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Old Thu, Oct-11-2018, 07:42:27 PM   #11
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Default Re: The Resurrection M3 Touring Build

Alright so I decided to hold off posting until I get one complete task at a time done. I’m running everything in parallel, so it’s a lot to keep track of and make cohesive. I’ve also learned the XI chassis is different and the M3 transmission mount will not fit, front subframe bolts up just fine though – more on that later.

This write up is fairly long, but I’ve done my best to provide enough information and justification on what I did so that you guys can critique and add to it in your own builds’. Here’s my journey on the subframe reinforcement and restoration. I’ve done things differently than most, but I think it should be solid for many years to come.

First off, materials used:

1 x PSDesigns/Redish RACP Plate Kit (thanks Pete!)
1 x Epoxy Dispenser Gun 10:1
1 x Pack (10) mixing tubes
4 x Henkel H4710 Epoxy (5 recommended)
1 x Spot Weld Cutter
1 x Zep Industrial Cleaner Gallon
1 x Spray Bottle
2 x Cans of Brake Clean
2 x Stripping Discs
1 x Wire Wheel
1 x Pack of Drill Mounted Brushes
1 x Roll of Paper Towels
1 x Weld Thru Primer
2 x Wurth Sprayable Seam Sealer
1 x Wurth Seam Sealer Spray Gun
2 x Wurth Gravel Guard
1 x Wurth W-Sol Cleaner
1 x Spray Paint (final top coat)

After reading up a ton about RACP failure over the years, I decided that I really didn’t like welding, or some of the kits that have a smaller footprint, albeit thicker plates. The top side bar solutions to me are still largely non-validated and I have seen a reinforced car with the bar develop cracks in new locations. In my personal opinion, the top side bars are great if they’re being tied into a roll cage – otherwise the load is just being pushed off to other areas, often going through large panels of single layer sheet metal.

So on to welding vs epoxy. Welding is great, but at the end of the day, we’re welding to 1-3 layers of 20 gauge sheet metal. Heat and stress concentrations are major factors as well, largely operator dependent. In terms of loading, we’re now putting all the load through the throat of that weld bead at the perimeter. To make matters worse, thicker plates are far more rigid than the floor and rather than flexing with the floor, they may end up creating larger perimeter loads on the welds. For this reason, as far as I’m concerned, the PSDesigns plates are the best solution, utilizing the largest footprint and relatively thin 14 gauge sheet metal that can flex with the RACP.

Now to be perfectly fair, I spoke to Pete about this matter, and they only recommend welding their plates and have not validated any epoxy. Credit where credit is due, Pete knows his stuff, and they have yet to report a single failure after repair, so I tip my hat to Pete at PSDesigns for the great work and product – don’t cheap out, these are in my opinion the best plates on the market period and well worth the coin. As an Engineer, what you’re paying for isn’t just steel, it’s the time and development put into these plates and after looking at and dissecting the RACP, these plates are VERY well thought out from a loading point of view. I will say, they are also the most work to install, but again, this isn’t somewhere you want to cut corners. My choice to epoxy, is purely my own preference, and I do think that it is perfectly acceptable to weld in these plates.

Numbers don’t lie, so I wanted to validate, in theory, that epoxy should be in the same realm as welding. Doing some math, I roughly approximated the total perimeter of weld per plate, and compared it to the bonded strength of the whole surface area of the plate of a few epoxies. I made the assumption that in shear, the weld strength was based on the root of the full reinforcement plate thickness and that in tension it was simply the cross section of the weld on either surface, I did not apply any stress concentration factors so this is likely a generous estimate. The numbers show that epoxy is right in the ballpark in terms of overall bond strength, but it’s distributing the load over the whole area of the plate rather than just the weld line. Keep in mind these are crude numbers at best and I did not go through great length to validate the loading vs calcs.

In terms of epoxies, I looked at a few different brands and lines, and these 3 were on the short list. I have personally worked with all 3, and done a fair bit of validation and testing, including ultimate shear, environmental, thermal shock etc. on another product. I did contact 3M and spoke to my rep at Henkel/Loctite, and these were their recommendations. These 3 products are seriously outstanding, their capabilities in the correct application are truly impressive.
  1. 1. I eliminated 3M DP420 from the list on the basis that it lacked specifications. I asked for additional information and none was provided – maybe I didn’t have the right contact. In testing I have also found the substrate itself to not be as strong as the others – where it did well was bonding materials that had significantly different rates of thermal expansion – not the case here. Of interest, it’s likely that all the adhesives used on the car are from Henkel anyways that swayed my decision a little as well.
  1. 2. H8000 is an absolute work horse and arguably one of the strongest and most versatile toughened structural epoxies in the Henkel line up. If I’m not mistaken, it looks as though this epoxy is actually used on the front frame rail sheet metal where it bonds at the cab. What I didn’t like is that this epoxy has no bond line thickness control, and is susceptible to running. In a manufacturing controlled process, this is not a problem. But on the car, I have no way of controlling the bond line and we are bonding nearly vertical in 4 places.
  1. 3. H4710 is a methyl acrylate – the same family as H8000. I’m lead to believe it’s nearly identical to H8000, except that it has 5mil beads in the substrate (think like toothpaste), better peel, better anti-sag, and gap fill properties. This was my epoxy of choice for its strength and bond line control.

