German engineering

[TheOldHokie]

My son was changing the oil in his BMW X3 today and as he was removing the drain plug the shank snapped off just below the head. When I went to help I noticed the shank was hollow and there was a pronounced undercut just below the head. I thought that was really bizarrre until after I removed the broken off stub and gave it some thought. Its actually very clever. engineering. Anybody else see why?

Dan

 

[Elliott in GA]

I prefer Toyota/Lexus, Honda/Acura and Subaru, because I do not have to wonder about some engineering design after a failure of something that should not fail. My son has my old Honda: 14+ years with no repairs.

I presume the undercut allowed the head to separate to reveal the hollow threaded portion, and that allowed you to use an EZ Out without any drilling.

Of course, you could also conclude the design made such a failure more likely.

 

[TheOldHokie]

I prefer Toyota/Lexus, Honda/Acura and Subaru, because I do not have to wonder about some engineering design after a failure of something that should not fail. My son has my old Honda: 14+ years with no repairs.

I presume the undercut allowed the head to separate to reveal the hollow threaded portion, and that allowed you to use an EZ Out without any drilling.

Of course, you could also conclude the design made such a failure more likely.

Your analysis is close but misses the central element.

Dan

 

[Henro]

Two observations…

Are you contradicting the fiction as claimed here at OTT by some that easy outs don’t actually work?

Could this be an indication that the Head of Engineering at BMW is actually an accounting major interested in dealership profits driven by repair work?

I’m having trouble visualizing from the two pieces sitting on the white paper what actually broke from what. It’s obvious that the threaded plug accepted the easy out to get it out. But the other piece doesn’t look like it offers much for something to grab onto it.

 

[Dustball]

Functions similar to a shear pin- prevents overtightening and thread damage.

 

[mcmxi]

My son was changing the oil in his BMW X3 today and as he was removing the drain plug the shank snapped off just below the head. When I went to help I noticed the shank was hollow and there was a pronounced undercut just below the head. I thought that was really bizarrre until after I removed the broken off stub and gave it some thought. Its actually very clever. engineering. Anybody else see why?

Dan

View attachment 140207

Oops .... @Dustball beat me to it.

From your description of the bore in the drain plug and the undercut above the head, it sounds like the drain plug is designed to shear before the threads in the oil pan are destroyed. It's a lot cheaper and easier to replace a drain plug than it is to replace and oil pan. Some oil pans require pulling the engine in order to change.

 

[TheOldHokie]

Two observations…

Are you contradicting the fiction as claimed here at OTT by some that easy outs don’t actually work?

ABSOLUTELY

Could this be an indication that the Head of Engineering at BMW is actually an accounting major interested in dealership profits driven by repair work?

ABSOLUTELY NOT - JUST THE OPPOSITE

having trouble visualizing from the two pieces sitting on the white paper what actually broke from what. It’s obvious that the threaded plug accepted the easy out to get it out. But the other piece doesn’t look like it offers much for something to grab onto it.

The hex head is not pictured and the plastic pin has nothing to do with the plug.

Give it some more thought.

HINT - There have been two recent threads on OTT dealing with oil pan plug failures on the highly regarded Kubota engineered tractors.

Dan

 

[mcmxi]

Functions similar to a shear pin- prevents overtightening and thread damage.

You beat me to it but I think you're right, if not about over torquing the drain plug, but about not stripping out the oil pan threads in the event that it's seized in there.

I still think that German engineers are the best in the world.

 

[TheOldHokie]

From your description of the bore in the drain plug and the undercut above the head, it sounds like the drain plug is designed to shear before the threads in the oil pan are destroyed. It's a lot cheaper and easier to replace a drain plug than it is to replace and oil pan. Some oil pans require pulling the engine in order to change.

Winner winner chicken dinner.

My son said there was almost no resistance when he loosened it.

My guess is the plug was overtightened to the verge of breaking during the last oil change.

