01-05-2024, 11:51 PM | #1 |
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Rod alignment
This is a short story about rod alignment tools and broken rods caused by them back in the late 40's and early 50's..The story was inspired by a 12 pack and a long session about old dayz bench racing.
The tool under discussion was basically a small 2 ton hand pumped hydraulic jack mounted in a "C" frame. It had various fixtures that could be changed on the base and ram for bending in all directions. It was very popular in the day and most of the big shops had one. I forgot the brand but I remember a few guys on this forum saying they had one. If the rod was twisted the base fixture supported the rod on the edges of the I beam at 4 places. These 4 supports were mounted so that they could move independently when down force was applied and thus the rod was twisted. It worked great except for one thing, the supports always left a small dent in the edge of the rod. So a buddy of mine and I stopped by a friends shop one day. A quick word about this buddy, he had just graduated from getting a masters in mechanical engineering with a specialty in analyzing metal failures in aircraft. Anyway the guy that owned the shop had a sprint car with a model B engine in it with an OHV head on it. He was using Pontiac rods which was the standard of the day and he had one of the alignment tools under discussion. As we walked in he was just about to start assembling the engine. He had blown 3 engines in the last 3 weeks. Never finished a program. Broken rods every time. He attributed it to a bad batch so had gotten new ones each time from different parts of the country. My buddy asked if he could see one of the broken rods. It took about 3 seconds for both of us to see the problem. 4 distinct dents where the alignment fixture pressed on the rod and the break was right through the center of 2 dents. We showed him how to polish out the dents with a spiracone and the marks in the longitudinal direction. That set of rods lasted him all season. |
01-06-2024, 07:54 AM | #2 |
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Re: Rod alignment
What Pete is talking about is called a stress riser and any mechanical engineer knows about them. The notch concentrates the stress and thereby the stress is more than in the bulk of the material at that point. A sharp notch will make more concentration. You can think about this in the way glass is cut. Glass is a brittle material. The glass cutting tool makes a sharp groove in the surface of the glass and it breaks easily at that notch. Another way to think about it is how flowing air would crowd around a protrusion in a duct. The flow of stress works the same way, crowding around the notch.
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01-06-2024, 08:49 AM | #3 |
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Re: Rod alignment
Does this small "notch" work the same when someone notches the rod on the shank to indicate whether it is for number one, two,three or four cylinder.? What i have seeen is usually done with a small grinder or chisel. ken
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01-06-2024, 12:01 PM | #4 | |
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Re: Rod alignment
Quote:
The analogy about how glass is cut is 100% right. I remember watching an automotive machine shop tech doing that back in the 60's and I thought it didn't seem like a good idea but I was young and not going to challenge him. |
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01-06-2024, 12:48 PM | #5 |
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Re: Rod alignment
Interesting story Pete, and very true. If the guy had babbitted the Pontiacs, as most did, they should have been straight when the babbitt was cut.
I don't know if you knew Wes Cooper? I was at his shop one day when he was building B engine using Pontiac rods, he simply used Pontiac inserts with another, different, insert inside of the Pontiacs. Not the way I would recommend, but I guess it worked. I've also seen him lighten valves by just using a common bench grinder, with just holding the valve head against the wheel. For some reason he had a pretty good reputation as an engine builder, he specialized on Cragars. |
01-06-2024, 02:53 PM | #6 | |
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Re: Rod alignment
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01-06-2024, 07:29 PM | #7 | |
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Re: Rod alignment
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01-06-2024, 07:44 PM | #8 | |
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Re: Rod alignment
Quote:
The usual place for a rear axle to break is right at the differential end of the hub and that is a result of worn hubs and bearings.
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01-06-2024, 08:29 PM | #9 | |
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Re: Rod alignment
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01-06-2024, 10:49 PM | #10 |
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Re: Rod alignment
One of my professors in engineering school told us a story about the meticulus aircraft mechanic aboard an aircraft carrier during WWII. He would paint the tips on prop engine planes white. In order to make sure the edge of the white was neatly straight he would scribe a line into the surface of each prop. Soon thereafter they began to experience the loss of a large number of planes due to prop failure. They determined the scribe line created a high concentration of stress which ultimately cause the blade to fail.
Similary, a more familar phrase to many is "tear along the dotted line". |
01-06-2024, 11:18 PM | #11 | |
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Re: Rod alignment
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And as to whether the seal groove in the rear axle creates a sufficient stress riser to be an issue: the grooves I've seen have been relatively smooth with no sharp edges. You mechanical engineers out there can correct me, but my recollection is that it's sharp corners, either internal or external, that create potential problems. That's one reason it's important that when resurfacing flywheels you maintain the radius at the outer corner of the friction surface. JB weld in the axle seal groove will neither help nor hinder this stress riser problem. I think it's more an issue of whether the JB Weld will hold up to the rotational friction from the seal. I have some minor grooves in the axles of the rear end I'm building right now. I plan to polish them, check the seals to assure they still make contact with the slightly smaller diameter, perhaps fill some pits with JB Weld but I'm not planning to try to restore the original diameter. If I was to try to restore I'd consider having a spray weld, followed by lathe work and polish to the original diameter, although since the axle appears to be hardened I'm not sure I want to put a large amount of heat into it. But, since new axles are $800+ apiece, I'm not anxious to replace them unless I really need to.
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01-07-2024, 12:56 AM | #12 |
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Re: Rod alignment
Doesn't the keyway end right at the end of the taper which is at the end of the hub?
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01-07-2024, 10:34 AM | #13 | |
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Re: Rod alignment
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01-07-2024, 10:52 AM | #14 | |
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Re: Rod alignment
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J
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01-07-2024, 11:52 AM | #15 |
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Re: Rod alignment
Great idea, katy, thanks, I'll look into it.
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01-07-2024, 01:28 PM | #16 |
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Re: Rod alignment
A friend ground a cam on a lawn mower sharpener, it ran pretty well but had more lift but not much more duration.
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01-07-2024, 06:24 PM | #17 | |
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Re: Rod alignment
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Quote: Originally Posted by katy View Post You could try putting a spacer under the seal, to move it to a new spot. We've done that on different machines with success. Great idea, katy, thanks, I'll look into it. I've been doing that for years on my Model As. I even have a length of copper pipe the right diameter that I put in the lathe and cut off a piece each time I need one.
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