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Rod journal tolerances? What is too much wear?


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I am in the process of using a micrometer to measure my rod journals on a 1922 Chandler Six. I plan on having the big ends of the rods re-babitted.

Is there an allowable amount of wear, taper, out of roundness that will not neccesitate regrinding the journals?

The manual states that the rod bearings (babbit type) should be tightened to a slight drag. That (from my research) translates to about 0.002 per bearing.

My initial measurements show the 2" journal approximately 1.9983 or off 0.0017.

I have not finished measuring for out of round or taper and I know that I must complete these measures.

Any suggestions as to what the bearing will tolerate as far as taper and out of roundness?

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How much are you planning to drive the car once it's running again?

If it's just going to idle in the driveway or gently drive a couple hundred yards between trailer and show-field, then some taper and out-of-round may be ignored...

If you want to drive it, then any taper or o.o.r greater than .002" probably should be corrected.

Taper on the journal will cause the rod to bear laterally towards the small end of the taper, and will concentrate the force from the piston towards one end of the journal; at best this will result in accelrated wear on said bearing/rod.

Out of Round will probabaly contribute to knocking.

If your journals are "undersize" from factory spec, the rods may have been babitted to match; get some plastigage (Sealed-Power makes it) and check the clearance of each bearing too. Or, if you've got the "mikes" and are carefull, you can mike the journal, then the rod bearing, do some math, and see how much clearance there is...if you're shaky on your math skills, stick with the plasti-gage...(Mathematics and I have never been on very good terms... crazy.gif )

If you're this far into it, might as well make things right while you're there...IMHO...

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Lincoln model L (1921-1930) which also is a babbited bearing engine has main bear specs at .0015. Like it was said, if the expected driving mileage is light to mild, I think I would leave well enough alone. True, good to resolve this while you're in there, but how far do you take it?

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I agree with the two above posts on low use engines. But, if it were my car, and I were planning to drive it at all, and you are going to the expense of having the rods rebabbited, I would regrind the crank if you had much more than 0.001" taper or out-of-round. You are probably spending $50 to $100 per rod for the rebabbiting; having the crank reground shouldn't cost more than $150 (Tucson AZ prices). Since you can have your rods machined to fit the new crank, why not have a tight engine? Incidentally, how are the main bearings; as long as you have the engine apart?

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  • 1 month later...

Thank you for all your responses.

I have checked and rechecked the crank pins and have come up with tolerances (out of round and taper) that are 0.0002 and 0.0001.

I am comfident in the micrometer measures (each pin was measured at 3 spots across the pin and in three locations around the pin) and each measurement was done at least 2 times.

Looks like the crank may be good.

Now to settle on a reputable rebabitter?

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If the crank pins are within .0002 and the bearings show .0015-.002 clearance , I would button it back up! I have read in many engine repair books that a used rod bearing that checks less than.004 is OK I regularly set up engines for drag strip use that have .002 on the rods and .003 on the mains and they run fine. I agree that if you are going to drive the car for 20-30 years and put a ton of miles on, that you may want to redo it. I built a 32 Buick engine 30 years ago that had 100,000 miles on it. I pulled shims and scraped the original babbett to give me .002 all the way. That engine was installed in my Buick model 97, and pulled a 1935 "covered Wagon" house trailer from Michigan to California and back. The car has been redone, and I talked to the owner last summer and asked him what he did to the engine He said " I cleaned it up and repainted it, as it runs too good to tear down"

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