I'm rebuilding an engine for my 82 300SD. The motor had 250,000 mi on it, ran OK (from my parts car). Should I replace the oil pump preventatively. I am replacing the chain and tensioner and obviously it's easier to do it now, but I don't know if these pumps last for 500,000 mi or not. How often do they fail or get worn? Any advice is appreciated.

You will find that the oil pumps are expensive. Since you can change the oil pump out with the engine in the car I would not change it at this time.

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Jim

I have always been one to put new oil pumps on my cars as a matter of principle.... VW's , Toyotas, Fords, etc... and get lots of miles out of my engines...
But they are really cheap compared to the oil pump for a MB diesel....
I love the idea of having a new oil pump,,, the heart of the engine, in a rebuilding situation.. feels good from there on...
But it also costs more typically to rebuild a MB diesel...
So this is a matter of considering the satisfaction of knowing you have a new oil pump and the potential protection it affords your new investment in your engine.... compared to accepting that the old one will probably do just fine for the next 250,000 miles....
One thing to be sure of is cleaning out the oil bore which is accessed by removing the steel balls on the rear of the engine...and be sure you put a new oil relief valve on the oil pump.... and perhaps a new thermostat in your oil filter if you have one.... and be sure the hole in the tube in your oil filter holder is clean and all the rubber grommets in good shape....

At the very least you should consider installing a new chain on the oil pump, with high miles that is a definate wear item.
Intalling a new oil pump is always a good idea with a rebuild. Too much to lose now.

Without a doubt change the chain tensioning rails, tensioner and the chains and you should be covered for awhile.

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Jim

thanks for all your thoughts. I'll use a new chain, rails, tensioner ( the spring wire was worn about 1/2 way through) and am making sure that all oil passageways are really clean. I'll probably change the oil relief valve, whereever that is. I know some of you will think I'm crazy not to change the pump itself, but you can't replace everything, and besides, using some judgement and trusting your instincts is what DIY is all about.

I don't think your cra$$$$$$$$$$$$$$$$y at all.

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Jim