Where are these things? What purpose do they serve? Do they need to be changed with any regularity? How hard is it to change them? How can you tell they're bad? What's "nailing" mean?
Car's an 83 300D 5 cyl turbo

These are where the fuel is injected-

Prechambers designs are less efficent(? 20-30%), but are quiter. They also allow the engines to be higher reving(I think).

Not hard to change if you have the right tools- slide hammer and locking colar remover.

Rarely replaced. You can remove them and inspect for cracking-rare. You could measure the bore holes too... some people replace them at rebuild time, maybe if you had one really carboned up due to a bad injector. They are expensive.

Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E