I ask because a friend just had to have his Jaguar XJ engine overhauled-it seems that the camshaft chains are held by plastic guides that deteriorated, causing the engine to lose valve timing. The replacement parts were all metal. I understand the need to save weight, but plastic parts inside the engine? Plastic gets brittle with time, and a cracked chain guide can actually cause destruction of the engine. BMW also uses a lot of plastic parts-the coolant hoses are held by plastic clips-so I imagine the same problems emerge after 5-8 years of running. Again, invariably, mechanics recommend replacement with metal parts. My question: will the use of such parts cause a lot of problems for these engines at high mileages? Or should you expect that these cars will require expensive service after about 150,000 miles?
Some GM engines in the 1970s had plastic coated gears on the timing chain. It made the engines quieter and was suppose to help the chain last longer. Yea right. I was in a buddy’s Pontiac when the timing chain jumped then jumped again causing the engine to overheat then stall. Again this was a timing chain and not a timing belt.
Recently I had a lawn mower with plastic gears inside the engine. I replaced the gear the first time it broke and dumps the lawn mower when the gear broke again within a year.
Plastic has its places and when done properly should work fine. However there are exceptions.
It depends upon the plastic. Some are extremely tough and r4esistant.
When I worked with Acetal plastics (Polyoxymethylene, brand names Delrin and Celcon), the companies advertised them as “metal-like”. They’re extremely tough (as long as you don’t use very thin pieces), resistant to most solvents, and don’t swell under water. Delrin is the stuff they use for that white rotor in a can opener. You can also incorporate fibers into it, which makes it even more resistant to coming apart. It’s routinely used for plastic gears, chains. and screws.
It deteriorates under mineral acids, alkali, and chlorine (and, I might add, the application of alkali metal suspension), but otherwise its pretty tough. The Wikipedia article claims that acetal pipes and fittings have failed in hot water, though, which I find surprising (as did the people who made the pipes, I’ll bet), but as long as the temperature stays under the melting point (about 175 C or 350 F) it should be Okay.
I doubt if anyone is making engine parts out of Delrin, but there are plenty of other high temperature polymers that I could see being used without risk.
Speaking directly to the subject line:
“Are Plastic Parts Inside Engines a Good Idea?”
Yes, provided they meet design requirements, one of which is service life. Some plastics, in some areas of the engine, are able to do this. But sometimes, a mistake is made and a part that appeared acceptable under limited durability testing turns out to present an unacceptably high failure rate IRL. Given that your friend’s replacement parts were made of metal, it seems his parts were an example of the latter. This does not mean that for all places and all times, plastic parts inside an engine are a bad idea. I will note that metal parts sometimes fail in service and are replaced with redesigned parts. Many BMW motorcycles, for example, are currently under recall to replace the spindly aluminum spider that secures the rear wheel with a much beefier steel one; the aluminum spider on some bikes has developed cracks (and in a few cases, even outright failures) at the bolt holes where the wheel and brake rotor are bolted on.
I have had bikes with plastic timing chain guides and have not had or heard of durability problems, and I would bet that there are plenty of cars out there that are likewise fitted with plastic guides that work well and last the life of the engine.
There’s a bit of confirmation bias when you ask a question like this.
I had a bad experience with a plastic engine part in a well-used 1986 Oldsmobile Calais with a 3.0 liter v-6. The plastic timing chain gear broke and the engine failed. I believe that it was an interference design and I recall that my mechanic had to replace some valves. I don’t know what kind of polymer the gear was but he complained that is was a cheap Delrin gear. My father was a GM assembly plant employee at the time and he thought the gear was Delrin too but I don’t know if he really knew. My mechanic replaced the gear with a metal one.
So, for me, that plastic engine part seemed like a terrible idea because it failed and it cost me money. But, in all the years since, I have never had another plastic engine part fail. I don’t know how many plastic engine parts in my cars have served perfectly, while offering lighter components, lower cost, smoother operation, a quieter cabin, and better performance than competing metal parts. I only noticed the plastic part that failed. I have had metal parts in my car fail but I didn’t complain that engineers should have made those components out of better quality plastic instead.
There are bushings made from metal shells with Acetal Copolymer linings called “DX” bushings. They routinely out perform “Oilite” sintered bronze bushings in things like earth mover pivot bushings. There is a plastic called Torlon 4435 that I made parts for a defense contractor for. This stuff is hella tough and will withstand 500 degrees Farenheit. It has a tensile strength higher than many steels. It costs $100 for a 1" diameter bar 1’ long. Not cheap, but very strong. It wears out carbide tools.
All plastics are not equal, but some are costly enough that the choice to use them is not an easy one. Some of the considerations revolve around expected life of the mechanism, replacement costs should failure occur, and consequences of failure.
BMW uses “plastic” water pump impellers which were prone to breaking (it happened to me). Apparentley at some point they switched to some kind of much stronger composite called PPS (PolyPhenelineSulfide, an advanced semi-crystalline polymer reinforced composite).
PPS impellers are used in Ferrari, Lamborghini, BMW, Alfa Romeo, Fiat, and Maserati cars.
Super weird… this showed up on the message list; I certainly didn’t go looking for a six year old thread, and didn’t notice it was so old.
Yeah my 1998 Ford Contour (Mondeo in the EU) had a plastic water pump impeller which led to the death of the 2.5L Duratec engine when my sister was driving it and didn’t notice it was overheating.