jandreas

These are press fit oil plugs so don't you want them to have the same coefficient of linear thermal expansion as the aluminum tube that they fit into?

target39

I'm fairly certain that the rocker axle that the plug fits into is steel (shafts inside the engine are typically steel for strength and its surface finish matches more closely with steel), while the original plug is aluminum (bad design by Audi/VW, not unlike using sub-par plastic for oil cooler pipe, intake manifold linkage arms, etc.), which would mean they originally had different thermal expansion properties, causing them to easily fall out due to extreme temperatures in the first place.

Having the new plug created out of stainless steel would only improve its properties (in terms of not shrinking as much to slip out). It would be better if it was made of the same alloy of steel, but even if it does not match 100%, it should still function better than the original aluminum.

dvs_dave

Aluminum's coefficient of thermal expansion is about twice that of steel. That means in an interference fit situation like this, the hotter it gets, the tighter it fits.

So presuming it was originally installed by cooling it in liquid nitrogen, what you now have with the push fit SS won't be anywhere near as tight a fit as the intended OEM install provides at any operating temperature you might come across.

kishoreh

What was interesting is that it required a press to remove the aluminum plug from the used rocker shaft. So I still don't understand how the oil pressure even slightly greater than 75 psi could have caused the plug to dislodge. Also the design of the plug has two places where there is a press fit the outer body and the nipple in the back .

We will see how the test goes with the stainless steel plug and the loctite.

dvs_dave

I suspect the initial failure was due to a manufacturing/assembly defect or falling foul of tolerance. Couple that with x number of heat/cool cycles being enough to gradually loosen it until the oil pressure on a very cold day was enough to pop it out. I.e. Your shaft hole is at the top of the tolerance range, and the plug size at the low end. Nothing you can do about that, just luck of the draw.

kishoreh

That maybe the case, but two of the members in this post have put the original plugs back into the original rocker shaft and it has resolved the issue.

I think jandreas had to reinstall the plug twice.

One of the failures was on the intake rocker shaft, my failure was on the exhaust rocker shaft.

My luck I could not find the original plug so I had to purchase a used replacement.

I did let the car sit for 4 days in February and the temperature in South Florida dropped from 75 F to just below 40 F so the drastic temperature could have caused the plug to shrink enough for it to dislodge.

I am still going to replace the PCV Valve even though several people feel this could not have caused the issue.

target39

Unfortunately, it almost certainly wasn't installed by cooling to <-200°F via LN2 and I doubt they would cool it down anywhere well below freezing during installation. It just would not make sense from a manufacturing standpoint in terms of cost and practicality. They probably just tap them in as with any engine plugs. I would know since I was able to install my plug back in without cooling it in a rather confined space.

In theory, while it sounds more beneficial that aluminum expands more than steel and would fit better as it heats up, it also shrinks more as it cools, potentially making it fit looser in the cold than steel. Not only that, but since aluminum is softer than steel, the installation of an aluminum plug may cause deep gouges/surface imperfections that allow oil to penetrate and lubricate the plug over time. My aluminum plug came out with scratches deep enough to be seen and felt with a fingernail. Since they were on the lateral faces far enough away from the sealing surfaces, I figured I could reuse it without serious refinishing and it has worked since; however, I would not be surprised if it comes out again when winter rolls around, so I have to ensure it stays in place with a retrofit of my own by then.

Also, you have to consider that the engine builds up oil pressure nearly instantaneously, at least well before it builds up temperature. Factor in the fact that the surface of the plug exposed to oil pressure only equates to roughly 30 square millimeters (0.0438 sq in) and 75 psi at WOT/Max RPM equates to 1,700 pounds of force. Which would make sense as to why the used rocker axle with a good plug required a hydraulic press to remove.

Essentially, using stainless steel for the plug as opposed to aluminum should not cause any issues that an aluminum plug would not, assuming similar/proper design, manufacture, and installation of the plug.

elektr0

Shooot my car has been sitting for almost a year because i couldnt find where oil was coming from thanks for the info im going to take a look at this plugs in the morning that might also be my problem