Flat 6 just developed a good knock

edhd58

edhd58

Well-known member
My G series 226 has now developed a knock.
It sounds kinda bad cold, but will disappear if I pull #5 plug wire.It also seems to go away when its warmed up, however I think I still hear it driving it under load.
Is it possible to overrev this thing?
I did kinda do a stupid thing, maybe. I was in traffic and needed to merge lanes so i kinda ran it up to 45 or so in 2nd gear. That had the motor sounding revved pretty hi. A couple days later when i started it up again is when i noticed the knock.
What i am wondering is, is it possible to stretch a rod bolt?
I am being told thats what it sounds like.

And next, how hard is it going to be to get new ones and replace them?
 
A rod knock will be louder under load, and may not be heard when the engine is loafing. If that is what you have, it could be a stretched bolt or simply a worn rod bearing. Don't let it bother you too much. I have put a lot of miles on flatheads with rods knocking; - gingerly. I am sure the 6 is equally durable.

How is your oil pressure? Even if good, 50 weight oil may quiet it down.
 
I think it is possible that you have a rod bearing going bad by what you describe. you could be lucky and just have a piece of carbon come loose, and caught in there. I have had this happen on a chevy 454 once. I would have sworn it had a bad rod, but tore it down , only to find A large chunk of carbon had come loose and would not pass though the valves and was hitting the top of the piston. If you have a bad rod bearing, it will most likely need to have the crank turned and the rod replaced or resized. usually if a rod bolt "stretches" it breaks. either way it will need new bearing and the crank turned.
 
I was always of the understanding that a bad rod bearing will quiet when under load and then start squawking once the engine is "slacked up". How's your oil pressure? I've had a couple modern 6 cylinder engines (Jeep 4.0) come to me with similar symptoms and in both cases it was piston slap. They both quieted down once warmed up...on cold start up they sounded like they we're going to blow up right then. One had a broken piston skirt and the other was just worn out. I've never experienced a stretched rod bolt and have always attributed that idea as if it were a unicorn sighting. In my (maybe flawed) thinking, the rod cap doesn't do a whole lot in the 4 strokes of an engine. On the intake stroke it's pulling the mass of the rod/piston down and whatever air/fuel resistance there is in the intake. Compression, power and exhaust are all working on the upper bearing. How can rod bolts stretch based on what he did??
 
Capt Kirk, if its piston slap its still a tear down, right? And wont piston slap ruin a cylinder if not treated fairly soon?
As for oil pressure its when warm 20 at idle and 40 to 50 running down the road.
 
If you're not ready for tear-down get familiar with STP The Racer's Edge. Or and as already mentioned thicker oil. STP works.
 
A bones, it has a quart of Lucas oil additive. It helped some. It's not gonna be torn sown yet, there are a few local shows I want to make. Might I try less 30wt and more Lucas ??
 
For certain Lucas makes a good product. I however would avoid an over dose. Once upon a time I tried too much, and wound up with a stuck lifter, ohv. Good luck at the events.
 
Capt Kirk said:
How can rod bolts stretch based on what he did??
Click to expand...

RPM...
Think about what those bolts go through at high RPM.

At 600 RPM, the piston and rod is moving up the bore being pushed by the crank. Suddenly the crank has to reverse the momentum of the rod and piston and yank it down to the bottom of the bore. This all happens 10 times every second.

Now bring that engine up to 3600 RPM and the exercise is repeated 60 times every second.

So yes, the rod bolts have a rather big job in the grand scheme of things.

---

Did he stretch a rod bolt? Good question. Generally you see a drop in oil pressure. The worse part of stretching bolts is that the rod bearing can become unseated from the rod and can actually start to spin inside the rod.
How hard would it be to pull the oil pan for an inspection?
 
edhd58 said:
My G series 226 has now developed a knock.

What i am wondering is, is it possible to stretch a rod bolt?
I am being told thats what it sounds like.
[/QUOTE

Who told you it's a stretched rod bolt? Was it someone with real life experience, or an 'armchair expert'? My guess is it was an 'armchair expert'. I've never seen it in over 50 years of working on engines. First of all, when a rod bolt is torqued to specs it is actually being stretched, but within its elastic limits. This 'stretch' is what clamps the cap tightly to the rod. Any external force from over revving has to overcome this preset tension (stretch) before it has any effect on the bolt. And this clamping force in in the thousands of pounds range. I don't think 45 MPH in second is really revving that high (maybe 4,000-4,500 RPM?) although it may sound like it when you're not used to that RPM.
Since it gets quieter when it's warm, I'd say it is a cracked or worn piston. Aluminum expands as it heats up so quiets down. Oil thins out as it gets warm, so a rod would knock louder as the engine warms up.
Click to expand...
 
