plug weld penetration?

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Hi folks, longtime listener first time caller here. I am restoring my 1966 ford mustang and have a ?. I welded the front aprons in and they are 20 gauge. They are welded to the shock towers which are 14 gauge. I plug welded them in using the settings for the 20 gauge metal. I can look at the backside of the shock towers and see a blue circle roughly 3/4" in diameter. I can also feel a very slight bump as well. Is that enough penetration? I have pics of the backside but I am at work right now so I will have to post them later. I am using a miller 140 with autoset, 23 wire and 75/25 gas. I searched and didnt find my answer. Thanks!!!!
Reply:This isn't a smarta## answer.  If you can, find the burnthrough point then dial back.  Then you know absolutely for sure.Discoloration, and the distortion you mention is generally a good sign.  Even slight signs of the base metal melting on the backside(wo burnthrough) is also a good sign.There's guys here that know a lot more about this."Any day above ground is a good day"http://www.farmersamm.com/
Reply:The more I thought about my scenario the more I wanted to do my own testing. So I took a piece of 14 gauge metal and plug welded 20 gauge metal to it. I then put it in the vice and performed a destruction test. I am glad to say that the metal broke away before the weld so that puts my mind at ease. If anybody is interested I can try and post some pics of my "test". Thanks for the reply
Reply:That's probably the best thing that anybody could do.  Then you have the feel for what's right because you know the results.I'm not the best at this, and It's always been kinda learning by doing."Any day above ground is a good day"http://www.farmersamm.com/
Reply:A slight bump on sheet metal is ideal.  It can always be ground down.Esab Migmaster 250Lincoln SA 200Lincoln Ranger 8Smith Oxy Fuel setupEverlast PowerPlasma 80Everlast Power iMIG 160Everlast Power iMIG 205 Everlast Power iMIG 140EEverlast PowerARC 300Everlast PowerARC 140STEverlast PowerTIG 255EXT
Reply:I too have learned by doing and screwing up so there are times when I am not confident in my welds. Thankfully all the welding I have done so far has been sheet metal and not real critical. Thank god I didnt sign up to build bridges
Reply:Twice you promised pics. We always want pics.  Please resize to 800 x 600.9-11-2001......We Will Never ForgetRetired desk jockey. Hobby weldor with a little training. Craftsman O/A---Flat, Vert, Ovhd, Horz. Miller Syncrowave 250
Reply:napaguy,Here are  a few shots of a firewall modification my son and I did to his '83 S-10 Blazer to allow installing a 4.3 in place of the original 2.8.  The later '80's and newer models with the 4.3 had the steering column moved outboard slightly to allow the shift linkage to miss the back of the cylinder head and the steering shaft slip coupler to miss the exhaust manifold.  A wrecked '91 Somoma supplied the needed firewall section and steering column and after cutting out the original firewall the doner 'organ' was grafted in using the plug spot weld technique of your project.  Plug spots were approximately 1" apart and the results were as you described as seen in the photos yielding at least as strong as original bond.Don't mean to hi-jack your thread but I just wanted to demonstrate I believe you're 'spot on' with your project results. Attached ImagesMM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!