After removing the panels–you can read about how I did that here: 46 BMW 1977 R100RS Remove, Disassemble Bodywork–I need to fix all the cracks before I take everything to the painter.
After I made the repairs as shown, my painter found the repairs did not hold up as he sanded the panels. He added fiber glass mat reinforcements across the repaired cracks. Therefore, I recommend you do the same when you repair large cracks.
The worst panel is the headlight panel which is the top middle panel that surrounds the headlight. It’s so badly damaged with many small hairline cracks I decided to pickup a used panel.
I’ve had good success fixing cracks using Plast-aid which is made here in Colorado in Estes Park.
It is a polymer plastic that bonds to many plastics, but not to PET or other “oily” plastics. It creates a chemical bond between itself and the original plastic material that is as strong as the original material. When the two parts (liquid and powder) are mixed, the consistency starts out as a runny liquid. As the chemical reaction proceeds, the material thickens slowly into a paste and then a putty like consistency which I can mold with my fingers. This property of changing consistency allowing shaping and molding before it hardens is very helpful when fixing rounded or curved areas as I have on the edges of the fairing panels.
I’ve written about various repairs I’ve made using Plast-aid in the following write-ups and blogs.
- 46 BMW R100RS Repair Panniers & Plastic
- BMW Fairing Plastic Repair With Plast-aid
- 1973 BMW R75/5 Rebuild: Using Plast-aid To Attach Electric Connector in Windjammer Fairing
By the way, for all these repairs, including the ones I made to these R100RS body and side panels, I’ve used about 1/2 of the material in a single package.
Repair Large Crack and Missing Edge on Right Top Panel.
You can see there is missing plastic and a large separation of the rounded edge on this panel along with a web of cracks radiating from the edge.
I use a Dremel tool with a tapered stone bit to make a v-notch along all the cracks and past the end of the cracks to provide stress relief.
I did not use a fiberglass fabric or other woven patch on the back side of the fairing to reinforce the repair. This was a mistake. The repair failed and had to be redone with fiberglass fabric on the back. Another material I have used successfully for reinforcement is aluminum window screen mesh. As with the fiberglass fabric, I apply a layer of Plast-aid and then embed the fabric in it. I let that harden and then apply a second layer of Plast-aid to fill in the gaps in the mesh and to smooth out the Plast-aid.
I mix up some Plast-aid with enough liquid to make it soupy. I put a little of the liquid into the v-notch to improve adhesion with the panel. I use a toothpick to fill the v-notch with the soupy Plast-aid. When it hardens in about 10 minutes, I mix up another batch and work on the large crack at the edge. I use tape and a clamp to close the gap while the Plast-aid sets up. I make a third batch but let it harden to the point its about the consistency of peanut butter. I apply it to the edge and as it hardens and I shape it to match the rounded profile of the edge. Its very easy to shape Plast-aid as it gets thicker which I can’t do with fiber glass resin or plastic epoxy. It makes it very easy to make a repair to contoured or rounded parts of a panel.
Even though Plast-aid hardens in 10-15 minutes, I let the part sit overnight before I sand the panel to smooth out any high spots in the Plast-aid. Here is the repaired panel. The painter will apply putty over this area to make it perfectly smooth.
Repair Lower Right Panel Broken Boss & Missing Threaded Insert
The lower panels attach to the frame at the bottom with a metal strap that attaches to the front engine mount as shown here. The strap is just above the clutch cable as it passes next to the nut on the engine mount stud.
The other end of the strap connects to the panel with an M6x1.0 bolt. The right lower panel boss that holds the threaded insert is broken and the insert is gone. The panel was attached using a sheet metal screw into the remains of the plastic boss.
Here is how the original boss and metal insert look on the left panel which is not damaged.
I bought a package of M6x1.0 brass threaded inserts from MSC. They require a 5/16″ hole to be drilled into the boss and the insert is tapped into the hole. The serrated end goes into the hole. It has a split so it will compress as it is driven into the hole. When the bolt is screwed into the insert, it expands the serrations into the plastic to keep the insert from spinning.
I have to build up the boss to the same height as the original one. It’s the same height as the adjacent boss. I mix up the Plast-aid and again use a little of the liquid applied to the boss to promote adhesion. I build up the boss when the Plast-aid is easy to form into a ball. I press the ball onto the boss and use a putty knife to flatten the top of the ball so it is the same height as the adjacent boss. I use my fingers to form the edge of ball so it goes over the base of the boss and ends up rounded like the boss
I let the part dry over night before I drill the hole for the brass insert. I use some tape on a small drill bit to set the depth so it is a bit deeper than the insert and then I drill a pilot hole in the center of repaired boss.
I drill the final hole the size specified for the insert using the tape on the drill bit to maintain the proper depth.
I use a plastic hammer to drive the insert into the hole. I test the strength of the repair by driving an M6 bolt into the new brass insert. The Plast-aid does not split and the insert does not turn. 🙂
Results of Other Repairs
Here are pictures of the other cracks and repairs I made with Plast-aid. I use the same process of cutting out the crack into a v-notch with a Dremel tool and then filling the crack with Plast-aid when it’s soupy. After a day, I sand out the high spots of the Plast-aid. There are some small voids you can see in the pictures but these will get filled by the painter with putty when he prepares the panels for painting. I tested the repairs by bending the panels a bit and did not hear any cracking sounds, so I believe these repairs are as strong as the original material.
The side covers had a number of cracks. The left side cover near the hand hold has a rounded cutout and part of it was missing. I formed the Plast-aid with my fingers to match the curvature.
2017-05-25 Added post script about using fiber glass mat to reinforce repairs.