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Old 06-16-2019, 11:17 PM   #1
bdtutton
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Default Checking rod and main bearings

I am doing some valve work on my engine so the head and the valve cover will be off the engine. I was thinking about dropping the oil pan and taking a look at the rod bearings while the engine was half apart anyway.

Here is my question....is there a way to do a general check on the bearings without pulling the caps and using the plastic stuff to check clearances? I remember a couple of my grandfathers friends just reaching in and grabbing and pulling on the crank and the rods and declaring the engine good or bad. When I asked the old guys they said they could feel if anything was loose and if it wasn't you were good to go. If it was loose they would pull the engine apart and fix it.

My engine sounded good and did not burn a drop of oil before I started pulling it apart to fix the burnt valve. I always worry that I will break things that don't need to be fixed when I start pulling them apart to check them. Any opinions ?
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Old 06-17-2019, 03:53 AM   #2
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Default Re: Checking rod and main bearings

If it aint broke, don't fix it. Let sleeping dogs lie. Don't tempt fate.
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Old 06-17-2019, 07:18 AM   #3
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Default Re: Checking rod and main bearings

Its really a big job to mess with crankshaft bearings. Rods not so much. Plastigauge helps, but when your crank is severely "egg shaped" then plastigauge is of much less use.

My "original" engine (an engine which was painted red and probably bought from Sears Roebuck) is so worn (rod bearings about 6thou egg-shaped) the last time I tightened this engine I shaved the caps until each cap started to "bind" at one spot in the rotation. The engine made a most disconcerting "whuppa-whuppa" sound in operation - but it ran.

I think I've told my story of traveling at 55 mph on the Mid-Cape (Cod) highway in this Model A and having the engine suddenly sound like it was coming apart - I babied it home (I lived a short distance off the Mid-Cape) and on removing the oil pan I found a connecting rod nut below the oil pan tray. It sounded like it was coming apart because IT WAS.

I think you'll know from the sound when it is time to "tighten-er-up."

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Old 06-17-2019, 07:26 AM   #4
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Default Re: Checking rod and main bearings

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If it aint broke, don't fix it. Let sleeping dogs lie. Don't tempt fate.
Do you smell what I'm cooking?
This is really good advice. I wouldn’t be looking for problems that don’t give any indication of being there. Enjoy.
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Old 06-17-2019, 08:06 AM   #5
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Default Re: Checking rod and main bearings

Checking bearing clearance can extend the life of your engine,if you wait till it knocks its too late.A trick I use instead of plastigage is I mic a piece of tin foil.(heavy duty foil folded in half) to gain .002,then take a square of it and put it on the cap,torque the bearing and gently try to rock the crank...if its tight Im right,and on to the next one.The tin foil will squeeze flat as well.
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Old 06-17-2019, 08:48 AM   #6
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Railcarmover; I assume youre talking about aluminum foil. Only us old timers know about tin foil. Jack
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Old 06-17-2019, 08:48 AM   #7
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Default Re: Checking rod and main bearings

I agree, if you wait until you hear it, its too late. It can be tedious, actually a royal PITA to lay under the monster checking the bearings, but, I think its worth it.

I took .0045" out of a center main and .0025" from the rear of one that did not make any noise. I like to set them at .0015" on all except the rear which I like a bit tighter.
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Old 06-17-2019, 09:10 AM   #8
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Default Re: Checking rod and main bearings

The use of modern diesel grade engine oils allows a little more than Ford spec for bearing clearance in my opinion.It has higher load and shear values,which means it will "cushion" better than the oils of 1927

And yes you are correct Jack,I meant that new fangled space age aluminum
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Old 06-17-2019, 11:07 AM   #9
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Default Re: Checking rod and main bearings

The "whuppa-whuppa" sound I referred to earlier is (was) an extreme wear signal. I should not have been running that engine at all with that potential for failure.

But I was young and stupid then. Now not so young.

Funny though - the "whuppa-whuppa" disappeared above about 35mph. Probably because the centrifugal force keep the rods against the rod mains. At 55mph it did not "knock" - just fail I guess.

This one probably qualifies as a "Model A Story." Not quite to the level of "Grapes of Wrath" bacon rind - but almost.

