John Deere No 12A Combine
John Deere No 12A Combine

John Deere No. 12-A Straight-Through Combine

(66-INCH CUT) (SERIAL No. 12-47519 and Up)



The Setting-Up Manual

Almost all field troubles can be traced directly to improper setting-up and starting. It is, therefore of the greatest importance that extreme care be taken to see that the machine is set up and started properly. To reduce transportation cost the combine is partially disassembled for shipment; in spite of this, John Deere Combines are more completely assembled at the factory than most combines.

The Setting-Up Manual should be followed very closely. The printed copy in the Setting-Up Manual should be read carefully. It is not always possible to illustrate the assembly of all units. In such cases, an explanation is given in the printed section. If you read and follow the directions given, there should be no trouble experienced due to the machine being improperly set up.

John Deere No 12A Combine

Breaking-in the New Combine

After the setting-up is completed, the entire machine should be gone over and bolts tightened. All bearings should be generously lubricated.

All V-belt drives should be checked carefully with the diagram stickers on the side of the machine. Belts should be tight enough to eliminate slippage because a belt can be very quickly ruined if it is allowed to slip in the grooves of a sheave for any length of time. Be sure all shafts turn freely.

Platform and feeder canvases should be installed before the combine is run off.

The clean-out doors in the bottom of the clean grain and tailings elevators, and grain tank unloading auger (see Figures 66 and 70), should be opened. Run combine slow for some time to allow the parts to work in gradually. After a short run at idling speed, stop combine and inspect completely, making a careful check for loose bolts, heating bearings, binding parts, loose belts, etc. After a complete re-lubrication, the combine should again be started and run for a brief period at a slow speed. It should then be brought up to a tractor fast idle speed and operated at that speed for at least two hours, preferably longer. After this full-speed run-off, another careful check should be made for loose bolts, heating bearings, etc. New belts will stretch slightly after the first run-off and tension should be increased.

Remember, the breaking-in period is just as important with a new combine as it is with a new automobile. Don’t try to “step it down to the floor” right at the start. If you do, trouble is sure to develop later.

John Deere No 12A Combine


Understand Function of All Working Units

Before starting to combine, turn to Cross-Sectional View of John Deere No. 12-A Combine figure, where you will find a cutaway view showing the working units of the machine. Study this carefully to understand thoroughly the function of each unit and become familiar with the adjustments necessary to obtain best results.

Don’t Start Combining Until Crop is Ripe

It is only natural for the owner of a new combine to want to try his machine as early as possible. This results in most new combines being started in the field before the crop is ready for combining. As soon as a binder is seen in the neighbor’s field, the urge to start becomes uncontrollable. When grain is ready for binding, it is not ready for straight combining. No one would think of picking up bundles right after a binder and hauling them in for threshing, yet, in too many instances, the combine is expected to satisfactorily handle grain in the same state of immaturity.

A combine should not be started until the grain is dead ripe, which is usually 7 to 10 days after it is customarily cut with a binder. If the threshed grain feels damp or is easily dented with the fingernail, the moisture content is usually too high for safe storage.

Grain crops containing 14% moisture or less are considered dry enough for safe storage. Arrangements can usually be made at the local grain elevator for necessary moisture tests. The maximum moisture content for safe storage depends upon the crop to be combined and in part upon atmospheric conditions, storage facilities, foreign material in the grain, whether handled in bulk or sacks, and whether the crop is for market or feeding.

The first round in the field is usually the hardest. The forward speed of tractor should be as slow as possible to reduce the volume of material entering machine. The tractor should be run at full throttle to keep the combine mechanism up to full speed thus guarding against slugging and clogging—shift the tractor to a lower gear to obtain slower travel speed but do not throttle down tractor motor.

In certain sections of the country where grain ripens unevenly, and where green weeds present a serious problem, the practice of windrowing is followed. There are advantages as well as disadvantages in this method of harvesting. In the Field Operation, Windrow Pick-Up Method section you will find detailed information on windrowing. Read this very carefully, before deciding upon your method of harvesting.

Do not start combining until crop is ripe.

Limitations of a Power Driven Combine

The operating efficiency of any power driven machine is directly proportional to the tractor power available. Steady, smooth power is of vital importance—any fluctuations in tractor motor speed is reflected in the speed of the combine—uneven speed results in loss of grain, inferior threshing and, in extreme cases, complete plugging of the machine. Every precaution should be taken to maintain uniform speed.

John Deere No 12A Combine

The Operator

The degree of satisfaction given by this or any other combine is directly dependent upon the carefulness of the tractor operator. Once the combine has been adjusted to meet the crop condition, the rest is up to the operator.

