1972 Porsche 914

Repair and Restoration - Part 14 - Transmission

1/29/2008

Ok, I really am not digging in and trying to do any R&R work on the transmission.  At this point I am not even sure which one I will use.  I have three 914 transmissions.  One tailshifter that came with the car.  I may either keep this for historic reasons (if for some crazy reason I decide to put the car by to stock some day in the future) or use it for parts.  I have the side shifter from my older 914 and another side shifter that was given to me.  I probably will tear apart both side shifters and rebuild them both.  Then use one in the car and use the other as a spare.  Or if I win the lottery (which is not going to happen as I don't play the lottery) I may buy a 915 transmission and not use any of the 914 transmissions.

Anyhow, regardless of what I do, I became very curious about how to clean and protect the magnesium (and to a lesser degree aluminum) cases on Porsche transmissions (as well as other parts such as intake manifolds, cooling fans, etc.).  I will not go into all of the details here.  This thread I started on the Pelican Parts forum has good info...

http://forums.pelicanparts.com/showthread.php?t=389279

Anyhow, regardless of what I do, I became very curious about how to clean and protect the magnesium (and to a lesser degree aluminum) cases on Porsche transmissions (as well as other magnesium items such as cooling fans)

Q: What do you use to clean a Magnesium part?
A: Probably the best way is to use something known as "Stoddard Solvent", a soft brush and elbow grease. Stoddard Solvent is a type of mineral spirit. That is probably the least destructive way to get it clean. Other methods include media blasting and various cleaners that may have some type of acid in them. Those obviously will remove some amount of metal.

Q: What type of coating is on the Magnesium transmission cases.
A: A waxy protective coating known by the brand name "Tectyl" coats the completed transmission. That information is commonly known. What is less commonly known is that the magnesium parts most likely had some type of "chromate" conversion performed on them during the manufacturing process. It is likely that this was done to provide some level of basic corrosion protection between the time the parts were cast and machined and when the transmission was assembled.

Q: What type of chromate process was used if any?
A: Most likely one of the trade name "Dow" processes. With processes 1, 7, 9 or 19 being the likely candidates. However we don't know for sure.

Q: Can you reproduce this finish?
A: Yes you can. Chromate processes are being phased out due to environmental concerns. Newer processes exist, and may in fact be superior, but they may not reproduce the look of the original process. You can still find places that will do these processes. It is not easy to do these at home.

Q: Should you reproduce this finish?
A: Porsche doesn't expect you to do so if repairing a transmission. The only expect you to reapply the Tectyl coating. So unless you are building a concorse car, it is probably not worth the hassle.

Q: Should you use something in addition to Tectyl?
A: Not sure. Some people do. Some people use a lubricant known as "Gibbs Brand" as a coating on bare Magnesium.

As it turns out, in my parts stash, I had an extra 914 transmission differential side cover.  It had some corrosion on it and I felt it might be a "test subject" to validate what I had learned above.  The photos below are those that are in the Pelican Parts forum thread above...
 

In this close up shot of the outside you can see a bit of the iridescent color deep in the webbing on the outside.  So this indicates to me that the process was done to the entire cover (inside and out)

In this close up shot of the inside, you can see more of it on the inside.  Because of the oil bath, the inside was better protected than the outside so more of the chromate conversion remains

Inside

Outside

5/26/2008

Ok, I have started my experiment on how to protect magnesium.  I am going to try a few different methods of cleaning and coating magnesium.  I wanted to use commonly available materials.  Two materials that I wanted to try, I can't easily find.  The first is "Tectyl" which is a waxy sealant that is used by Porsche .  It seems to be impossible to find in small quantities.  The second is "Stoddard Solvent" which as far as I can tell is a special formulation of Mineral Spirits.  I am sure I could find this if I searched, but am not able to find any locally.  What I have found is "CRC Heavy Duty Corrosion Inhibitor" which looks to be a Tectyl clone.  It comes in a spray can and probably can be found at any FLAPS that caries CRC products.  I ended up mail ordering mine. This is my substitute for Tectyl.  I also am using regular Mineral Spirits as my substitute for Stoddard Solvent.

My cleaners are...

(A) Soap and Water (dish soap)

(B) Simple Green (straight)

(C) Mineral Spirits

(D) Glass Bead Blasting

My coatings are...

