At 250,000 miles, I'm a believer that camshafts and valve tappets are on borrowed time. I'd replace them around 200-250,000 proactively. The bottom end is probably good for 500,000, but I'd do the cam and tappets half way there. The other major top end wear parts to worry about are the valve guides, but you can't do those without head removal. So as long as the guides will go a long time in this engine (I don't know if they are documented to.), then cam and lifters alone will help to guarantee many more carefree miles,
Brett
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