Please excuse my ignorance but.....

[Licensed to kill]

I know when a bearing goes in a motor with shells it knocks and, especially with the throws will soon turn and score the crank and rod journal but what happens with poured babbit??. The babbit on shells is supper thin and things so sideways quickly once yet gets down to the copper but with poured babbit being relatively thick AND the fact that these are intended to be adjusted for clearance occasionally, what happened exactly???. What sounds do we listen for to indicate that the clearances require adjusting and at what point is it too far gone.In other words, since they are not shell that will turn, what happens, does the babbit start to fall apart or what?. How often should the clearances be checked?.

 

[Brentwood Bob]

Babbit wears oblong. I have read that a maintenance practice was to drop the pan and adjust the fit of the rods and mains. A knock would indicate a clearance issue.
Not sure of the interval for this but would guess 5000 miles. Rings and reground valves after 10,000 miles, not everybody could afford to have the local ford dealer repair their model a with new ford authorized replacement parts.

 

[BRENT in 10-uh-C]

I know when a bearing goes in a motor with shells it knocks and, especially with the throws will soon turn and score the crank and rod journal but what happens with poured babbit??. The babbit on shells is supper thin and things so sideways quickly once yet gets down to the copper but with poured babbit being relatively thick AND the fact that these are intended to be adjusted for clearance occasionally, what happened exactly???. What sounds do we listen for to indicate that the clearances require adjusting and at what point is it too far gone. In other words, since they are not shell that will turn, what happens, does the babbit start to fall apart or what?. How often should the clearances be checked?.

This is a loaded question, ...but properly installed Babbitt will basically pound itself flatter & flatter until it disintegrates into tiny pieces. Again, the keyword on this is the bearing was properly cast. If the damage is caused by lack of lubrication, the friction of the crankshaft journal will cause the metal to become fluid where it melts & smears away.

As far as checking clearances, properly cast bearings that are burnished to a properly refurbished crankshaft will likely go 20,000 - 25,000 miles without needing adjustment. If your engine is abused either with timing or RPM control, -and/or if the bearing was poorly cast or machined, you may need to check it every 1,000 miles or so.

.

 

[SAJ]

Licenced. You mention the Babbitt on shells is super thin. But not all shells use Babbitt on the bearing surface. Trimetal shells have a very thin babbitt layer (King bearing shells use 0.0005 to 0.0008 inch layers), but bimetal Silicon Aluminium shells run on the hard silicon particles embedded in the aluminium alloy matrix. This quite thick layer (maybe 0.012 inch) is usually bonded directly to the steel outer shell using a thin layer of pure aluminium. the Aluminium alloy matrix can have tin and copper in small quantities. Rich Faluca in Skokie uses this type. Snyders con rod shells use trimetal, or I was told they did when I fitted them when they first came out.
Some engine builders use the softer trimetal bearings in one position, in rods especially, and the harder Silicon Aluminium in the other. The belief here is that the bimetal half shell can better take the pounding in the top of the big end eye and the thin babbit layer over nickel then copper embeds any dirt more effectively, offering better crankshaft protection in the less demanding lower position. That is one reason for doing this that I was quoted and there may be others. Obviously any dirt particles larger than a half a thou cannot be sequestered in a trimetal bearing with such a tiny babbit thickness, so this is a questionable advantage.
Some say the bimetal bearing takes no prisoners if it fails due to poor oil supply etc and the crank will be scored, whereas the trimetal bearing may spare the journal surface. The babbit layer is awfully thin though when unworn and fails quickly when worn away.
SAJ in NZ

 

[Joe K]

I have found that a Model A engine worn beyond normal life but "tightened" to the maximum by removal of shims (or filing caps) makes a "whuppa-whuppa" sound when at idle. At speed it can sound "ok."

I have told my experience of bringing an engine to the "near binding" condition (think two ovals at 90 degrees one within the other) and then driving it at speed on the Mid-Cape Highway at congested traffic times (for the Mid-Cape now most any daylight time of day.) And having the engine suddenly sound like it was coming apart - which it was when the vibration apparently wore through the cotter pin and un-did the rod cap nut on one side.

Not an experience for the faint of heart.

But that engine was well into "whuppa-whuppa" stage.

Joe K

 

[Bob Bidonde]

On several occasions I have found remnants of cotter pins in the oil pan of Model "A" and "B" engines. My practice is to use modern nuts without cotter pins on the rod caps to replace the ******* size OEM nuts.

 

[rotorwrench]

The #2 main takes a good beating and wears faster than the other two due to the force distribution. They tend to wear oblong and off center. This is the most problematic bearing in the engine. If you tighten a bearing that's off center, it will pull the crank off center and change the way forces are distributed on the crankshaft. This can lead to cracks and premature bearing failures.

The connecting rods can be a bit off center and have very little real force change to the way they function with exception to failure of the thrust surfaces.

How well this design will last largely depends on how the vehicle is operated. They may last a lot longer for the operator with a light touch on the throttle and experienced operation of the spark control & GAV adjustments. Folks that are hard on them will be pulling the pan more often as a general rule.

 

[Jim Brierley]

The sound of a bad bearing is about the same, babbitt or shell bearings. The difference is babbitt will usually not harm the crank. I once heard the rods making noise on the way to Vegas, stopped in Barstow and tightened the culprit, and drove it the 200 miles back home. Later had the rods re-poured and as back in business. Didn't even pull the engine!

 

[Synchro909]

On several occasions I have found remnants of cotter pins in the oil pan of Model "A" and "B" engines. My practice is to use modern nuts without cotter pins on the rod caps to replace the ******* size OEM nuts.

I agree with you 200% on the ******* size OEM nuts. Les Andrews can't even work out what size they are.
I do pretty much the same with new nylock nuts each time after using old ones for the first "test" assembly.