I'm about to purchase an engine from a car that was wrecked. The engine still ran well after the accident, but I have a friend whose TDI was totaled up front, and his engine blew up 3 weeks after the car was repaired. When they took the head off, everything inside was torn up. I'm just curious as to how much shock an engine takes from a front end collision. Thanks

If the collision is not severe enough to touch the engine, the forces involved will most likely not damage it. It's what happens afterwards that you have to worry about. If it was driven for a short time after the collision, it is possible that the oil cooler and radiator were damaged and leaking, possibly overheating and/or oil starving the engine.

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The G-force shock can break the engine mounts.

I suppose it depends where the force goes. In a FWD theres a risk I suppose of forces being transmitted through the front wheels, through the gearbox, and onto the flywheel.

In a RWD automatic I'd say theres less chance of that.

You also have to wonder if the engine was run without coolant or oil for a time.

I'd think 2.5 has it right. If the car overheated or starved for oil and was driven, that's where you'll end up with internal "hidden" problems. Have a look at the underside of the engine /trans for leaks, also look for antifreeze leaks or green stains anywhere low down on the car. Also if the impact was enough to damage the front end, you may have some trans damage. Try to drive it in all gears, or negotiate the price down. Have a look under the car at the trans mounts. see if they look pushed back or out of whack. A look at a similar model just before looking at the purchase car would be of great help.