Rod alignment

[Pete]

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.

[nkaminar]

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.

[kenparker0703]

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

[Flathead]

Originally Posted by kenparker0703

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

That technique is no longer acceptable, If you have marked rods polish those marks away and don't leave any new marks.
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.

[Jim Brierley]

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.

[Pete]

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Originally Posted by Jim Brierley

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.

I didn't know Wes but had heard of him. I can understand lightening valves on a bench grinder. I ground my first cam on a bench grinder when I was in high school. It was faster than my friend's Winfield in the same engine.

[ursus]

Originally Posted by nkaminar

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 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.

I have to ask about another area where a notch often forms: on rear axle shafts where the seal riding on the shaft can form a concentric groove. Would this create a stress riser that might cause the axle to separate at the groove, with possibly catastrophic results? I've seen guys fill in the groove with JB-Weld and sand it smooth in order to restore the surface dimension but how risky is that?

[Synchro909]

Originally Posted by ursus

I have to ask about another area where a notch often forms: on rear axle shafts where the seal riding on the shaft can form a concentric groove. Would this create a stress riser that might cause the axle to separate at the groove, with possibly catastrophic results? I've seen guys fill in the groove with JB-Weld and sand it smooth in order to restore the surface dimension but how risky is that?

JB weld is no where near as strong as the original steel. IMO, filling the groove with JB weld is OK only as part of fitting a speedy sleeve over it for the new seal to run on but the axle is still weaker at that point
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.

[Pete]

Originally Posted by Synchro909

JB weld is no where near as strong as the original steel. IMO, filling the groove with JB weld is OK only as part of fitting a speedy sleeve over it for the new seal to run on but the axle is still weaker at that point
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.

On race cars using an early Ford rear end, most of them broke at the weakest point, the end of the keyway.

[CT Jack]

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".

[JayJay]

Originally Posted by Synchro909

JB weld is no where near as strong as the original steel. IMO, filling the groove with JB weld is OK only as part of fitting a speedy sleeve over it for the new seal to run on but the axle is still weaker at that point
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.

Synchro - I've been unsuccessful finding a Speedi-Sleeve the right size for the rear axle seal (~1.135" or so). Do you have a source?

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.

[Synchro909]

Originally Posted by Pete

On race cars using an early Ford rear end, most of them broke at the weakest point, the end of the keyway.

Doesn't the keyway end right at the end of the taper which is at the end of the hub?

[katy]

Originally Posted by JayJay

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.

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.

[johnneilson]

Originally Posted by Synchro909

Doesn't the keyway end right at the end of the taper which is at the end of the hub?

The keyway on the large end of the taper causes a stress riser and that is where it breaks.

J

[JayJay]

Originally Posted by katy

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.

[Jim Brierley]

Originally Posted by Pete

I didn't know Wes but had heard of him. I can understand lightening valves on a bench grinder. I ground my first cam on a bench grinder when I was in high school. It was faster than my friend's Winfield in the same engine.

A friend ground a cam on a lawn mower sharpener, it ran pretty well but had more lift but not much more duration.

[Synchro909]

Originally Posted by johnneilson

The keyway on the large end of the taper causes a stress riser and that is where it breaks.

J

I agree with what you say here and it is not contrary to what I am saying. IMO, the stresses that are concentrated at the end of the keyway arise from the flex in the axle caused by a worn hub. The hubs always wear at the nut end, allowing the wheel to lean inwards at the top. That puts a bend in the axle and as it turns, the stresses eventually cause it to fatigue at the stress riser - the end of the keyway.

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.