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I had to abort the oil change project this past weekend. I ran an errand to warm up the engine, drove the car up on ramps, removed the brace above the engine, removed the engine cover, and removed the air intake. The next step was to remove the center panel below the engine. This metal panel is held in place with five M10x1.5 flanged 17mm hex head bolts and four M8x1.25 flanged 13mm hex head bolts. The M10 bolts are threaded into inserts pressed into the chassis. The M8 bolts are threaded into spring clip "nuts". The threads on one of the M8 bolts at the rear edge of the panel were very rusted and the spring clip broke while trying to remove the bolt.
I reinstalled all of the parts I removed in the engine compartment and deferred the oil change until I could figure out how to remove the bolt which was spinning freely. After searching through the available tools and other things in my garage I came up with an approach to drill a 6.75mm pilot hole in the center of the bolt head. I made a drilling fixture from a 13mm long reach socket with 6.75mm drilling bushings inserted into both ends of the socket. The foam spacers kept the socket lifted against the bolt head, but compressed a bit as I drilled the pilot hole. Once I made a 3mm deep pilot hole in the center of the bolt head, I switched to an 8.5mm drill bit and held the bolt head with a 13mm box end wrench. Lesson learned is to remove the engine panel from the bottom of the car first.
I ordered M8x1.25 x 20mm long stainless steel bolts with flanged hex heads to replace the stock 30mm long zinc plated bolts. I also ran a tap through the spring nut threads on the other M8x1.25 clips which required excessive force to insert the bolts. I will start another thread later this week with details for doing the DIY oil change.
Broken spring nut clip with rusted bolt and head removed.
13mm long reach socket, 6.75mm drill bushings, Silicone sleeve on socket, foam spacers.
Assembled fixture for centering pilot hole. Adjustable wrench for grabbing on to Silicone sleeve to keep the socket from spinning.
8.5mm drill. 13mm box end wrench holding bolt head.
Wow, what an ordeal. Surprised you had a rusted bolt on such a new car. My E450 has a thick plastic cover under the engine with 10 bolts all the same size that screw into similar spring clip nuts that you have. All of them came out easily and not any signs of rust or corrosion. It's surprising for MB to use bolts or any hardware susceptible to rust under the car where it will get very wet in rainy conditions and not dry off quickly. I wonder if the cover is not installed properly from the factory and water is pooling up where that bolt is.
Wow, what an ordeal. Surprised you had a rusted bolt on such a new car. My E450 has a thick plastic cover under the engine with 10 bolts all the same size that screw into similar spring clip nuts that you have. All of them came out easily and not any signs of rust or corrosion. It's surprising for MB to use bolts or any hardware susceptible to rust under the car where it will get very wet in rainy conditions and not dry off quickly. I wonder if the cover is not installed properly from the factory and water is pooling up where that bolt is.
The E53 Hybrid has a relatively heavy painted steel panel under the engine. The rusted bolt was at the rear edge of the panel. I am certain it gets wet when driving in the rain, but it's unlikely that any water is pooling in the area which in a raised section of the panel. There are several drain holes in the panel. There are two tabs on the front edge of the panel that rest on the forward-most plastic panel. The one M10 bolt near the center of the rear edge and the two tabs at the front edge will hold the panel in place while it is being removed or installed.
The plastic panel under the transmission is secured with sheet metal screws with an 8mm hex head with large captive washers. These screws are also threaded into a spring clip. They are probably the same screws used on your E450.
I am planning to use 10mm shorter M8x1.25 x 20mm stainless steel bolts with anti seize compound on the threads when I re-install the panel after the oil change. There will only be one or two threads protruding above the spring clip with the 20mm long bolts. I could not find any M10x1.5 bolts with a flanged 17mm hex head like the five M10x1.5 bolts used to hold up the panel. So, I will continue to use the factory M10 zinc plated bolts with anti seize.
Metal panel under engine with tabs on leading edge to support the panel while being removed or installed.
Eventually I will probably replace the broken spring clip U-nut with a genuine MB part. I was able to find an MB part number for the sheet metal screw U-nuts, but not for the one for M8x1.25 bolts. I found some USA made steel U-nuts that were limited to a 4mm thick panel. I ended up using a stainless steel U-nut I found on Amazon since it worked with up to a 5mm thick panel. I replaced the 30mm long zinc plated M8x1.25 bolts with 20mm long stainless steel bolts.
Black oxide and stainless steel U-nuts for M8x1.25 bolts.
Finished up the repair job removing and replacing the frozen M8x1.25 bolt today. I replaced the 30mm long zinc plated bolts with 20mm long stainless steel bolts which do not extend very much beyond the U-nut threads. I also applied a thin film of anti-seize compound to the ends of all nine bolts securing the panel below the engine.
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