Back to the car, I stripped the rear subframe bare, and any suspect spot weld was drilled out and re-welded along with the marked rosette locations of the PSDesigns plates. I am realizing now that a lot of people have been stripping the bottom with grinding discs and flap disc, if this is the case STOP NOW! That sheet metal is so thin, there is no way you aren’t removing a significant portion of the material thickness. Instead, grab yourself a couple of these stripping discs and a wire wheel.

I’m glad I went through the spot welds, the front right corner, the spot weld had failed and the inner layer of sheet metal had moved – I drilled a hole through it and used a self-tapping screw to pull the two layers together, then of course tacked and rosette welded the joint.

Next up, I ground the spot welds flush. In this case I used a small 3” 120 grit flap disc on a die grinder and very carefully removed material until it was flush.

Forming the plates was the biggest pain of all, bar none. On a welded application, I don’t think this is as much of a problem since you can tack, form move on. Without welding though, the front plates teeter and don’t want to form. Furthermore, the steel needs to be over bent and allowed to spring back which you can’t really do on the car. This meant going back and forth between the car, a vice, an anvil and an assortment of hammers to get it right – likely not exaggerating when I say 50 trips for each front plate. The rears were pretty close right off the hop. The goal here was to have no gap larger than 1/32” as the bond strength of the epoxy begins dropping off. You will need a TON of patience here. Once they were well formed, I welded up the seams on the front mounts and ground them flush.

Surface prep is key for epoxy. The best you could possibly do is have sandblasted etched surfaces, so I used a medium glass bead etch on the plates. Of course on the car, this isn’t really a possibility. I cleaned the surface with brake clean, abraded it as best as I could with 60-40 grit sand paper. Then a final clean all the way around with brake clean and dry air.

I then applied the epoxy in a zig zag pattern to both the plates and RACP and distributed it across the whole surface. I couldn’t take any pics here as I was working fast, this stuff sets up in 10 minutes and it needs to stay cold (stored in fridge). Also it smells quite strong, a respirator is recommended if you have one and definitely wear gloves. I started with the front plates which used about of an epoxy cartridge. The rears required about 1 , so it was just enough. I would have bought just one more cartridge to be safe.

Plates were then quickly placed on the car and subframe mounting bolt tightened to clamp the plate vertically. I had pre-cut a couple pieces of 2x4 at an angle and used a pair of jacks to press the vertical walls in place – a bit janky, but it worked very well. I used a rag to clean up the excess and smoothen the edges.

I gave each corner at least 2 hours to set before removing the wood blocks. Once all 4 corners were done, I let it all cure for 24 hours then started cleaning up using the stripping discs and wire wheel.

Next up was the messy job of cleaning the whole underside. I used Zep industrial cleaner, combined with the drill mount cleaning brushes, paper towels, and a bottle of water to rinse. Some patience and it actually cleans up quite well. Gave it a final wipe with brake clean and Wurth W-sol, then masked the area and laid 2 coats of primer.

Then sprayed on the seam sealer generously, it’s very important to seal the seam around the reinforcement plates as best as possible. Any water and corrosion getting in behind the plates will eventually fail – this is one of the drawbacks of epoxy. I went ahead and covered the whole plates, but kept it thin on the bottom where the subframe bushings will mount.

The final step was a coat of Wurth Black Gravel Guard, then Wurth Beige Gravel Guard. I wasn’t happy with the tone of the Wurth Beige, so I picked up some industrial spray paint to top coat to a more OEM finish. The reason I went with black as a base coat and a lighter coat over top is that should the floor ever shift or crack, I will hopefully be able to see a clear black line. That’s the theory anyways, which I hope I’ll ever test. Disregard some of the unpainted bits, the rear trunk floor is coming out and I’ll blend the two together when the time comes. Here’s the finished product!

At this point I’ve just slipped in the new brake lines for the MK60 swap and dropped the engine/trans in. Will hopefully have an update soon, but progress may be a bit hampered from slicing my finger open… oops. Feels great to be hitting some proper milestones!
2002 M3 CB Coupe 6MT|||

M3_Ed's Journal RIP

The Resurrection M3 Touring Build

Last edited by M3_Ed; Wed, Oct-17-2018 at 05:08:20 PM.
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Old Thu, Oct-11-2018, 09:24:04 PM   #12
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Default Re: The Resurrection M3 Touring Build

Nice epoxy job sir!
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Old Fri, Oct-19-2018, 08:27:37 AM   #13
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Default Re: The Resurrection M3 Touring Build

Fantastic work and a great alternative application method for our plates
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For PSDesigns/Redish Motorsport BMW E46 Chassis Repair/Reinforcement Kits, drop me a pm or email.
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Discussing The Resurrection M3 Touring Build in the Member Journals Forum - Do you have a long term project you would like to share with the community? Use this forum to create a single thread which you can update over time to document the progress. at BMW M3 (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)