Home Monroe 8N-213 Mechanical Calculator Repair and Operation.

Monroe 8N-213 Mechanical Calculator Repair and Operation.


Monore 8N-213

The Monroe 8N-213 calculator is a highly advanced automatic mechanical calculator. It began production in 1954 and continued through 1966. A government book for purchasing Monroe mechanical calculators describes the 8N calculator line as being best suited to mathematical and statistical operations due to its advanced functions and quick handling of large figures. It goes on to describe the “light touch” of the control and speed of the machine.

Old Ad

These machines can even be found in old photographs from NASA. (With a Marchant machine in the background as well)

Old Ad

The pricing booklet listed the cost for this machine in the 50’s as $901.


Adjusted for inflation, that would be almost $10,000 today for a calculator. And it’s easy to see why. This calculator was incredible compared to others of the time and included a lot of advanced functions.

The 8N-213 had a 10 digit keyboard, a 10 digit counter without carry, an 11 digit counter register with carry, a 21 digit accumulator register, and a 10 digit multiplier register that could be used for back transfer from the accumulator. This calculator can perform automatic division, multiplication, and automatic squaring.




This machine has the serial number B745066.




The carriage has several different registers. The 10 digit counter register on the top left with the white dials is the counter register without a carry. This register counts at the same time as the other register on the left. During multiplication and division, this register remains the same as the other. However, if doing successive additions or subtractions that get higher than 9, the counter on the left will roll over to 0 again instead of displaying 10. This is useful in certain statistical calculations. The 11 digit register in the top right is a normal counter register with carry.

Above the 10 digit counter register are the tabulation keys. These allow the register to stop at a certain place when shifting the carriage or performing division. The rightmost tabulator key clears the others.

The levers to the left of the registers prevent clearing of the registers. The topmost lever prevents the 11 digit counter register on the right from being cleared by the Upper clear button. If the switch is up, both registers will clear. If the switch is down, only the left register will clear. The lower two levers prevent sides of the accumulator from clearing. If both are down, the entire register cannot be cleared by the Lower clear button. If the left one is up and the right one is down, the rightmost 11 digits will not be cleared and the left 10 digits will be cleared. The C lever on the right stands for Constant. If this lever is in the down position, any number in the multiply register will remain after a multiply operation. If it is up, the number will be cleared after each operation.

Below the accumulator is the multiplier register. This will be discussed in depth with multiplication.

Keyboard, Buttons and Levers.


The 8N-213 contains 10 columns of keys, with 9 rows for digits and another bottom row for 0 or clearing that column of an entry. To the right of the keyboard are the addition and subtraction keys. To the right of those keys are the carriage shift keys. These keys can be quickly pressed for a single placement shift ort held down until the desired shift is reached.

Below the two carriage shift keys is the Enter Multiplier key. This key places the number on the keyboard into the multiplier register. If pressed and released, the keyboard will be cleared for entry of the multiplicand. If held down, the keyboard will not be cleared, meaning the same digit is in the multiply register and left in the keyboard. This is done for quickly squaring a number.

Below the addition key is a small switch. The is the Repeat switch. If in the up position, the addition or subtraction key being held down will perform successive additions or subtractions until the key is released. If in the down position, the addition and subtraction keys being held down will still perform only one operation.

To the right of the Repeat key is the Multiply/Divide switch. This switch reverses the direction of the right counter register. If the switch is up in the X position the counter will count upwards. If it is down in the ÷ position, it counts downwards. The NON Entry switch immediately to the right disables the right counter register so that it does not count at all.

Below those levers is the Enter Divd button. This button shifts the carriage all the way to the right and enters the number on the keyboard into the accumulator register for division.

Below the Enter Divd button are the the Upper and Lower clear buttons. The Upper button clears the two counter registers and the Lower button clears the accumulator register. Below those buttons is the KB Clear button used to clear the keyboard. All three of these buttons can be pressed at the same time.

To the right of the Lower clear button is the Lock lever. This lever prevents the keyboard from being cleared after the Enter Multiplier key is pressed.

