How to unplug black friction pin connectors Black friction pin without slots connectors are common in older Technic sets. Note that these were quickly replaced by the current version with slots that is much easier to remove.

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Especially in older sets, it is not uncommon for these connectors to get completely stuck (as if glued) in bricks with holes. I have to resort to pincers and brute force. This typically results in the connector being damaged. 

The orange brick separator is often not tough enough to do the job.

So, how can you separate these things?
Take your cross axle:



And push it into the hole where your pin connector had stuck