Hold the rpm at 2000. Now feather the throttle between 2000 and 2500. Does it knock as the rpm goes from 2000 to 2500? Do this a few times without letting rpm drop. If the knock is there it has been my experience you have a rod knock that will get progressively worse until catastrophic failure. Depending on clearances that may be sooner than later.
Also, a knock that goes away when combustion is removed generally indicates a rod knock.
 
40 Deluxe said:
edhd58 said:
My G series 226 has now developed a knock.

What i am wondering is, is it possible to stretch a rod bolt?
I am being told thats what it sounds like.
[/QUOTE

Who told you it's a stretched rod bolt? Was it someone with real life experience, or an 'armchair expert'?

It was a fellow that supposedly has built many high performance - 600+hp engines for drag racing, for both others and himself. I say supposedly because I was not present when he did them, And this motor has more stroke than is race car does, its over 800 hp naturally aspirated no nitro
my motor has 4-1/4" stroke.

We were discussing stretched rod bolts because the engine has been apart who knows how many times, a few just with me, and always used the same rod bolts. I know one of the pressure plate bolts broke torquing it down this last time, and actually i was just snugging it getting ready to torque them all.



MOON, on this car it's a pull the motor event.
Click to expand...
Click to expand...
 
I'm in agreement with Capt. Kirk that a rod bearing is loudest when decelerating, while a main bearing makes most noise during acceleration.........Bob L
 
fordy_nine said:
I'm in agreement with Capt. Kirk that a rod bearing is loudest when decelerating, while a main bearing makes most noise during acceleration.........Bob L
Click to expand...

This is correct! A rod knocks when not under a load and a main knocks when under a load!
 
edhd58 said:
Capt Kirk, if its piston slap its still a tear down, right? And wont piston slap ruin a cylinder if not treated fairly soon?
As for oil pressure its when warm 20 at idle and 40 to 50 running down the road.
Click to expand...

That oil pressure seems pretty good to me. If you want to know for sure, you can pull the head and check the #5 piston. In my cases (one was with my daughters jeep) with piston slap, once the head was off, I was able to move the piston around with my thumbs...the one with the broken skirt, I was able to rock a bit. My daughter must have drove 8-9 months with it clattering around when cold till she could afford the parts for me to rebuild for her.
 
I had a G series 6 cyl. engine in a 47 Ford pickup when I was a kid. It developed a knock and it was a bad rod bearing. I had the crankshaft turned and put it back together. It was fine after that but it always had a rattle in the engine for a few seconds when starting it cold ( especially in the winter) The pickup tube on the oil pump ran the entire length of the oil pan. If I remember correctly, the oil pump was at the front of the engine and the pickup tube ran approx. two feet to the sump in the rear of the pan. It never gave me any problems and I just figured it was due to the length of the pickup tube and the fact that there was no check valve. The 226 flathead 6 was a good engine with lots of low end torque. It did not have the sound of a flathead V8 but it was a good powerplant, especially for trucks.

Bill in Conn.
 
Cecil/WV said:
This is correct! A rod knocks when not under a load and a main knocks when under a load!
Click to expand...

Which is why a rod knock can be heard at upper rpms as the engine is "floating". Not so under deceleration or acceleration. A load either way and it will not knock. Main knock is a deeper sound pitch and apparent under load.
 
I think i will baby it to the end of the month and tear it down. As i do I'll post pics and be asking more questions I'm sure.
 
I learned a lesson today, at least I think it was a lesson. I had been asking another time about ignition timing. There are no marks anywhere, so for the average guy like me replacing the points etc there is no way to set the timing.
WELL, I had set the adjustment to 3/4 way to full advance. Today I decided to set it more retarded, just because. I had no real reasoning I just wanted to see what would happen.
I warmed the car up some, not a lot, and took it for a drive. WOW what a change,the motor is a lot quieter now and the whole ride was much more enjoyable. In fact it was like a different car. Under load I could hear a VERY slight noise but no where near what I was hearing before.
I dont think I am going to tear it down anytime soon, the noise changed that much.
Oh yeah, I was running 65 to 70, air temp is around 80 and the thing ran a temp of 185 with 40 psi oil pressure.
 
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