You can make as much money as you wish and still collect SS. The amount you earn impacts whether that income is taxable or not. So keep earning!
Re the 401k contributions? Not taxable so does not impact SS at all.
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There are 2 different things to know about social security. People get them mixed up all the time.
1. Your actual SS checks
If you are over full retirement age your actual ss checks won't be reduced. Otherwise they will actually reduce your payments if you make too much other income in the prior year. See SS FAQ for working after retirement
2. Income Tax
For any age up to 85% of Social Security becomes taxable when ALL your other income plus 1/2 your social security reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0