What size are the larger underbody retainer clips?

9mm works for me on those random big ones

 

You can lookup the Toyota part numbers below which will get you aftermarket equivalents.

 

It's also worthwhile to anti-seize all the (2) bolts, most of mine have broke the head off over the years. I drilled them out and use sheet metal screws for now, but I may re-tap them one day.

 

Totally agree... The sooner you can get away from overpriced and quick to fail underbody panel fasteners the better. Even random screws from your junk drawer spun through random parts of the plastic is more reliable than Toyota's lame design standards that are more about lowering costs on the assembly line than lasting durability.

 

Lowering costs AND weight. A few grams here and there do add up.

 

I have an '09 here that somebody took real painstakingly amount of time to put in some very nice stainless steel fasteners Phillips head on one side I'm imagining a washer and a nut on the back they look like about 2 mm very small heads and they did a very good job they are in very ideal places I have pictures they don't like the post here without a bunch of resizing but the people did a really good job It's unbelievable every piece of plastic under this car is in perfect condition not flapping not wind noise nothing It is unbelievable whereas the rest of the car is in okay condition but it's incredible how well they've kept the plastic under pieces especially the far frontal piece that goes from right under the front bumper cover lip under the radiator usually that piece is destroyed by parking curbs and what have you All of these pieces have been very painstakingly remounted with seemingly proper sorted hardware for the long term.

 

A few thoughts on the engine under panel and its fasteners in the oil change link in my signature*. I’d keep them up, with the stock fasteners, clean and dry.

I’ve had the panel off completely about 25 times for oil changes, and at least another 5 times for other reasons. Replaced 3-4 fasteners due to a missing jaw. Ordered a full replacement set through Amayama quite reasonably priced.

* on a phone turn it landscape to see signature

 

Sounds like a typical dealership: rushed mechanics don't have the time nor patience to deal with the underpanel.

For the engine under panel fasteners here's my all-in price through Amayama, in November of 2019, for 11 of the regular and 2 of the larger (grey cap) ones, basically a full replacement set. Approximately $20 USD:

 

Dealers are not using mechanics for oil changes in most cases. Around here they are separate even at the "maintenance advisor" level with separate driveways and waiting rooms from the service/repair guys.

 

I've been using the the Rexka branded ones from Amazon, and have found them to be pretty well built and true to the OEM designs. I did also get a kit that was not Toyota specific (mainly for the upholstery clips it also contained), but those look a lot more shoddily manufactured.

 

They're a lot less expensive from Amazon, but Amazon descriptions often stupidly neglect to reveal the size hole they fit. I bought a box of the smaller (7-mm) size, and they seem just as good as the originals.

When I bought my Prius used from the original dealer, which had also done all oil changes to that point, one of those fasteners was missing from the oil-change flap. They replaced it, no charge, when I complained.

The oil drain plug has to have metric threads to fit the original threads in the oil pan. Perhaps you have a non-OE drain plug with proper metric threads, but a 16-mm head (vs. OE 14), which coincidentally fits a 5/8" wrench.

 

I just bought 5 from the my dealership in Toronto, Ontario Canada. It costs $5 CAD each. Pretty expensive.