Repairing an ancient plastic item - what's my best bet?

When I have had to fix tiny plastic bits that undergo physical stress (like, say, the whammy bar yoke hidden inside a Guitar Hero guitar, or the spray lever cam inside a kitchen tap) I have used http://www.jbweld.com. It's ugly, but it is very strong after 24 hrs of setting, and I find it is perfect for fixing plastic bits that are often cheaply made and easily broken. One caveat is that you may have to cut away any runoff afterwards, if the piece has to be inserted into something else and you need clearance where the epoxy would otherwise dry and get in the way. This is easy to do with a Dremel-style tool with a cutting or sanding bit.

Not sure how much tolerance you have for this kind of thing, but visit http://www.sculpt.com/ and check out their mold making supplies. With a little practice, you can make molds and castings fairly quickly, and replicate things in a number of materials. Tints and fillers can be used and TCS has them, too. Not too cheap, but cheaper than a machinist. (Alternatively, you can hire a sculptor to duplicate it for you. We make molds a lot, though Jag parts are not as much fun as nekkid peeps. Tech is the same.) O, and you can 3d print it. Check Shapeways.com Commercial printer of your designs. Cost reasonable, again, compared to a machinist. (i am currently involved in a small project to make 3d copies of antique telescope parts in bronze, using 3d printing tech. )

Without a picture or knowing what the shape of the object is, we can't advise you best how to proceed. For instance you may need to create an armature for the shape to help hold the parts while the cement dries. Yes plastic cements work differently than adhesives and they come in different flavors...take a few bits in to a local hardware store (not a no-nothing big box home supply) and ask for the correct type. Can you post a pic of a similar auto part?

Would http://sugru.com/ work?

superglue is an adhesive, not a void filler. it seems your specs require both adhesion and reinforcement. fiberglass and epoxy would be my uninformed choice, based on your limited description. it's strong, conforming, thin, temperature tolerant, relatively easy to use. pix necessary for further recommendations.

The part is definitely not something which is going to come squirting out of a 3-d printer, I don't think. This http://www.ebay.com/itm/1292-Jaguar-3-8-Etype-Fiberglass-Gearbox-Cover-/220977038917 shows one of the repros, which I sheepishly admit looks better than the crude one I have on hand (so there's another out). The reproduction covers don't come pre-drilled with holes, there are about a dozen around the perimeter. It's about 10" wide by 18" long by 6" high. I have considered JB cold weld--and may give it a shot, there's some on the bench. Like you say, it's a bit ugly to work with, but the flip-side is that it would be gap-filling, so repair bits would only need to be "close". The fact that I'm asking this rather than trying it says how much I "enjoy" working with it. sugru: Looks interesting, not sure if it would be suitable. But interesting! And, sadly, I already know more about this than my local hardware store. I'd like to fix this piece, of course, but I'm also interested in the repair as a more general learning experience, as broken plastic items are one of the hardest parts of restoring an old car or bike for me. Metal you can weld or machine or whatever, but plastic is always a puzzle. (And most of the time if reproductions are available, they aren't very good, or just don't fit.) Having a few techniques in my bag would make facing plastic challenges a lot more fun.

To be clear, the cracks don't really need filling, as they press tightly closed...hence my idea of superglue or the proper equivalent. I feel fairly confident I can cut repair pieces that won't need much filling for the missing areas. Fiberglass I have also considered, but I prefer working with epoxy, given the choice of the two.

Just a word of warning: some plastics become brittle with age (and/or exposure to heat and light) - the cracking raises a little warning flag for me about this. If you repair it, the part might just end up disintegrating further when you fix it and put it back in to place. Not very helpful, I'm sorry, but something to think about.

I think we need a picture to give you stronger recommendations. My gut says your part might be celluloid rather than some more modern plastic. My gut also says you should consider Gorilla Glue rather than epoxy or superglue; epoxy is thick and needs space, which you don't have, and superglue is brittle and doesn't like flexible substrates. Testors Cement will happily bond celluloid BUT because of the way it works (melting) it is not great for fixing cracks, more for sticking bits to other bits -- testors repairs are often thin and brittle and just snap again.