Connecting Rod-Crankshaft Journal Clearance

R

Ron/IA

Well-known member
I hope the illustration will give you an idea of what I am trying to say. It has to do with the clearance of the connecting rod to the crankshaft journal.
Let’s say the journal is 1.500” diameter, and the inside diameter of the connecting rod is 1.5005”. You shim the connecting rod to get 0.015” clearance.
The shims only separate the top and bottom of the connecting rod, but not the sides near the shims. That being said, we have a clearance on the sides of only 0.0005”. Is this going to be a problem?

Note-the clearance dimension near the shims is incorrect on drawing; it should be 0.0005".
 

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Yes, the 0.015" is also a typo (I guess I could not label my drawing correctly); should be 0.0015". The connecting rods are bored with shims in place. What I am trying to say is that connecting rod bore is only 0.0005" larger than the diameter of the crankshaft journal.

So, if you shim it so there is a clearance of 0.0015, to me that means the shim area is still only 0.0005". That is why I was wondering if there would be a problem?
 
If the rods were correctly grooved for oil, there should be a V bevel all the way across the rod and cap babbitt at the parting line. That eliminates the .0005 measurement at the parting line.
 
0.0005 at any point is too tight
always measure 5-6 points vertical and horizontal and 45 deg amd 30 deg
 
The rods have the correct oil grove, and they are V-beveled all the way across the rod and cap.

Anybody have any other thoughts?
 
Did you have a crankshaft was ground undersize without the grinder knowing the journals were to be .0015'' under the nominal size?

I suppose it's a moot point now, as you need to survive the error regardless of what or who was the cause. I agree that is too tight and would likely wipe the oil off the journal at the parting lines.

Short of a machinist that can handle the oddity, the only thing I can think of is carefully mate the babbitt to the journals with Timesaver compound which has been around forever. While that will likely make the babbitt guys cringe but I've seen one example that is still great after 7k miles. However that was with NOS rods with Ford babbitt. Ford rods were done undersize as that is how they were done in production.
 
How it it being measured? ---what method?


a babbitt scraper would make short work of that minor fitting.

The top one is my favorite, feels perfect for the A sized journals
 

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Marco - You are correct, there was a misunderstanding (I perpetrated it), and I will let go at that. What grit of Timesaver compound would be used? I am more curious about the compound right now, as I am going to consult a machinist I have a lot of confidence in, and ask his suggestion.

Kurt - I am not sure I would have the skills to use a scraper. Is it hard to keep the babbitt surfaces flat? Or is that where a person wants to have a jar of machinists blue handy?
 
Ron/IA said:
Marco - You are correct, there was a misunderstanding (I perpetrated it), and I will let go at that. What grit of Timesaver compound would be used? I am more curious about the compound right now, as I am going to consult a machinist I have a lot of confidence in, and ask his suggestion.

Kurt - I am not sure I would have the skills to use a scraper. Is it hard to keep the babbitt surfaces flat? Or is that where a person wants to have a jar of machinists blue handy?
Click to expand...

I don't recall but I believe they spell it out in their instruction sheets available on their web site. It's interesting that Timesaver is mentioned (without conclusions) in my 1936 Dyke's manual.
 
Maybe I am totally missing the point but my understanding of the clearance is that it is 0.0015 on the diameter, not 0.0015 on the radius as shown by his drawing. Or 0.00075 on the radius when centered in the bearing making the 0.0005 he notes pretty close or maybe not.:confused:
 
Somebody correct me if my thinking is wrong, but when setting the clearance of the connecting rod using the aluminum foil method; the foil is spacing the cap 0.0015" away from the journal while the other side of said journal is resting on the rod side. Thus, you would end up with an inside diameter of the connecting rod 0.00075" larger than the crankshaft journal.

Does that make sense?
 
No, the clearance of .0015" means the crank diameter is .0015" less than the rod journal. If the rod stay centered due to an oil film, then the rod would have a clearance of .00075" on ALL sides. Or, a radial difference of .00075".
 
clearly there has been no improvement in methods and technology in 80 yrs.
Foil has been discussed in a very detailed manner here, search it. It is easy and accurate. I use plastigage (a buck a strip) on rods and mains and foil on the mains if the engine is in the car.
 
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