Start by doing a compression test. If compression is good follow Toms advice and fix the leaks.

If it is indeed overhaul time there is no magic involved but overhaul parts are relatively expensive.

Changing liners is no big deal for a competent machine shop, but if you buy oversize pistons there is no need to change liners, simply bore to fit new pistons like any other engine. If pistons are in good shape, the machine shop can change liners and bore to fit original pistons.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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