I've got a 67 250S that I am working on restoring. Motor runs, but figure I have it out already to get the car painted and engine bay painted, might as well do something with it. It's got roughly 120k miles, so it wouldn't hurt to rebuild. You guys think it is worth it to get the motor rebuilt or should I just replace it with something else like a crate motor or another w108 motor? I've gotten some quotes of around 2500 alone for labor of the rebuild. I could just clean it up and throw it back in as well, just trying to weigh my options and see what others think who have gone down this path before. Thanks!

I rebuilt my M108 from a 67 250S, to eventually go in a Unimog. So from my experience I think the first question would be, does it really need it?

After looking at the standard things (compression, oil pressure, etc.), I would do a partial disassembly and closely check for corrosion in the water jacket and plugged oil cooler. The M108 is known to run hot, my had an overheating problem and coolant leak from the previous owner, lots of crap in the system. Afterwards, I would check the oil pump bushings and camshaft.

The M108 is kind of an odd ball, it is very much like the early M180 and later M114, but the differences and that it was only produced for 3 years means that specific internal parts are very hard to come by (at least, at a decent price). Water pumps, head gaskets, and other wearables are common and easy to find. Things like oversize pistons and oil pumps, on the other hand, can be extremely expensive (i.e. 82.5mm piston: $350 each, oil pump $800). It took me 3 months to find a set of pistons hidden away at an old parts supplier reasonable price (got the last 6 on the shelf). A year later I found a good used oil pump on Ebay for $15! I'm doing the parts sourcing and rebuilding myself, still the machining was ~$1500 by the time it was all done.

Very interested in the answer as I'm mulling the same question...

Just take into account the overall condition of the engine. Compression, condition of the camshaft and the cam followers, chain tension... A total rebuild might bring piece of mind but the main thing to consider is: Is it necessary?

I have seen Mercedes engines with lots of miles that were in great shape and I have seen them rather worn out at only 200,000 miles. It just all depends on the care the PO took with them.

If the engine is running now then it can always be rebuilt later. I would say to just go with what you have and deal with it when the compression starts to fall.

Do a leakdown test to determine the condition of the engine and whether the rings or valves need refreshing.
On my m117 the bottom end was like new after ~100k miles, but most of the valve guides were worn, so I had all the guides and valves replaced.

If the engine is out, at the very least I would replace the crankshaft seals, oil pan gasket, and any other seal/gasket that will be hard to get to once the engine is reinstalled.

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Csaba
1972 280SEL 4.5, silver