- Home
- About Us
- Join/Renew
- Member Benefits
- Member Pages
- Log In
- Help
- Museum Store
I am in the process of checking the rod bearings on my series 81 and am wondering if anyone can tell me the acceptable Tolerance. One of the Rods appears to be slightly looser then the others which caught my attention so I figured I may as well check them all as long as I have it apart.
Hi Charles, the suggested clearance is .001″ . That is new tolerance.
You can have .002″ and you won’t hear any noise..
But is you plan on driving your car a lot, and driving it hard, I’d recommend keeping the clearance under .002″.
The Connecting rods on S81 engines are aluminum,, so getting them rebabbitted will require finding someone familiar with the process, The Franklin cars had aluminum rods as well, you might check with someone in the Franklin club for a recommended babbitting company to use..
The crankshaft needs to be round, and not have any taper, so check the rod journal in multiple places, to be sure the journal has not worn to be oval or egg shaped.. the normal maximum out of round is .0005″.
The bearing size is quite large, the S81 got larger bearing sizes, and with the stiffer crankshaft, the vibration dampener on the S80 engine was eliminated.
I hope this information helps with your engine work.
Greg Long
Greg,
Thanks for the information this is indeed very helpful. By chance have you found any time to dig out that cowl?
Greg, forgot to ask what the tork for the rod bolts is?
I would err on the side of looser. Automobile engines, particularly luxury car engines specified very tight clearances for basically one reason: to make them silent. Industrial/commercial engines specify much looser clearances to minimize friction, bearing temperature, and life. They make a lot of noise but ironically last longer.
As per my discussion on oil viscosity, tighter clearances also generate more friction and heat which is the enemy of babbitt bearings.
The following quote from J. F. Taylor’s seminal text “Internal Combustion Engine in Theory and Practice”:
“The usual recommendation for crankshaft bearing clearance is .001 times the journal diameter. The author’s experience, confirmed by most bearing experts, is this is a minimum value, and that values up to twice this amount are acceptable and often desirable. Small clearances are dangerous both because they do not allow enough for distortion and because they have less safety against wear and scoring by foreign particles…bearing coefficient of friction decreases as clearance/diameter increases.
The objections to large clearance include lowering of the value of the Sommerfield variable and hence a lower factor of safety against oil film breakdown. However, most bearings fail for reasons other than direct oil film failure. The present author cannot recall a bearing failure due directly to excessive clearance, whereas he has seen a great many bearing failures attributable to inadequate clearance….No journal bearing in any engine, no matter how small should have a minimum clearance less than .001 in.”
On my 845 the minimum recommended by Taylor matched the maximum (.0025) specified by Pierce. If you set clearances super tight it is extremely important as Greg mentions that the journals be round and have virtually no taper. Check the clearance in multiple positions to make sure there is no position where it has inadequate clearance. Plastiguage is useful in making sure that the journal and rod bearing aren’t tapered, and that the rod sits flat against the bearing The slightest bend or machining error of the rod bearing can make the rod ride one side and reduce its load capacity. A tight clearance has virtually no room for error. I wouldn’t trust any rebuilder to do a better job than Pierce-Arrow, and that means it is safer to err on the looser side of the clearance. It is also better from a heat and bearing survival standpoint.
I think many of the bearing failures on antiques attributable to bad babbitt may actually be from rebuilders proudly setting them up at the minimum clearance with the notion that is the measure of rebuild quality, and then putting in thick oil to give it that extra measure of protection.
Jim
Thanks, Jim, good information.
In addition to measuring the journal for being round and no taper, I always install a connecting rod on the journal and rotate the crankshaft to make sure there is NO drag, or contact of the babbitt to the crank journal.
Also, when I start a new engine, I use a separate oil pump, that is attached to an external oil line, to feed oil into the engine’s oil passageways, the crankshaft passageways, feed passageways to the cam bearings and on ’33 and later engines, to the hydraulic lifter bodies. While the assembly lubrication is very important, having an engine run for even a few seconds on only the assembly lubricants, while the engine’s oil pump is filling the passageways, filter canister, and pushing any air in the passageways through the rotating bearings.. well this just can’t be good for an engine’s bearings..
In order to promote full lubrication as immediately as possible, I use a light weight oil for the start up, usually a 5w-20 oil. This oil is in the engine only for it’s initial run on an engine stand, or in the car prior to it’s first drive on the road. I think of this first oil as a wash and rinse following the previous reassembly of the engine, There is ALWAYS some bits and pieces of ‘stuff’ that you just can’t fathom HOW it was missed during all the cleaning, washing etc you did during the rebuild.. I also cut open the oil filter to open up the paper element to look for contaminants that the filter caught.. It is pretty amazing what it catches.
Greg Long
Thanks guys this is all really helpfully information. I will follow up with more information and questions as once I get back in the garage. Unfortunately work has kept me really busy lately and I certainly don’t want to rush this part. My guess is that I am in tolerance because my dad said when he shut this engine off for the last time several years ago it ran very quietly.