Mine is leaking. How hard are they to replace?

Shouldn't be difficult to replace, assuming you haven't buried it yet...

Ohhhh, gasket! Can't help you for relative difficulty with other head gaskets, but it should be pretty straight forward, but it will take quite a few steps and a special tool or two. It depends on your mechanical experience. Have you taken a peak at the FSM or Haynes procedure? Are you it's your head gasket that's leaking? The last thing you want to do is replace the gasket and have it not fix the problem.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition