Fixing broken/missing screw bosses on plastic parts

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pauly

pauly

MOPAR or no car!
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I'm trying to fix up a pair of 1970-1971 Duster tail light lenses and finding that a common issue is broken off screw bosses. I could put longer screws in to hold them on some places, but I'm wondering if anybody has had any luck with rebuilding the missing bosses? I've done some internet surfing and have found a number of products to repair cracked plastic but not to rebuild the bosses when missing. Hoping someone can point me in the right direction. Thanks!
 

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You could build it back up with some epoxy for plastic, and then re-drill the hole. Probably wouldn't be red, but it would work.
 
I wondered about epoxy. I was gonna put a nail or end a drill bit (waxed up) in the hole and see if I could build up around it. I read somewhere where a guy did a couple with a fiberglass kit. He cut the glass mat into littering parts, then soaked them in resin and stuffed in the boss area
 
is you do decide to build it up with epoxy, I would suggest drilling three or four little holes in the housing, just big enough to put some wire in, maybe from a paperclip

almost like the way you install rebar in concrete
 
epoxy is not the best.Cut small bits of abs plastic from a broken grill or whatever and mix with acetone until it's like toothpaste.The acetone will liquify the abs bits and may take a bit of time if the pieces are big.
....use to build up the area or repair broken grills etc...very strong!
 
Couldent you make studs out of stainless steel machine screws. Maybe 10/32 thread, and glue them into the lens with JB weld making sure they go down inside the holes for the old screws. This would eliminate the problem of self tapping screws into brittle old plastic. Then you just use nuts and lock washers on the lightbulb reflector housing side. Granted its not OEM looking, but will eliminate the problem.

I have a nice used set of factory 67 cuda taillenses with the same problem. I am gonna stud n nut em w stainless studs and JB weld.
 
Looks like about 1/3 of the hole remains. I wouldn't try to rebuild the missing portion. Any pressure from a screw would just crack it again. Instead I would remove the rest of it to a L shaped mating surface with a well roughed surface for bond tooth. Then build the boss with Plasticast which is basically the same as making your abs paste like described above. Drill proper size hole for screw in new boss.
 
Thanks for the input guys. Moparmatt2000: great idea! Wasn't thinking that way but would make it a helluva stronger and easier to work with!!
 
ir3333 said:
epoxy is not the best.Cut small bits of abs plastic from a broken grill or whatever and mix with acetone until it's like toothpaste.The acetone will liquify the abs bits and may take a bit of time if the pieces are big.
....use to build up the area or repair broken grills etc...very strong!
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I have to agree here, I did the same thing for my 71 Barracuda grill. I bought a piece of gray abs plastic off of Ebay for a few bucks because I didn't have any grill pieces and people want a small fortune for 71 grill pieces.

This is a chemical welding process and works great. My grill turned out great using this.
 
pauly said:
Thanks for the input guys. Moparmatt2000: great idea! Wasn't thinking that way but would make it a helluva stronger and easier to work with!!
Click to expand...

What i am going to do with my cuda lenses is prep the lenses i have for the studs, glue the studs in with the JB weld, and put some wheel bearing grease or vaseline on the mating surface of the reflector/bulb holder part where the studs go through. Then fit the pieces together maybe tape em together so they dont shift while the adhesive is drying, and adjust the studs while the JB weld is wet to center them in the holes of the reflector. When dry the grease acts as a mould release to get it back apart.

Dont forget to cut the studs to the length your going to need before gluing , since it will be pretty hard to do it afterward, and a cutoff wheel will get the metal studs too hot and melt the plastic.
 
I've got some of the screws so I should able to figure out the length easy enough. I think I can glue them in and the run through the housing to keep them straight.

Grease should a great mold release- or car wax.

I'm a plastics engineer by trade- I can get lots of ABS runners from the parts we mold! I think the lenses are actually acrylic. If they were polycarbonate, they would have turned yellow from the sun over time
 
pauly said:
I've got some of the screws so I should able to figure out the length easy enough. I think I can glue them in and the run through the housing to keep them straight.

Grease should a great mold release- or car wax.

I'm a plastics engineer by trade- I can get lots of ABS runners from the parts we mold! I think the lenses are actually acrylic. If they were polycarbonate, they would have turned yellow from the sun over time
Click to expand...

Cool deal, as a plastics engineer you know what your doing.

I use an aerospace version of JB Weld. Its Araldite AVHV 1258 by Huntsman chemicals. It comes in a kit with 2 tubes the size of toothpaste tubes.

I have reinforced grilles from behind by using plastic pieces like splints, and gluing them in using solvent adhesive such as plastiweld. That works great too. If the cracks are not all the way across its also a good idea to stop drill the end of the crack with an 1/8" drill bit before repairing it. This helps keep it from continuing further.
 
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