I have a 401k through work. Payroll department diverts 6% of my gross pay as an employee contribution to my 401k, plus the employer chips in a 3% matching contribution toward my plan.
I have some overtime available in the near future, a significant amount of extra hours available paid out at time and a half.
My question is, if I ask my payroll department to pay all of the overtime toward my 401k, will that extra amount be subjected to reduction by Medicare and social security?

All the pay you direct to your 401k should follow the same rules unless you exceed the limit 30% I think it is

No.  The amount going in to a retirement plan does not skip Medicare or Social Security taxes (FICA).  It does avoid Federal and State Income tax.

This is correct. 401k is a pre-tax deduction for Federal and State only. FICA taxes (Med and SS) will still be deducted off of gross earnings.

OP - your wording is a little confusing. Are you talking about requesting that your employer move your overtime earnings into a 401k or just increasing your percentage while you are earning extra in overtime pay? Make sure you check your plan documentation and company rules. Some companies don't defer based on overtime compensation and I don't think that you can typically just tell them to put a certain amount or a certain payroll check into your 401k. You would have to adjust your contribution %.

Are you saying there is a limit to the amount you can put in your 401K per paycheck?

There is a 30% yearly limit of your income I believe, unless you are of a certain age and have saved under a certain amount then there is a "catch up" clause where you can put more in.  But I think 30% is the limit for a year.


 

EDIT: Nevermind I just looked it up, it's just $18,000 in a year maximum period.  Not sure where I remembered that 30% number from.

You all have been very helpful. Thank you.