Are you referring to the M116 3.5 litre (iron block)? I suspect you might be. Unless abused (ie. infrequent oil changes) they will last a long time (like most MB engines). I have owned my '73 350SLC for almost 21 years. I don't know the exact distance it has travelled as I purchased it cheaply showing much lower kilometres than it had obviously travelled. Whilst I have only put about 150,000km on it, many have been short trips in heavy traffic. I also drive it fairly hard. To compensate for this I have usually changed the oil (Castrol GTX2 20W-50) every 2,500km and the oil filter every 5,000km. I suspect it may now have covered at least 350,000km (especially when considering the other work it has needed including transmission and diff). About 120,000km (18 years) ago I removed the heads for a valve job and to replace the valve guides and seals (this is typically a 200-250,000km job) and fitted a new timing chain and tensioner. At that time the bores looked like new with good cross-hatching and no lip. Since then the engine has required no major work and is showing no symptoms of wear in the pistons, rings or bores. It could probably use a new timing chain again and possibly another set of valve stem seals if I wanted to be really fussy. An iron M116 with worn pistons rings or bores has either travelled high mileage or has been abused.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.