Engine knock - What fails ?

[suprastanger507mgte]

Hi,
What part of the engine mechanical fails which causes the dreaded rod knock in a 7m enigne?

Is it worn bearings? Which particular one?

I DON'T need to know what causes because I already know, but I DO need to know what fails before we get rod knock?

Thanks,
Moe

 

[trydrew]

Hi,
What part of the engine mechanical fails which causes the dreaded rod knock in a 7m enigne?

Is it worn bearings? Which particular one?

I DON'T need to know what causes because I already know, but I DO need to know what fails before we get rod knock?

Thanks,
Moe

Most often its the rod bearings (the bearing that floats on the crank at the bottom of the rod.) Actually it is always.

'Which one' cant really be justified as a clear answer, it's just which one decides to let go, dependant on many other variables.

EDIT: Not to clear on what your asking. You ask which part fails that causes rod knock, then you say you dont need to know what fails (the cause)?

 

[JustAnotherVictim]

Hi,
What part of the engine mechanical fails which causes the dreaded rod knock in a 7m enigne?

The bearings.

Is it worn bearings?

That can be a cause but it's more likely contamination.
Improper clearances can attribute to rod knock as well.

Which particular one?

Not sure what you mean by this.
I assume you're asking rod or main. IIRC it's the rod bearings most of the time.
The bearing that has had it's oil layer contaminated or run dry.

 

[trydrew]

Is it worn bearings? Which particular one?

The more worn a bearing is, the more clearance there is between the rod bearing and crank. With a bigger clearance, they will not float, and oil will pass through, rather then staying there and creating pressure, holding them floating. This can be explained by saying that flow and pressure are opposites which im sure you know. When pressure is lost, the crank can strike the bearing and make a knock.

Now I'm sure you know the ways the bearing can wear down in the first place, so i'm not going to explain, unless you want.

 

[suprastanger507mgte]

Most often its the rod bearings (the bearing that floats on the crank at the bottom of the rod.) Actually it is always.

'Which one' cant really be justified as a clear answer, it's just which one decides to let go, dependant on many other variables.

Okay...so it is usually the bearings you stated. kool

 

[trydrew]

Okay...so it is usually the bearings you stated. kool

Unless it's the mains, like JAV said. But then its not RK

 

[suprastanger507mgte]

The bearings.

That can be a cause but it's more likely contamination.
Improper clearances can attribute to rod knock as well.


Not sure what you mean by this.
I assume you're asking rod or main. IIRC it's the rod bearings most of the time.
The bearing that has had it's oil layer contaminated or run dry.

Yeah JAV I meant which one of the bearings. THe rod or the Main.

 

[JustAnotherVictim]

Ok, that's what I thought.
Rods are usually the culprits.

 

[suprastanger507mgte]

Unless it's the mains, like JAV said. But then its not RK

lol, Well I put a used engine in my cressida. Started it up and it sounds like there is someone inside the engine hammering away. :biglaugh:

Reason I asked was because I want to decide whether I should just get another engine or rebuild this one if not too many other parts failed due to rod knock.

 

[trydrew]

lol, Well I put a used engine in my cressida. Started it up and it sounds like there is someone inside the engine hammering away. :biglaugh:

Reason I asked was because I want to decide whether I should just get another engine or rebuild this one if not too many other parts failed due to rod knock.

Why not send it off to Doriftomotion!! lol

 

[lagged]

lol, Well I put a used engine in my cressida. Started it up and it sounds like there is someone inside the engine hammering away. :biglaugh:

Reason I asked was because I want to decide whether I should just get another engine or rebuild this one if not too many other parts failed due to rod knock.

eh....it really isnt the best to resize one rod and replace one bearing. youre better off replacing all of them.

 

[JustAnotherVictim]

I think he was going to replace them all.
He's worried about if the rod knock affected anything else I believe.

 

[suprastanger507mgte]

eh....it really isnt the best to resize one rod and replace one bearing. youre better off replacing all of them.

That is why I asked a specific question in the first post. To Not get responses like this. lol .
I never said I was thinking about only the one or two bad bearings or part.

The whole engine is going to be rebuilt. JAV got it right...

 

[GrimJack]

The reason they fail is nothing special - pretty much the same reasons that every car has - oil starvation, caused by low oil or occasionally blocked oil passages. I've seen a case of high G force turns starving the oil pickup. I've seen a bunch of cases of BHG causing milkshake oil and that in turn roasting bearings.

I haven't heard of any other causes that were even vaguely common.

 

[gtsfirefighter]

lol, Well I put a used engine in my cressida. Started it up and it sounds like there is someone inside the engine hammering away. :biglaugh:

Reason I asked was because I want to decide whether I should just get another engine or rebuild this one if not too many other parts failed due to rod knock.

Somebody, and I don't remember who, told me to assume all used 7M's have a rod knock or blown head gasket. I went through three used engines, the first two, rod knock, the third, bhg. So I'm going to replace bhg and anxiously await the rod knock! I kept one of the knocking engines to rebuild in my spare time. So my advice, just rebuild the engine and be done with it.

 

[RacerXJ220]

I haven't heard of any other causes that were even vaguely common.

Forged internals + Detonation = beat up bearings

 

[GrimJack]

Forged internals + Detonation = beat up bearings

I've always wondered if that would kill bearings... seems like it should.

 

[tekdeus]

I'd highly recommend using a bonding lubricant as a failsafe. Either Prolong or the Bi-Tron in my signature. One Supra owner recently had his brother start the car not knowing it had been drained of oil and ran it for several minutes. He later checked some of the main bearings and there was no wear. You can read that story in the link in my sig. Cheers - Brad

(P.S. There is a debate between the believers in this product, and a few who feel it is snake oil. That thread is here: http://www.supramania.com/forums/showthread.php?t=32678 . There is a heap of quality user feedback that this product does great things and does not harm anything, short term or long term.)

 

[phoenix6]

Oil pump can go out, or u can get what happened to me, someone hitting something on the bottom of the oil pan and hitting your oil pump in the process cuasing its internal mounting bracket to break off and fuck up all your shit.

You cant tell which one unless you do a compression test or just pull the engine like I did. Its a lengthy process to do a full rebuild ont he bottom end, theres alot of things to make sure you double check, even tripple.

Anyways dude. Oil being blocked or the pump going out, OR not enough oil in the system will cause this to happen.

I do recommend rebuilding instead, toss in some brand new parts, and a new OEM HG with ARP Head Studs, Im happy so far. I just turned 1100 miles and the cars great!! (Knock on wood) except the bad 02 sensor fucking up all my timing.

 

[suprastanger507mgte]

I'd highly recommend using a bonding lubricant as a failsafe. Either Prolong or the Bi-Tron in my signature. One Supra owner recently had his brother start the car not knowing it had been drained of oil and ran it for several minutes. He later checked some of the main bearings and there was no wear. You can read that story in the link in my sig. Cheers - Brad

(P.S. There is a debate between the believers in this product, and a few who feel it is snake oil. That thread is here: http://www.supramania.com/forums/showthread.php?t=32678 . There is a heap of quality user feedback that this product does great things and does not harm anything, short term or long term.)

I already have a pretty loud knock. I don't think any snake oil can fix a busted bearing.

I think I will pull the engine out again complete without taking anything else out and just access the bottom end. If it is just bearings, I will change them all as long as there ais no damage to the crank or the rods.