Doorsill patch - What is it for?

autobonrun · autobonrun's Garage

In order to install my stainless doorsills, I lifted my rubber ones for the first time. The sill has a couple holes drilled in it ( I assume to drain water from underneath) and the piece in the photo welded to the front. It looks like an access way, but access to what?. I found a little rust around the welds which I sanded down and treated with naval jelly. I'll spray paint over it before I install the new doorsills.

My question, is what is the patch for? It doesn't appear to be structural and since it's welded in place, I don't see what function it serves.

silverc4s · silverc4s's Garage

Drug cache? ;-)

Yellowbird RS · 02-07-2005, 01:56 PM

I don't know my 74 Carrera have the driver side w/o the patch and the pass side is just like yours, I was tempted to patch it.

CarreraS2 · 02-07-2005, 02:00 PM

I don't know what it's for, but there should be a little metal plate that fits right into that rectangular indentation.

Maybe check under your rubber sill piece, it might be stuck on it!

autobonrun · autobonrun's Garage

Quote:

Originally posted by SoCal911SC
I don't know what it's for, but there should be a little metal plate that fits right into that rectangular indentation.

Maybe check under your rubber sill piece, it might be stuck on it!

Actually, the little rectangular plate is the part that shows up in the picture. It appears just to be an indention but it is the plate welded down flush with the sill. I just checked the other side and it is not in as good shape. I'm going to need to do more sanding and treating on the passenger side. Just trying to figure out what it's for.

autobonrun · autobonrun's Garage

Quote:

Originally posted by Yellowbird RS
I don't know my 74 Carrera have the driver side w/o the patch and the pass side is just like yours, I was tempted to patch it.

Does yours have some rust around the weld?

Yellowbird RS · 02-07-2005, 02:54 PM

Quote:

Originally posted by autobonrun
Does yours have some rust around the weld?

No, no rust

ausdilecce · ausdilecce's Garage

Don't have a clue what it is for but ( I *think* ) it has a name and a part number..

Compensating Plate - 911 501 642 01

autobonrun · autobonrun's Garage

Quote:

Originally posted by ausdilecce
Don't have a clue what it is for but ( I *think* ) it has a name and a part number..

Compensating Plate - 911 501 642 01

Thanks Ausdilecce, I think we're getting closer to an answer. Now to find what it is "compensating" for.

I guess no guts no glory so using my dremmel, I started to attack what appeared to be rust on the doorsill. The very good news is that the hollow sound that I thought was rust-through was simply a factory punched opening in the sheetmetal beneath the plate. It was sealed with a piece of rubber and the plate glued, not welded on top of it. The plate appears to function as a sacrificial corrosion coupon, but at this point, I don't have a clue. But I'm determined to find out its purpose. In this photo, you can see the hole it covered, the rubber plug, and the corroded plate. The sheetmetal beneath is completely intact with no corrosion whatsoever. I checked the price for the part on Pelican and it is $1.00. Can't beat that. If I find out what the plate is for, I'll post an answer.

North Coast Cab · North Coast Cab's Garage

Anyone? I have two that have come loose as well. Just curious.

John

john walker's workshop · 02-12-2005, 09:32 AM

maybe a locator point for the assembly jig.

autobonrun · autobonrun's Garage

North Coast, I ordered the part recommended by Ausdilecce but have not gone by the dealer to pick it up yet. I'll try to do so tomorrow. If they are the correct parts, I'll post back. For a buck twenty five each, I thought I'd just chance it.

I sent an email to PCA's tech section but have not heard back from them as to what this part is for. The dealer did not know either. It's not a part that you would even see unless you lifted the rubber sill plate.

I did however speak with the chief metullargist at my company and he felt that the piece is indeed a sacrificial anode as I first thought. In brief, it is a dissimilar metal to that of the sheetmetal body of the car. Water is a good electrolyte and conductor (even more so in the presence of salt). If water were to get into the doorsill area, the sacrificial piece of metal would act as an anode and give up its electrons and rust rather than having rust form on the car itself. Oxidation/rust basically occurs when a metal gives up its electrons. By having another metal with a lower electronegativity, it rusts instead of the car. This means the sacrificial piece needs to be tight enough against the door sill metal that any water that gets into the area will form an electrical path for the electrons to flow between the anode piece and the doorsill. This explains why the part is not painted on the bottom.
I guess mine is proof that it works.

I found this article that helps support this theory. See section 5 paragraph 17. The entire article is fairly long and highly technical but it is a good discussion of corrosion.

http://www.dstan.mod.uk/data/03/030/00000300.pdf

Anyway, my theory that this indeed is a corrosion coupon functioning as an anode is the one that is making the most sense to me at the moment, although I don't pretend to be a metallurgist. Now I need to find out if the adhesive used to attach the piece is anything special. I know I'll use as little adhesive as possible under the coupon so I don't interfere with the electron flow from the part to the car body.

CarreraS2 · 02-20-2005, 08:06 AM

Except isn't the sill that the metal piece sits on painted? It's been a while since I've seen it, but I thought it was.

If so, could electrons flow between the piece and the painted sill?

autobonrun · autobonrun's Garage

Quote:

Originally posted by SoCal911SC
Except isn't the sill that the metal piece sits on painted? It's been a while since I've seen it, but I thought it was.

If so, could electrons flow between the piece and the painted sill?

Mine was not painted underneath. Take a look at the photo above. Neither the metal coupon nor the car body was painted at that point.

CarreraS2 · 02-20-2005, 08:11 AM

Yes, I remember that now!

abisel · 02-20-2005, 08:13 AM

Quote:

Originally posted by john walker's workshop
maybe a locator point for the assembly jig.

Your not too far off.

Actually, this rectangular area is an access point for the assembly spot welders to weld the forward edge of the rocker panel to a bracket extending toward the rear off the door post. Then they simply "glue" on the cover plate. It isn't welded.

autobonrun · autobonrun's Garage

I should have known John wouldn't be off on the answer. Is it possible the part could serve two purposes? If there's an additional function, I want to make sure I don't do more harm than good and replace it correctly if it does also serve as an anode in reducing any rust. I'm curious why they wouldn't also paint the area underneath if it is just a cover.

I always have this suspicion that Porsche engineers have motives that aren't immediately obvious to me. It could simply be an error rather than a planned design.