I have removed taken apart 2 engines, 1 turbo from an 83 300D and 1 non turbo from an 81 300D. I intend to rebuild one of them. However I need to remove some carbon form the piston ring gaps and the valves from the head.
The pistons from both engines are resusable and ill see if the valves are reusable, which most likely they are.

What are some good methods of getting the carbon off them without removing any material off the pistons+valves?

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Where I worked to clean valves Valves we had a Pedestal Grinder with A Grinding Wheel on one end a a 10 inch fine Wire Wheel on the other end. You just wire burshed the Carbon off of the Valves. (Always wear Eye Protection as even a New Wire Wheel will throw off pieces of wire and a old one will be even worse.)

For getting the Carbon out of the inside of Turbo Charger Housings we put them in Carburator Dip (this would work on the pistons). Carburator Dip is not the Carburator Cleaner you get in the spray can. It is like a 1 gallon can (or in our case a 5 gallon can) that you the taken apart whole Carburator goes in.

Do not soak Aluminum parts in the stuff overnight; I would not soak Aluminum longer that 2 hours in a new batch. So as long as the Pistons are Aluminum and Steel you could use the carburator dip (Bronze or Brass Bushings are also OK).

Of course the time honored way to clean a Piston Ring Groove is to break one of the old Piston Rings and use it to scrap off the carbon from the Grooves. I is a slow process but the real Piston Ring Cleaning Tool is too agressive and if you are not careful can remove some metal from the groove.

Of course the steel head of the Piston can be brushed with a Wire Wheel assuming there is not some special plating on the head.

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84 300D, 82 Volvo 244Gl Diesel

When I bought my 300D, it got about 22 mpg. It smoked badly on acceleration, and was sluggish.

I started using Lucas Injector cleaner and Fuel Treatment. After about ten tanks of treated fuel, the mileage started to climb. It is now 30 mpg with the a/c off, and 25 with the a/c on. I prefer it on. It is cold and comfortable.

Now it does not smoke on accerlation. There is no black smoke. The mileage is up, and it starts easier and has much more power. I think that the Lucas worked. I have gone four tanks without it, and I notice that it is starting to smoke when I floor it.

uncut 100% d-Limonene and maybe a fine brass brush.

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1999 E300DT (131,800) 154,000 Black on Black SOLD

2006 CLK 500 coupe Capri Blue on Grey (zoom,zoom)
47,000mi

04 VW TDI Passat 80,000mi
(Techno)

How to eliminate oil dependency through market-driven approaches.
“We could cut oil use in half by 2025, and by 2040, oil use could be zero,”
The Sound of Diesel Speed
Ode to MB

"I prefer to empty a few bags of sand into the intake while running at full throttle to clean out the carbon. It also ports and polishes the head at the same time."

why work so hard, your S.I.S. (soot injection system) is already fulfilling that purpose!

3lpuller's post is humor.
Never introduce abrasive into the engine.