The Stovebolt Site Forums The Shop Area Engine Shop Knocking engine

I thought I had a babbit pounder engine, but when it developed a "knock", I pulled the pan and dropped all but # 4 cylinder cap. Found inset bearings! The engine is blue and what I thought was a painted over truck engine just might be a Power Glide car engine? Anyway this is what the Plastigauge revealed: # 6 = .0015, # 1 = .003, # 2 = .002, # 5 = .003, # 3 = .002. I did not pull # 4 as it was getting dark and I was tired.
Question: would this slight difference in clearances cause a very audible "knock"? # 4 felt as tight as the rest of the cylinders and I found no scoring or discoloration of the crank on all the others.
My shop manual does not cover inset bearings, so I am a bit puzzled at this point. What ever you can suggest will be a great help. Thanks, Bob W.

I guess that the jury is out until #4 comes off. Usually with a knocking bearing, you will see some clues on the culprit. You can buy .001" bearings and you could buy one pair for fitting.

A low pressure motor will have dippers on the rods and dipper troughs. Also, a badly worn main bearing can contribute to low oil pressure.

What was the knock like? A rod bearing knock is harsh and metallic and is present as the RPM starts to drop when you rev the engine a little. This is where there is no load on the bearing during part of the cycle. A main bearing knock will be there at low RPMs and definitely when under load.

It is definitely a "rod" knock. The knock is present at idle and louder when engine is "revved". Under a load (clutch released / brake on) the knock subsides somewhat. I was surprised to find insert bearings as the casting numbers on the block indicate a 1950-51 engine.
The engine was painted Blue, however. Might be from a Power Glide car? I will pull # 4 tomorrow (weather providing) and repost results. I just can't think that # 4 is the culprit - "snaps" from side to side on the shaft like the others.
Maybe the slight "tolerance" difference is causing the noise - a real puzzle for me. BW

Just a thought.....yesterday on the bolt I read something about a guy sending the rods off to be machined to accept bearings???? Maybe possible your rods had it done???

Do the rods have dippers and dipper troughs? If not, it is a full pressure motor.

One other possible cause for a knock would be a bent rod. I have never seen a bent rod but I would imagine that if it is bent in the front to back axis, it would cause the piston skirt to smack the bore.

One other possibility is a cracked piston. Can I observe / determine this from under neath? The most play I have found in the insert bearings is .003 (# 4 has not been checked - seems as tight as the others). Thanks for the input. BW

Are you sure it isn't a bent pushrod? Have you had the sidecover off?

Check your valve clearance. If a valve is excessivly out of adjustment your lifter will have a severe knock sounding lake a rod.

You've been asked several times, and haven't answered- - - -DO THE RODS HAVE DIPPERS? If not you've got a full-pressure engine. Also, if the rod journals on the crankshaft have oil holes, it's a pressurized-rod engine. The babbit-pounders have troughs built into the oil pan, and dippers on the rods. A cracked piston skirt, bent connecting rod, or worn wrist pin hole in the piston can all cause a "rod" knock, or even a piece of broken piston ring lodged in the top of a piston hitting the head can cause a knocking noise.

Generally, it takes .005" or more clearance to make a knock audible at idle. The rods might rattle a little at light load just before shifting to a higher gear with less clearance, but a knock at idle needs a pretty wide clearance to be noticeable.
Jerry

The engine has dippers. I expected babbit bearings, but found insert bearings instead.
The knock appeared rather suddenly, and since it is a 216 engine, I assumed a bad bearing and found the greatest clearance (so far) .003. I will check out all the other suggested possibilities and keep posting the findings. Thanks all! Bob W.

OK, so you have dippers. Your rods must have been converted to inserts. The bottom shells should have a slotted hole and they should be installed so that the offset is correct for the dipper feed. That is to say, they are offset in the direction that the dipper feeds the oil. I assume that the motor was put together properly but it is possible to put the dippers on backwards.

Were there any shims on the rods?

Are you sure it isn't a wrist pin knock? That can be almost the same noise as a rod knock.

Bottom inserts have slotted hole and dippers were / are positioned properly. Found no shims in the inserts. Will order a new set of inserts (Std.) from Jim Carter (?) to tighten things up on the lower end. For a cracked piston / loose wrist pin, I will have to pull the head. May as well get the valves reground then, too. New piston rings (Piston, if bad) - what else for a bottom / top treatment? - No sense in buttoning
it up without considering all the vital areas.
Regards, BW

I know that Kanters will sell the rod bearings by the pair, if you want. that would allow you to get a standard and a .001" for trial fitting purposes. That is what I did. If you indeed had a rod bearing knocking,you would most likely have some metal tranfer onto the crank which would be visible. You might want to examine the rod throws on the back side of the crank. I am referring to that area that faces inward as the crank goes around. Often there will be tiny ridges, maybe only a few thou in width, not height. The backside of the throws undergo the most stress when the revs are in the higher region.

Have you checked your fuel pump? I have heard of fuel pumps making a knock. Some people have ever replace engines because something as simple as a bad fuel pump.

Thanks for all the input on what was causing the "knock". I tried compression tests, pulling wires to "isolate" the cylinder, checked the rod bearings (OK), removed the side pan (nothing amiss). Couldn't get to / solve the problem. Got tired of working under the carport in the WV, December (cold) weather and had the truck towed to a local garage / trusted mechanic. After engine removal and a complete teardown, the fiber gear on the camshaft was found to be missing 5 teeth! Don't know how it started and ran under that condition, but that was the cause of the noise. No damage to cam, crank or vitals, but a rebuild is in the works. BW