Retired Engineer With $1.6M Discovers Tax Bomb Hiding Inside 401(k) He Never Rolled Over

A 66-year-old retired mechanical engineer sits on $1.6 million in total assets, with $1.1 million still parked in a former employer’s 401(k) he never rolled over. He has not touched it because nothing felt broken. But at age 73, things break. That’s because the IRS forces him to start emptying the account on its schedule, at his marginal tax rate, whether he needs the money or not.

This is one of the most common avoidable mistakes in retirement planning. Vanguard’s How America Saves research notes that participants who leave balances with former employers often face limited menus, higher fees, and lost flexibility compared with an IRA.

Every dollar in that $1.1 million balance is pre-tax. The IRS has a claim on a portion of it at ordinary income rates whenever it leaves the account. Under SECURE 2.0, required minimum distributions begin at age 73. Left untouched, a $1.1 million balance compounding at a modest rate could easily push past $1.4 million, generating first-year RMDs in the mid five figures, stacked on top of Social Security and any pension income.

That stack is what creates the bomb. It pulls the engineer into a higher federal bracket, increases the taxable portion of Social Security, and triggers Medicare income-related surcharges that are easy to miss until the bill arrives.

The Hidden Penalty: IRMAA

Medicare premiums are means-tested. For 2026, a single filer with modified AGI at or below $109,000 pays the standard $202.90 Part B premium. Cross $109,000 and the premium jumps to $284.10. Cross $137,000 and it climbs to $405.80. Above $171,000 it reaches $527.50. Part D surcharges stack on top, starting at $14.50 monthly and rising to $91.00 at the top tier. A large RMD can vault a retiree several brackets higher in a single year.

Some retirees opt to offset RMDs with qualified charitable distributions, which let people aged 70½ and older send up to $108,000 of RMD money directly to charity tax-free. But they are only available from IRAs. 401(k) plans do not permit QCDs.

The years between 66 and 72 are the engineer’s planning runway. Earned income is gone. RMDs have not started. Taxable income is likely at the lowest point it will ever reach again. That makes this the cheapest window to voluntarily pull money out of the pre-tax account.

There are two options for most people in this position:

1. Roll the 401(k) into a traditional IRA. A direct trustee-to-trustee transfer is a no tax event. This unlocks QCDs at age 70½, opens the full investment universe, typically cuts expense ratios, and enables clean Roth conversions.
2. Run bracket-fill Roth conversions every year from now through age 72. Convert just enough each year to fill the 22% or 24% federal bracket without spilling into 32% or tripping the next IRMAA tier. Pay the tax from taxable savings so the full converted balance keeps compounding tax-free.

Start with the rollover. It costs nothing, takes a few weeks, and eliminates the QCD restriction immediately. Once the IRA exists, model one variable: how much can be converted this year while staying under the $137,000 IRMAA threshold for a single filer or $274,000 for a joint return. Every year of inaction shortens the conversion runway and raises the eventual tax bill on a balance that will only grow larger.