 

[D2Cat]

Built ready for an easyout if broken. No need to drill a hole to start it.

 

[North Idaho Wolfman]

I hate to burst all of the happy bubbles, but the hole is designed for a magnet, not stress relief.

 

[Mark_BX25D]

But what about the undercut?

I hate to burst all of the happy bubbles, but the hole is designed for a magnet, not stress relief.

But what about the undercut?

 

[MapleLeafFarmer]

depending on the year BMW and many others like my range rover use single use / disposable and torque limiting oil drain pan plugs.

 

[TheOldHokie]

But what about the undercut?



But what about the undercut?

Exactly. The undercut in the steel bolt is designed to yield if overtightened and prevent pullout of the threads in the aluminum oil pan. FWIW - there is no magnet.

Dan

 

[Henro]

Winner winner chicken dinner.

My son said there was almost no resistance when he loosened it.

My guess is the plug was overtightened to the verge of breaking during the last oil change.

Well I still can’t visualize the initial situation, but you used the easy out to remove the plug. How is that different than just removing the plug if it didn’t have that undercut and nothing broke to begin with?

I guess you could be feeling that the design is such that the plug should break off if it is over torqued before the threads in the oil pan are damaged, but that obviously didn’t happen in this case… And possibly set up a catastrophic event if the plug head had fallen off after the fact, unless the electronics is smart enough to shut the engine down if the operator doesn’t notice what’s going on.

Perhaps your son turned the plug the wrong way and broke it off and doesn’t want to admit it…LOL In that case I totally see how your evaluation makes sense. IF I am envisioning the situation correctly without seeing all the broken parts.

 

[TheOldHokie]

Well I still can’t visualize the initial situation, but you used the easy out to remove the plug. How is that different than just removing the plug if it didn’t have that undercut and nothing broke to begin with?

I am not sure what the question is.

I believe the plug was compromised (partially fractured) by overtightening it on the previous oil change. The male and female threads were not damaged or seized and the remnant was easily removed with an EZ out and tap handle. Not even finger tight. Why is it also hollow?? Who knows.

 

[Henro]

I am not sure what the question is.

I believe the plug was compromised (partially fractured) by overtightening it on the previous oil change. The male and female threads were not damaged or seized and the remnant was easily removed with an EZ out and tap handle. Not even finger tight. Why is it also hollow?? Who knows.

I guess it comes back to the original question. Was it clever engineering or not?

The plug seems to have been designed to fail before the oil pan threads were damaged, in case it was over torqued. But the assumption is that the plug only failed partially, since it was not over torqued enough to break off. If this was the case it seems like it set up an opportunity for a potentially catastrophic failure to occur, if it broke off due to vibration when the car was being driven, and if the car was not smart enough to shut down due to the loss of oil pressure, if an alarm was not noticed by the operator.

So I guess I’m not sure whether that is a clever design or not. Not really a question, but rather an observation. Nothing more…

 

[TheOldHokie]

Here is an after market plug showing the undercut. It is not drilled out like the OEM plug.

Dan

 

[Elliott in GA]

Your analysis is close but misses the central element.

Dan

No, I just did not articulate specifically, and I guess that I did not because this is poor way of protecting threads.

First, use a torque spec to avoid any problem
Second, use a threaded insert of tougher material in the pan or a plug made of weaker material than the pan

Using a pre-scored/weakened plug of a stronger material is like using a low grade screwdriver tip to avoid stripping a screw head. The fact that you believe/realize the plug was partially compromised by the last oil change reveals the problem inherent with this "safety" design. The plug can easily be compromised without any visual warning, and then the plug can fail during operation due to vibration/expansion-contraction/etc. The result would be a catastrophic loss of oil while driving.

I do not see this as a better design in any way.

 

[North Idaho Wolfman]

I'll agree the undercut is possibly a stress relief or just a place for the sealing washer to expand into???
But the hole is for a magnet that they have left out because it's of little use, it's a left over design from years past.