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what should you look out for? When people talk about shaft play, do they typically mean axial or lateral, or both? How much play, if any, is acceptable?

Yes, shaft play would be (when looking at the front of the compressor wheel) in-out and side to side. As for how much play... not much maybe 1mm overall. My 120 000 miles 14b turbo has no in-out play whatsoever but 1 mm of side to side play. Still boosting strong.

Depending on the center bearing type, side to side play is normal even on a new turbo. The shaft "floats" on a pressurized film of oil (when the engine is running) with turbos built with float journal bearings. So at rest, it's normal for them to move a bit

Look for scratches on the compressor housing and turbine housing left by the wheels from too much play. Also, check for oil stains inside the housings meaning the seals have failed.

Hope it helped.

Hey, thanks for the reply. The turbo in question is a k27. It has a very slight amount of what you call in-out play, and no side to side play that I can detect.