Engine Knock

[Guest SLO Ken]

Well, I finally got my engine back together. It took four months to collect parts and find time to get it done, but Wednesday afternoon I fired it up. It's a 364 Nailhead in a '61 LeSabre. In February I broke the connecting rod on the number 6 cylinder. This required the replacement of the rod, piston, and valves on that cylinder. Now there is a very pronounced knock when the engine warms up. It's fine when it's cold, or even when idling while hot... but if I give it even a little gas while it's hot, KNOCK-KNOCK-KNOCK-KNOCK. And then it knocks some more when I let off the gas as it returns to idle. I haven't changed the timing or messed with the carburetor since the last time it was running in February. Is there anything else that is adjustable on this engine? If not, does that mean I'm going to be pulling the whole thing apart again to check and replace the rest of the bearings? Thanks for any advice you may have.

-Ken-

[Bhigdog]

My opinion without hearing it.............No & Yes.

[Guest SLO Ken]

Thanks. That's what I was afraid of. Although, I was talking with my dad and he said it might be the four-month-old gas in the tank causing detonation. It doesn't really sound like that kind of knock to me though... One can only hope!

[pirate]

It definately sounds like the engine has to come apart again. But before doing so do this. With the engine running short out the spark on each cylinder and while engine is in the mode that makes the knock. You will find that when the troubled cylinder is shorted out the noise will either be gone or noticable different in sound. This way when you tear down the engine you will know where the problem is. It looks to me to be a rod bearing problem but difficult to really know over a computer.

[Bhigdog]

4 months isn't "old" for gas. Detonation sounds more like a rattle than a knock and it would not detonate during DEceleration. If the noise follows the engine RPM in frequency then you have something in the rotating mass that has a definate problem.

If you threw a rod at high RPM then I think a complete tear down/inspection is in order. There are issues such as a bent crank, bent rods, broken piston skirts, metal pieces in the oil system, etc, etc.

There is another possibly though. One time I had my young daughter crank my 327/350HP Corvette while I did something under the hood. She was just supposed to jog the engine while I looked for the timing mark. Well, the engine started and in the confusion she held her foot to the floor. Before I could get to the key the engine severely over-reved and shut down with a bang and lots of loud knocking. It took lots of cranking to get it started again and then it knocked painfully loud with hard metallic ringing. I figured it was toast and just pulled the engine. When I pulled the heads I found the source of the knocking. The engine had over-reved so severely it actually pulled all the carbon build up off the pistons and then smashed it between the piston top and cylinder head. Lots of junk also got under the valves and that's what actually shut it down. I did a rebuild but there was actually no damage from the episode.....Bob

[Guest SLO Ken]

Thanks for the insight/experience. I agree that I need to tear it down again. What is really bugging me is that it only knocks once it warms up. I would think that if there is any temperature dependance, any mechanical noise would be worse when everything is cold, not when it's hot.

[The Old Guy]

The oil is much thicker when it is cold. this cushions the noise. I would almost bet that you do not have very good oil pressure at idle.

[RocketDude]

It sounds like it probably is a rod bearing. When you installed the bearing, did you use plastigauge to check the clearance? Did you replace all of the bearings, or just the one in the bad cylinder? What happens to the oil pressure after warm up?

You didn't give a lot of information on what exactly you did during your rebuild.

Knocking after warm up, as you describe, I would guess you put in the wrong bearing. I would start by pulling the pan and plastigauge the new bearing. You might well be able to get by without pulling the top end. It's doubltfull you did any damage if you didn't drive it much, or hard. You might be able to just change that bearing. No reason to touch the top end, unless you determine you need to pull that rod. If it didn't knock cold, then you just have some excessive clearence in that hole. Could be an easy fix. Just because it knocked a little, doesn't mean you did any damage.

Good luck.....

[Guest SLO Ken]

I would not call what I did a "rebuild" per se... more like "repair." I don't have a garage, so I wanted to leave the engine in the car. I only replaced the one rod and piston, and the valves in that cylinder becaus they were slightly bent. And I only replaced the bearings on the #5 and 6 cylinders, since 6 was destroyed and 5 shares the same journal on the crank. There were a couple of small nic's on the crank which I smoothed out with emery cloth. I did Plastigauge the new bearings and they were between 0.001 and 0.002 which is the tight end of tolerance according to the service manual. It was the first time I had used Plastigauge, however, and I may have read it wrong.

I don't have an oil pressure gauge, just an "idiot light" in the dash. So all I can tell you about the pressure is that it stays high enough to keep the light off.

