How to judge a welding job

[roGERK]

Hello everyone,

After three London winters, and a couple of Boston ones, my once rust free '73 911 T is in the shop for a welding job.

Its the usual problem area on 911s - the front pan, the cross-member between the torsion bars at the front, and (almost certainly) the fuel tank cross member/support as well.

I've entrusted this work to a Porsche specialist, but was wondering if there is any way to check if the work has been done properly?

I know nothing about welding, I've never done it, don't know anyone who does it, and have never had any kind of welding job done before. So any info at all would be welcome.

Second question, what's the best way of protecting the new metal-work so that I don't have to have this expensive job done again?

Thanks,

- roGER

[s_wilwerding]

First off, hopefully this person has already done this sort of repair - just because they're a Porsche mechanic doesn't mean they can weld body panels. If you still have time (i.e. haven't yet taken it in yet), call the shop and see if they've done this for anyone else. If so, see if you can get ahold of them and ask them what they thought of the service.

As to how to judge the quality of the job, you can take the car to an alignment place. The welding of the front pan, since it has numerous suspension mounting points, is very ticklish. If the welding job is off, the front suspension and wheels won't be lined up with the rear wheels. However, this is not something you want to find out after the fact - spend the time beforehand to find a good welder who has done it before.

In order to protect it from additional rust, the body panels you are using for the replacement should probably already be treated. However, for additional rust protection, there is a product that is very popular on this board called POR-15. They have a website - do a search of this forum and you'll find it. Make sure that the shop that's doing your welding uses appropriate joint sealer and underbody coating - I believe the best products for this are made by Wurth.

The reason I know all of this is because I too am shopping around to have a new suspension pan welded in. I am being very selective as to who will do it since it can easily be screwed up and costs a significant amount to fix.

[jpachard]

Roger, I would recommend West Foreign Auto in Natick. Barry is the owner and he's a great guy and has done lots of Porsche's . No affiliation but he has done some work for me and a few friends and is workmanship is first rate. In regard to welding it is a very big subject( I am certified in Mass. and NH for TIG welding on Stainless). If you have some specific questios I would be more than happy to answer them.

Cheers, James

[RSR.guy]

Roger,

First off, my apologies on your front end cancer. I agree with what Steve is saying on this one. Alignment and proper fit of the parts is very crucial to keeping your beloved car driving straight down the road.

Beyond that James is right, welding is a huge topic. Here are a couple of additional comments and things to look for in no particular order:

1. Cleanliness. Make sure every seam is clean and free of dirt, rust, undercoat, and typical 911 road grime prior to welding. It's pretty easy to contaminate a weld, which compromises its strength. Do not try to judge a weld if you have no experience on the topic. There are an incredible number of factors at play.

2. Types of welding: In this case it's likely to be either MIG (electric wire feed) or TIG (tungsten inert gas) welded. MIG machines get results faster, and generally produce a bigger puddle. Typically, a MIG welded seam gets a bit of massaging with a sanding or grinding wheel prior to paint. TIG welding is slower and far more precise. The heat affected zone is generally much smaller with a TIG machine, and will require less grinding (if any) after the fact. TIG welding (and gas welding) panels is slowly becoming a lost art.

3. Spot Welds: Porsche used spot welders to assemble most of the panels on 911's at the factory. More than likely, your metal guy will have to drill out many of these spot welds to remove your rotten sections. When replacing these panels, the welder typically uses a rosette or plug weld to re-attach them to the chassis. This technique involves punching or drilling a small hole in the new metal section, and welding 'through' the hole to attach it to the chassis. Make sure that there are a sufficient number of holes (and resultant plug welds) in your new panels to maintain strength in the area.

4. Parts Fit: Make sure you test fit your suspension and fuel tank before the job is complete, as insurance that the new panels are in the right place. Many times, the new stampings do not exactly mimick the factory stuff, and will require some tweaking.

4. Weld quality: In general, most welds fail because they are too cold. A good weld requires melting of the substrate material as well as melting of the filler rod. In the case of sheet metal, most welders will have no problem getting enough heat into the area. The converse may occur however. There is a chance to get too much heat into the panels which can cause warping and cosmetic issues. Not to worry in the case of 911 front pans; there are plenty of compound curves and bent edges to keep warpage to a minimum.

5. Finish and paint: Yeah, as Steve mentioned, it is very important to protect the new metal. Good paint/primer is key to a nice looking job that will hold up against the weather. Also, don't forget to seam seal and undercoat as necessary. I have used POR 15 and Wurth's rust preventative paints in the past, however, my experience is that these products are designed for rusty surfaces and do not adhere well to bare metal in good condition. If you are able to successfully remove all the rusty sections, I personally prefer to use a metal etching primer, followed by epoxy primer, followed by undercoat (if needed), and a color coat. A good paint shop will be able to point you in the right direction.

If you want more detail, there are several good publications on the topic, including 'Performance Welding', and Ron Fournier's Sheet Metal Fabrication book. If you can find an experienced metal shop that has done this before, it's definitely gonna' be your best bet. I hope this helps. Good luck!

Cheers, Chris

[Brad_H]

Keep in mind that the gas tank support is welded on top of the pan, hiding some of the work forever. You might want to request to have a look before the gas tank support is installed. Also, of course, you lose the ability to do any painting inside the space between the two parts once the gas tank support is installed.
Brad

[Tspringer]

Welding can be a funny thing. So many people judge a job by exactly how the finished results look, and less by how strong the weld may be. I can understand this if the car is going to be used in concourse competition, but otherwise my advice is to get it done with strength being the primary objective.

I see so many cars repaired with plugs welds or spot welds are are very limited in number and may even be ground down to look less intrusive. This can be very bad. Grinding down a weld weakens it. In the aircraft industry, they are not allowed to grind down the welds! I think you should have the parts spot welded into place, preferably on a factory jig and then have them at least partially seam welded. Large, wide seams welds with good overlap. this is best done in areas that are less visible on the car and has to be done properly to avoid warpage, but it can add a lot of strength to the tub.

Terry

[dean]

One way to prevent it from happening again is not to drive it in the salt.

I found that Por-15 is an excellent primer.

[TimT]

This is the repair I did to my car, I jigged the panels and took an excrutiating amount of measurements to locate the repair panels properly. I then punched holes about 1" o/c and plug welded the repair pieces. its not the prettiest but it is strong.

So many people judge a job by exactly how the finished results look, and less by how strong the weld may be. strong welds look good.. good welds look strong....... whats does a strong weld look like?

The inspection criteria for welds ALWAYS involves visual inspection. and for the most part pretty welds are acceptable. Undercut and overfill are obvious visually, porosity and crater cracking are also obvious.. arc strikes and other tell tales are obvious as well.... on stressed members they lead to local hardening and stress risers......

Mild steel sheet metal is very easy to weld, and kind of hard to screw up.The 911 was put together with spot welds.... in stressed areas they are more closely spaced....make sure your installer is at least replicating the amount of weld the factory used... but more importanly make sure they locate your suspension pick up points properly.

Im betting the repair panels are coated and galvinized. So just protect the welded areas with inorganic zinc. then top coat as needed

Tim

AWS CWI 94030471

[roGERK]

Great replies,

Very useful - thank you everyone.

Should have pointed out that I'm now back in London, England and the work is being done by a shop that does nothing but old 911s - nothing much post 1973 is seen there (like every garage on the planet, there are occasional exceptions that prove the rule).

I was just looking for some geneal guidelines so when I peer underneath I've got some idea what I'm looking for!

Thanks!

- roGER