(W) Nothing (unprotected)

(X) Gibbs Brand Penetrating Oil/Lubricant

(Y) CRC Heavy Duty Corrosion Inhibitor

(Z) Gibbs + CRC

My test subject is the same magnesium differential cover plate above.  If you look at the outside you see that the webbing creates ten pie shaped segments starting at the top right, I have labeled them in a clockwise fashion from one to ten.  The part number "914.301.311.02" at the bottom left is in the sixth segment  While the inside doesn't have the same webbing, I used the outside webbing location to create segments inside.

Here is the outside before I do anything.  I will say that I actually already washed the entire part with soap and water at this point.  I had done that months ago when trying to see what if any of the chromate conversion still existed on the outside and inside.

Here is the inside before.

Here is the outside after cleaning.  I used Simple Green (straight) and a nylon scrub brush for 10 minutes on segments 1 - 5 (right hand side).  I used Mineral Spirits and a nylon scrub brush for 10 minutes on segments 6 - 10 (left hand side).  Frankly when I was done, it was a bit cleaner than it was from the soap and water, but not much.  I can't tell any difference between the two.  Maybe the Simple Green is slightly cleaner.  I suspect that if the part had been greasy dirty, I would have found out which really cleaned better.  But the bottom line is that the part was pretty clean to start. 

Here is the inside after cleaning.  I used Glass Bead blasting on segments 1 - 5 (right hand side).  I used Soap and Water on segments 6 - 10 (left hand side).  Actually I didn't do anything to the left hand side as I had cleaned it earlier.  The bead blasting worked nicely.

Here is the outside after coating.  Here is the key for the segments...

1. B, W
2. B, W
3. B, Y
4. B, X
5. B, Z
6. C, Z
7. C, X
8. C, Y
9. C, W
10. C, W

Here is the inside after coating.  Here is the key for the segments...

1. D, W
2. D, W
3. D, Y
4. D, X
5. D, Z
6. A, Z
7. A, X
8. A, Y
9. A, W
10. A, W

This is the first time have use the Gibbs or CRC product.  Gibbs, goes on easily, but doesn't seem to want to evaporate, so it stays "wet" for quite awhile.  I think that days later it still has a wet feel.  The CRC product goes on nicely.  It is very soft until whatever solvents evaporate.  It may take a good day or two to really firm up.  Even then the it really is just a waxy covering.  You can scrape it off.  CRC did seem to spray OK over Gibbs.

I haven't done this yet, but I plan to either mount this outside to allow it to weather, or maybe even bolt it under my daily driver to allow it to get weather as well as road grit, etc.  The goal is to see what holds up the best after a few months of abuse.

1/19/2008

Ok, time to look in on how the differential cover is doing.  It actually sat in my garage for about two months after the cleaning and protection treatment in May of last year.  So it it was actually outside hanging under my back porch for about six months.

What I am seeing so far is...

1. CRC Heavy Duty Corrosion Inhibitor is working really well.
2. Gibbs Brand Penetrating Oil/Lubricant does provide some protection.  But not as good as the CRC product.
3. Gibbs + CRC together seem to work fine, but I can't tell any difference than just using the CRC product by itself.
4. Areas that were cleaned via media blasting corroded the most.
5. The chromate conversion (even if mostly gone) is still providing some protection.
6. Areas that might get abraded via handling, weather, etc. have the most corrosion.  Specially the Gibbs protected area show corrosion in raised areas that are most likely are due to the Gibbs being either soaked up or abraded/weathered away.  You can see this clearly on the inside on the right hand side (that was media blasted) at approximately 4-5 o'clock position.  On the raised ring, you can see magnesium corrosion in the Gibbs area.

Outside.  Top section that was un-protected has corrosion.  Both the Gibbs and CRC areas look pretty good with the CRC product looking better.

Inside.  Like the outside the top section has corrosion.  However the right hand side has more corrosion.  That side was media blasted.  If you look at the left side you can see that some of the original chromate conversion is still protecting the magnesium.

My conclusion so far are...

1. Media blasting is the best at cleaning, but it removes the chromate conversion that may still provide some corrosion protection.
2. Gibbs Brand Penetrating Oil/Lubricant works, but it's not a miracle product.
3. CRC Heavy Duty Corrosion Inhibitor seems to work the best.  This is not surprising since Porsche also used a waxy protectant (Tectyl) on the magnesium transmissions parts.

As I am not ready to start working on the transmissions, I expect the diff cover is going to go back out under my back porch until I am ready to start on the transmissions.

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