Section 6166(g)(2) provides:

(g) Acceleration of payment

. . . . . . . . . .

(2) Undistributed income of estate

 (A) If an election is made under this section and the estate has undistributed net income for any taxable year ending on or after the due date for the first installment, the executor shall, on or before the date prescribed by law for filing the income tax return for such taxable year (including extensions thereof), pay an amount equal to such undistributed net income in liquidation of the unpaid portion of the tax payable in installments.

(B) For purposes of subparagraph (A), the undistributed net income of the estate for any taxable year is the amount by which the distributable net income of the estate for such taxable year (as defined in section 643) exceeds the sum of-

(i) the amounts for such taxable year specified in paragraphs (1) and (2) of section 661(a) (relating to deductions for distributions, etc.);
(ii) the amount of tax imposed for the taxable year on the estate under chapter 1; and
(iii) the amount of the tax imposed by section 2001 (including interest) paid by the executor during the taxable year (other than any amount paid pursuant to this paragraph).

(C) For purposes of this paragraph, if any stock in a corporation is treated as stock in another corporation by reason of subsection (b)(8)(A), any dividends paid by such other corporation to the corporation shall be treated as paid to the estate of the decedent to the extent attributable to the stock qualifying under subsection (a)(1).

Nothing in Reg. section 20-6166-1 relates to the acceleration provisions of section 6166(g). Applicable regulations for purposes of the section 6166(g)(2) penalty are contained in Reg. sections 20.6166A-3(a) and (b), which provide:

§ 20.6166A-3
    Acceleration of payment.
    (a) In general. Under the circumstances described in this section all or a part of the tax which the executor has elected to pay in installments shall be paid before the dates fixed for payment of the installments. Upon an estate's having undistributed net income described in paragraph (b) of this section for any taxable year after its fourth taxable year, the executor shall pay an amount equal to such undistributed net income in liquidation of the unpaid portion of the tax payable in installments. Upon the happening of any of the events described in paragraphs (c), (d), and (e) of this section, any unpaid portion of the tax payable in installments shall be paid upon notice and demand from the district director.
    (b) Undistributed net income of estate. (1) If an estate has undistributed net income for any taxable year after its fourth taxable year, the executor shall pay an amount equal to such undistributed net income in liquidation of the unpaid portion of the tax payable in installments. The amount shall be paid to the district director on or before the time prescribed for the filing of the estate's income tax return for such taxable year. For this purpose extensions of time granted for the filing of the income tax return are taken into consideration in determining the time prescribed for filing the return and making such payment. In determining the number of taxable years, a short taxable year is counted as if it were a full taxable year.

(2) The term “undistributed net income” of the estate for any taxable year for purposes of this section is the amount by which the distributable net income of the estate, as defined in section 643, exceeds the sum of—
    (i) The amount for such year specified in section 661(a) (1) and (2),

    (ii) The amount of the Federal income tax imposed on the estate for such taxable year under Chapter I of the Code, and
    
    (iii) The amount of the Federal estate tax, including interest thereon, paid for the estate during such taxable year (other than any amount paid by reason of the application of this acceleration rule).

(3) The payment described in subparagraph (1) of this paragraph shall be applied against the total unpaid portion of the tax which the executor elected to pay in installments, and shall be divided equally among the installments due after the date of such payment. The application of this subparagraph may be illustrated by the following example:

Example.
      The decedent died on January 1, 1959. The executor elects under section 6166 to pay tax in the amount of $100,000 in 10 installments of $10,000. The first installment is due on April 1, 1960. The estate files its income tax returns on a calendar year basis. For its fifth taxable year (calendar year 1963) it has undistributed net income of $6,000. If the prepayment of $6,000 required by section 6166(h)(2)(A), and due on or before April 15, 1964, is paid before the fifth installment (due April 1, 1964), the $6,000 is apportioned equally among installments 5 through 10, leaving $9,000 as the amount due on each of such installments. However, if the prepayment of $6,000 is paid after the fifth installment, it is apportioned equally among installments 6 through 10, leaving $8,800 as the amount due on each of such installments.

Regulation section 20.6166A-3 applies to Code section 6166(g)(2) to the extent it is not inconsistent with the Code section. See, e.g., the statement in PLR 201403012 (section 6166(g)(1) acceleration question), which reads

Neither the Economic Recovery Tax Act of 1981 nor its legislative history indicates any Congressional intent that a disposition of an interest under section 6166A would not be a disposition under section 6166. As a result, the regulations under section 6166A are considered applicable to this ruling request to the extent those regulations are not inconsistent with the language of section 6166.