We have a dune buggy, street legal. Oil always fine. No leaks, really!
Car just started slowing down and eventually stopped.
Mechanic says main bearing failure and needs new engine.
Any idea what would cause this? Does this mean the cam shaft is not turning?

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."

Are you sure it didn't just drop a valve? They were notorious for that. That's how's I got my 72 Westy so cheap... it had burned a valve and needed head work.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

Yes, your cam shaft is not turning because your crankshaft is not turning. Remove the bottom oil pan and check for excessive metal. You can rebuild these engines quite easily.

Problem with these vehicles is the engine compartment isn't sealed, and airflow isn't really optimized. There are pieces of tin on the bottom of the engine which normally seal off the engine compartment. I don't know if yours were left in or not. In anycase, these kinda-sorta direct airflow over the cylinder heads. If they're not there, your engine runs hot and eventually it affects your oil quality.

It's also possible that the oil pump is bad also. Everything on an aircooled is run off the crank so yeah, your cam, your oil pump, no good, sitting in the bottom of that sludge. You can actually rebuild these things using pretty basic tools. When you get to the point where you need bearings you need to get some plastigauge and measure what size you need. If I remember correctly the cam is notched so you just line it up with the crankshaft. If you don't want to do that at least break it down and check it out. Pretty cool stuff.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman

Halfway up into the Escalante Range in Utah I had a similar problem from contaminating my oil. Ran half the day with my choke on. Saved my bacon by doing a quick oil change up off the road. If you let it go until the engine stopped your lower end is toast and a rebuilt switch-out is in order. Oil is always important, but moreso on an aircooled.

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89 300E
79 240D
72 Westy
63 Bug sunroof
85 Jeep CJ7
86 Chevy 6.2l diesel PU

"The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane."
Marcus Aurelius

I have the info I need to proceed. Many thanks for the quick reply.
This is the best place on the 'net.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."