The Stovebolt Site Forums The Shop Area Engine Shop 235 main bearings

Okay now bear with me on this one....is it prudent to change "only the main bearings" on this full pressured engine? The engine is 55' and has a lower end knock near # 6 cylinder. The mechanic pulled the ignition wires (one by one) while the engine was running and he tells me that this sound would have increased in sound if it was not firing, which he then implied indicated the noise was from the mains. Does this sound logical??? Also does one only replace the main bearings, or do the bearings come in sets which also includes the rod bearings? This here mechanic said that he thought the main(s) can be replaced while the motor was in the truck. the truck is a 47' 2-ton, with a changed engine.

You can change both rod and main bearings with the engine still in the truck. Once in a great while that will help. Usually the problem is deeper, like a damaged crank. On my 51 half ton, I pulled the rod bearings and put new rings on the pistons. The rod bearings were beat down to the copper, and I suspected a flat rod journal or two. Sure enough, #4 was .002 flat, and will talk pretty good even with 20W-50 oil at high temperatures.

Pull the pan and look at the bearings ONE AT A TIME!!! I would recommend replacing both rods and mains at the same time. Look the crank over good as well as your old bearings. You can use plastigage to get an idea of bearing clearances (.001-.003) on the mains. Look at the stampings on the back side of the bearings. There will be a date code (mo/yr) and some part numbers. If any of the numbers indicate undersize (.002, .010, .030 max) the crank has been ground and undersize bearings are needed.

Also, the rear main bearing cap contains the seal, which can be difficult to change sucessfully in the vehicle. Get a GM shop manual for the vintage of engine you have and go for it.

thadley: I assume you have not done this type of engine diagnosis/repairs before, hence you took your truck to a mechanic, am I correct in this assumption? Main bearings bad, not likely!

I would suggest you get a second opinion on the lower knock. This mechanic is giving you bogus information. The noise will DECREASE when the plug wire is removed from any suspect cylinder, regardless of the damage to the effected cylinder.
It does not matter if the damage is a bad piston, a wrist pin problem, lower rod bearing, whatever!

Secondly, the mechanic told you that it was a suspect main bearing, not likely. A 235 only has 4 mains, with number 3 being the thrust bearing and takes all the fore and aft motions. Number 4 is the rear main and has the rope seal. If either of these bearings were that bad, troubles elsewhere would be obvious. Heavy leakage of the rear seal, and trouble with both number 5 and 6 cylinders, as well as the others. Number 3 main controls crank thrust, and if it goes, everything goes, wrist pins, all the lower rods, will wear to an elipise, doublequick!! What I trying to say is that when a main bearing goes out on a 4 main engine, it takes everything else with it!

Having said that, the most likely trouble, and source of the noise, is the lower rod ends. Usually the rod bearings wipe because of bearing bore distortion. The lower rod bores distort because of the long rod length versus the size of the crankpin diameter, which is aggravated by the torsional vibrations (oscillations) of the crankshaft between the main bearings, and low oil pressure because of worn bearings. This causes metal to metal contact, and loss of the oil wedge. You have to remember that there are 2 crankpins between the mains, which promotes alot of give, and flex, in the crankshaft. Every power stroke of each cylinder pushes down on the rod with a force of something in the range of 2000/2200 lbs. That's what is causing the noise.

Racecarl is giving you good advise to follow.
Yes, main bearings and rod bearings are sold in separate sets. And yes, they can be changed out while the engine is in the truck. All thats needed are roll out pins, you can even use cotter pins to do this. This kind of repair would really be done much easier with the engine out of the chassis. I agree with Carl, that you probably do have out of round, or tapered journals. Need to check all rods and all mains. If you have no experience with this kind of work, might I suggest you seek out a reputable machine shop and they can fix your engine and get you going.
Good Luck, and I apologize for the ramblin on.

Think Chevy! :rolleyes:

Thanks Chevyman and Racecarl for the quick input, this has just added to my limited knowledge about these sorts of things...... as u might have gathered i am at best a novice in regard to mechanics, however I think now that I might just try to pull this motor myself an do the job. I feel that the resources within this forum can lead me along with the known literature available. My biggest concerns are the equipment needs in order to accomplish this. I realize that any machining is to be outsourced, but "what in the hey" else is gonna be needed?? Any one care to reply as to whether or not a novice should attempt this?

Now this post I can speak the voice of experience. hahahah :confused:

I have a '54 235 full pressure. One day out of the blue it started knocking. Loudest at idle, and even louder when the clutch pedal pushed in. Oil pressure was 20 PSI at idle hot. Motor still ran smooth and had power. First I thought it was the clutch, then I thought it was a rod. Turned out to be the #3 thrust main brng had taken a beating.

I pulled the engine and disassembled. Lots of things wrong that changing the crank bearings couldn't fix alone.

Check crank end-play with a dial indicator watching the flywheel. If runout exceeds .010" the thrust main is shot. And Chevyman is right. When a crank main goes, so goes the engine. If the mechanic says to change the mains, its time for a complete engine teardown and rebuild.

Get a second opinion from a shop that knows old engines. See what their diagnosis is, and how much the cost.

"Patching" an engine's bearings instead of a rebuild is usually a losing proposition in the long run. You may fix whats making the noise, but the rest of the motor is prolly worn out/damaged anyway.

Happy trails

Chip