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Old 06-17-2019, 11:37 AM   #10
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Default Re: Checking rod and main bearings

So the consensus is, once you’re center main or what ever starts to knock, it’s too late to tighten things up, and now you need new Babbitt? Assuming we’re talking Babbitt.
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Old 06-17-2019, 12:21 PM   #11
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Default Re: Checking rod and main bearings

I have a method to check the rear mains for excessive clearance without removing the caps. It is not highly accurate but it will point out abnormal movement and prompts further investigation. This is done while a car is on a hoist although it could be completed while on floor jacks.
(It would be a heck of a miserable job though when on floor jacks.) When I have the pan off the bottom of a car for a bearing check, I have often placed a sensitive dial gauge with a magnetic holder on the pan rail and placed the tip of the gauge on the crankshaft as close as possible to the bearing being checked. The crank needs to have the throws straight up and down. Using a tall support jack with a foot lever, I am able to apply a varying lifting force to the crankshaft. Because of the weight of the crankshaft, the crankshaft journal is typically in its lowest position in the bearing less the oil film. (But not always....) At times, I need to pry down on the crankshaft using the pan rail as the fulcrum to fully check the clearance. The dial gauge will show the movement of the crankshaft journal within the assembled bearing and that movement is the clearance less any residual oil film I find this method is accurate enough to confirm excessive clearance. The test actually works pretty good. Sorry no photos at the present time but next time I'll take some of the setup.
Good Day!

Last edited by Dave in MN; 06-17-2019 at 02:10 PM.
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Old 06-17-2019, 03:41 PM   #12
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Default Re: Checking rod and main bearings

I used that method to determin if I had Loose rod bearing and which one. I had the head off and put a magnetic indicator on the block.

I turned the cylinder to top center. I then turned it a little more which pulled down on the piston and seated the rod at the bottom of the crank throw. I zeroed the indicator and tapped the piston with a dead blow hammer. If it showed a reading that was the bad rod bearing.


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Old 06-17-2019, 05:24 PM   #13
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Default Re: Checking rod and main bearings

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Originally Posted by Chuck Sea/Tac View Post
So the consensus is, once you’re center main or what ever starts to knock, it’s too late to tighten things up, and now you need new Babbitt? Assuming we’re talking Babbitt.


I think its still worth a look. There is a chance it can be saved.

Noisy rod bearings don't last long but a main can go for quite awhile.

Last edited by Patrick L.; 06-18-2019 at 05:36 AM.
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Old 06-17-2019, 05:37 PM   #14
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Default Re: Checking rod and main bearings

You can do just about anything,I've epoxy glued cracked thrusts back together and soldered acid pits. Can you fix a thump?if you catch it before the babbit starts to crack or peel sure,but its better to catch it before it does.I consider bearing maintenance as important as changing the oil,you just don't have to do it as often.


The above methods for checking are sound...but if you suspect a loose bearing why waste the time?Nothing more satisfying than freshening up an engine and getting another life cycle out of it..set bearings,lap valves and rings,an old fashioned overhaul,instead of running it till it hammers and spending 5k..

Last edited by Railcarmover; 06-17-2019 at 05:43 PM.
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Old 06-17-2019, 05:52 PM   #15
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Default Re: Checking rod and main bearings

I have an engine on the floor of my workshop that I will reinstall sometime. When I got the car iot came in, the main bearings rattled. I took between 0.005" and 0.007" out of the shims. To say that if the bearings are loose, then it is ruined is not so in my book. I recall my father in law telling of how many times he and his brother in law took up the bearings in his Terraplane back in the day. If rattling bearings meant the engine was shot, they would have been walking years ago. Besides, the OP says that they are not even rattling. I see no reason to change my first advice.
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Old 06-17-2019, 09:19 PM   #16
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Default Re: Checking rod and main bearings

Wow...lots of advice out there. I have only had the car for about 10K miles and I don't know the history. Overall it seems to have been well maintained and the engine was not knocking when I started this project. On one hand I want to pull all the caps off and check the engine, but on the other hand I worry I will create a problem when there isn't one. I think I am going to drop the pan to make sure there is no sludge in it and maybe try to trick with the jack, but if things look good I may not pull any caps.
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Old 06-17-2019, 09:43 PM   #17
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Default Re: Checking rod and main bearings

If you drop the pan and clean out the pan including below the dipper tray, make sure to inspect/clean the oil pump screen. By dropping the pan you should have a better idea of how well previous owners took care of the A. If you find sludge, a good idea to also clean out the behind the valve access panel.
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Old 06-18-2019, 05:39 AM   #18
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Default Re: Checking rod and main bearings

5K doesn't seem like that much for an engine, try having an aircraft engine done.
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Old 06-18-2019, 07:23 AM   #19
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Default Re: Checking rod and main bearings

Model a's are very cheap to own and operate,they still meet the original premise of inexpensive quality..considering a decked out pickup truck goes for 90k..
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Old 06-18-2019, 07:26 AM   #20
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Default Re: Checking rod and main bearings

Leave the sleeping dog alone. If you look hard enough, you are going to find a potential problem in any 90 year old engine.

My parents got me a Model A in 1960 because I would just take anything apart. The A kept me away from our lawn mower, snow blower, and other mechanical things around our house.

If you own your Model A for the same reason. Go for it. There are a lot of old cars out there that are in pieces. Having been taken apart and never re assembled.
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