Bottom buttons

Below the keyboard are some more control buttons. On the left is the Stop button used for stopping multiplication or division. Beside that is the Div button. This method starts division once the dividend is in the accumulator and the divisor is entered on the keyboard.

Beside the the Div key is the Neg Mult key. Neg Mult stands for negative multiply. This key multiplies the number in the multiplication register by the number on the keyboard and subtracts the result from the number in the accumulator register. The Acc Mult key beside it stands accumulative multiply. This key multiplies the number in the multiply register times the number on the keyboard and adds the result to the number in the accumulator register.

Clear Mult has several functions depending on the machines state. If the carriage is shifted all the way to the left, the button clears the accumulator, and then multiplies the number in the multiply register by the number on the keyboard and places it in the accumulator register. If the carriage is not in the leftmost position, pressing the Clear Mult key will return it to it’s leftmost position. If the carriage is not in it’s leftmost position and the multiply register has been shifted any place to the left, the number in the accumulator will be back-transferred and placed in the multiply register, then the carriage will shift all the way to the left.

The Auto Clear switch below all of these buttons, if set tot the left, sets the machine to automatically clear the keyboard after multiplication and division.

On the far left is the Divd Align lever. If this lever is up, the carriage shifts all the way to the right when the Enter Divd button is pressed. If the lever is down, the carriage is not shifted and left in it’s current place for the dividend to be placed into the accumulator.


Addition and Subtraction

Simply enter the number on the keyboard and then press the + or - buttons to add or subtract it from the accumulator.


Make sure the carriage is shifted all the way to the left. Enter the multiplier onto the keyboard. Then press and release the Enter Multiplier button. The multiplier should be visible in the multiplier register. Next enter the multiplicand onto the keyboard. Then press the Clear Mult button to clear the registers and multiply the numbers. The result will be in the accumulator. If you want to add the result to the digits in the accumulator press the Acc Mult key. Likewise, if you want to subtract the result from the accumulator press the Neg Mult key.


First, decide how many decimal places you want. If you want a set amount, move the carriage to the right the number of decimal places required and move the Div Align switch to the down position. If it doesn’t matter or the most decimal places are required you can ignore carriage position and leave the lever in the up position. Next, enter the dividend onto the keyboard and press the Enter Divd button. This will place the dividend into the accumulator. Finally, enter the divisor onto the keyboard and hit the Div key. The result is in the right counter register.


Enter the number you wish to square onto the keyboard. Then press and hold the Enter Multiplier. Then press Clear Mutl. The result will be in the accumulator register.

Accumulate multiply constant

The number you want to multiply by must be at least one place from the rightmost place in the accumulator. Shift the carriage to the rightmost digit of the number in the accumulator. Then, move the metal digit selector in front of the multiplier register to align with the same digit. Using the lever on the right side of the multiply register, move it until it stops against the metal selector. Then select the Clear Mult button. The number in the accumulator will be transferred to the multiply register. Then move the C switch to the down position. Now the number from the accumulator can be multiplied as many times as needed.



Division on this machine did not work. It would divide correctly to a point and then miss an underflow and continue to subtract. This ended up being due to the underflow not carrying across the digits properly. A good way to test if this is the problem is to subtract 1 from a cleared accumulator. The entire register should be filled with 9s. In my case, the 9s one carried to the 10th digit.

This was repaired by adjusting the carriage placement. Pushing on one side of the carriage and subtracting 1 would put all 9s in the accumulator. So the fault was the carriage placement. The carriage has 5 screws on the back that can be loosened and used to adjust the placement.

Division repair

Once loose, push the carriage the desired direction and re-tighten the screws.

Multiply constant not working

The first digit of the multiply register would always return to 0 whether the constant lever was set or not. WD-40 several times followed by immediate oiling and working eventually got it working again.

General stickiness

All of the other problems with this machine were trivial and solved by oiling and working the machine.


Video of the machine performing division with the outer panels removed.

Video of the machine performing division ins low motion with the outer panels removed.

Internal Pictures

All of the outer coverings can be easily removed showing the internals of the machine and allow for servicing.

Inside front

Inside right

Inside back

Inside top

Inside carriage

Inside left

Inside bottom

This post is licensed under CC BY 4.0 by the author.