Thanks for the replies... It really boosts my confidence to know there are so many people willing to share their experience and expertise so freely. I hope I can return the favor someday. <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" />

[pphil]

i had a chevy 305 sounded good cold when it warmed up knock knock knock

i had spun a bearing turned the crank new bearings drove it 5 more years.

scott <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" />

1964 wildcat 425 2-4bbls 4-speed

[RocketDude]

On the offhand chance you still have the original bearing that came out (I always save old parts until I know everything is perfect). look on the back of the bearing shell. It should be stamped std. or .010, or .020, etc. that is at least a starting point to know what size the crank is. I would assume the engine was never touched before, but you just never know what happened before you came along. I have seen undersized cranks from the factory.

What happened to break the rod in the first place? Pretty rare for that engine to lose a rod under normal conditions. That might even hold a clue to your present problem.

[Guest SLO Ken]

The original bearing that came out has the following stamped on the back...

8943

GM M400

285HO

I don't see any undersize mark anywhere, so I assume they are standard. I used Clevite CB-762 P replacement bearings, which are supposedly direct replacement.

So, what lead to this whole situation continues to baffle me. I bought the car a year ago. I flew to Spokane, WA to pick it up and drove it back to San Luis Obispo, CA... a distance of about 2500 miles. Aside from a little carburetor adjustment, it ran smooth as butter. So I'd been driving around town pretty regularly for 6 months or so, no problem. One night, I was doing about 70 mph on Hwy 101 when the rod cap decided it's had enough and let loose. I immediately put the car in Neutral, shut off the ignition and coasted to the side of the road and called AAA. When I pulled the oil pan a couple of days later, I found what looked like the remains of a shop rag blocking the screen of the oil pump. I have no idea how it got in there, or why it didn't cause trouble before. <img src="http://forums.aaca.org/images/graemlins/confused.gif" alt="" />

[RocketDude]

GM Moraine 400 is a factory bearing, so it is standard. It is usally stamped, even if std, but not neccessarly.

That shop rag is a puzzler. It could have come from the factory. Perhaps on a hangover Monday! Humans build them. If the remains of the rag weren't blocking the pump, and you didn't get an oil presssure light, that may have just been a coincidence. If you were starved for oil, it would wipe out the bearings before breaking a rod cap, or bolt, which ever it was. Was it a rod bolt that broke?

It could have been a faulty rod bolt that finally reached the end of it's life. Those engines are usually bullet proof, good for well over 100k!

It might be a good idea to rent, borrow, or steal a manuel test gauge and replace the oil sending unit, so you can see actual pressure. You can make one, if need be. Buy a cheap gauge, get some neoprene hose, and the proper fitting to fit the sending unit hole. Just for temporary, so you know actual pressure, for trougble shooting. The more info you have before tear down, the better off you are.

You said you just replaced two bad valves. Was it done by a knowledgeable machine shop, that was able to properly install the valves, i.e. grind the seats, set the spring pressure and heights? How about the piston and rod you changed. Bore and piston checked for proper fit? Pin fitted properly?

The rod clearance you mentioned sounds good. You will just have to pull that rod cap and see what it looks like. I hope the pan comes off on that car, with out pulling the motor...

Keep the faith..... Every turn of the wrench makes you a smarter person.

[NTX5467]

I concur, knowing what the oil pressure is doing can be a key issue. I seem to recall that all it takes to turn the "OIL" light on is 7-10psi? Might even be a little less, but it's not much. You'll need a brass fitting to match the oil pressure gauge fitting, probably, to screw into the block for the oil pressure check.

Plastigage is pretty easy to use, but I think there are some issues with where you put it and at what relationship the rod is to the crank. With the failed rod/bearing, there had to be something that caused it. Was there any blackening of darkening of the rod big end, which would indicate a "heat" issue from "not enough oil"? How badly was the remaining bearing shell "beaten out", plus the condition of the other bearing on the same crank journal?

Also, in many engines, they use an "offset piston pin" orientation in the piston to decrease piston noise. I'm not sure if your 364 is that way or not, but there would be some designation on the piston crown of "Front" or otherwise--which would mean that the piston you installed would need to be from the same side as the one that you took out, or at least "pointed" the same. Sometimes, it's a stamp symbol and others have used a notch on the edge of the top of the piston. If that's the case, just turning it (the piston/rod combination) to be oriented correctly and putting it back together (again checking the bearing clearance) might be all that is needed. Might be something to verify in the service literature.

For what you're doing, I don't know that making sure of the valve spring pressures and such would be critical, but it would be nice if they were checked just as if a regular full valve job was being done. As long as they were pretty much the same as the others, unless one of them broke, that might not be a big issue. Dressing the valve seats would be good too, all things considered.

The shop rag is an interesting consideration. Perhaps they wanted a little finer "strainer" on the oil pressure pickup? You might also want to take the oil pump apart and inspect it for wear too. Maybe the engine had a plastic-clad timing gear failure and the mechanic added the shop rag to keep any residual plastic chunks out of the oil pump?

Please keep us posted on what transpires on the situation.

Enjoy!

NTX5467