take care of it .

I've become a faithful believer in breaker less ignitions, be they HEI or just simple Pertronix Ignitors .

All one needs do is : check the timing every oil change and drive happy.....

:chevy:

Re: Found the manifold leak, but....
 

A vacuum leak that caused a knock? Can't say I can wrap my head around that one, but glad it's fixed!

Re: Found the manifold leak, but....
 

Looking at the old gasket, when the #3 cylinder spit the exhaust out some of it would slam into the #4 exhaust port (and vice versa)...perhaps that was the cause of the knocking sound?