Excessive travel speed is one of the greatest causes of trouble. Traveling at a high rate of speed over rough ground can cause extra wear and breakage that would not occur if the combine was pulled at a more reasonable speed. Overloading, resulting in a loss of grain, is another evil of fast ground travel. More straw is taken in than the machine can handle. The heavy layer of material passing over the rack and sieves carries out grain.

The tractor motor must be operated at full throttle at all times. Any necessity for reduction in travel speed should be met by shifting to a lower gear instead of throttling tractor engine.

When stopping the tractor to unload grain tank, or for any other reason the combine should be cleaned out before disengaging power take-off. If necessary to stop in the middle of the field, the combine and tractor should be backed up a few feet before again going ahead. This will allow the combine to come up to speed before grain enters.

If ditches in the field necessitate throttling down motor when crossing they should be cut around.

By rounding the corners in the field more uniform speed can be maintained when turning.

The tractor operator should note very carefully the condition of the crop and adjust platform so just enough of the straw is cut to get all the grain. If, in a certain section of the field, the crop is extremely heavy and down badly, he should take less than a full swath.

The tractor operator should listen for the warning of clutches slipping. He should also listen to tractor motor for any evidence of slowing down, caused by cylinder starting to slug, and stop the tractor before the machine has become completely plugged.

Owner’s Responsibility in Proper Operation of Combine

The John Deere dealer is obligated to explain thoroughly, the adjustments that are built into the machine and give general instructions on when and how to make these adjustments. He should also explain to the operator the value of the Setting-Up Manual and the special Operator’s Service Manual you are now reading. The information given in these Manuals will afford a clear understanding of the fundamentals of combine harvesting. The best use of these fundamentals to suit the condition in which the machine is operating is a responsibility that is completely up to the operator.

Proper Preparation of Field for Combining Will Mean Less Trouble and More Profitable Operation

In fields where small grain follows corn in the rotation of crops, special care should be taken before seeding to clean up or cover cornstalks and large corn roots. They can be very troublesome should the crop go down.

When a cornstalk or root hooks onto the point of a guard, a great deal of grain is pushed ahead and run down. It is usually necessary to then stop, back up and clean off the cutter bar before going on. Raising the cutter bar to avoid this will mean a loss of some of the beaten down grain.

Another thing, large corn roots can be injurious to canvases should they be carried up the platform to the narrow feeder throat.

A little extra work done when preparing the field for the small grain crop will pay big dividends when harvest time rolls around.

John Deere No 12A Combine


  1. Keep all shields in place.
  2. Stop machine to adjust and oil.
  3. When mechanism becomes clogged, disconnect power before cleaning.
  4. Keep hands, feet and clothing away from power-driven parts.
  5. Keep off implement unless seat or platform is provided. Keep others off.


Cylinder Speed and Spacing Between Concave and Cylinder

When combine is run off at factory, the cylinder speed is set to operate at 1300 R.P.M. under load. At this speed, best results can be expected in handling most small grains. The cylinder and concave spacing is set at 1?4”. Remember, even though these are factory settings and need not be changed until field trial indicates a necessity for readjusting, it must be understood they cannot be expected to handle all crops in all conditions. Suggested changes for different crops and conditions will be found at the end of this manual.

The cylinder and the separator are driven by separate belts. The speed of the cylinder can be varied without affecting the speed of the balance of the machine. Instructions for varying cylinder speed are found in the Cylinder section.

Basic Speed of Separator

The basic separator speed can be checked at the end of feeder canvas drive roller. The speed at this point should be 540 R.P.M. with tractor throttle at fast idle. Tractor governor should be set so that speed of this roller is 540 R.P.M.

Beater Behind Cylinder

The beater behind the cylinder is set to operate at 650 R.P.M., with tractor operating at fast idle. Conditions sometimes make it desirable to change this speed. Full instructions for changing beater speed are found in the Beater Behind Cylinder section.

Triangular covers are installed over beater teeth. They prevent wrapping of green material on beater if combine is started in the field before crop is sufficiently ripe.

Cleaning Fan

The valves at sides of fan housing are set about two-thirds open and the blast deflector in fan throat is set to throw the blast about one-third of the way back from the front of shoe.

Cleaning Chaffer and Sieve

The adjustable chaffer lips are set about one-half open and adjustable sieve lips about one-third open when combine leaves the factory—these are average settings. A good rule to remember is to have lips of chaffer or upper sieve open as wide as possible without admitting too much coarse material and so grain works through before it passes over two-thirds of the length of sieves. Close the lips of the adjustable sieve as much as possible without carrying clean grain into tailings return auger.

John Deere No 12A Combine


The cutting, threshing, separating and cleaning units must operate at very near a definite speed at all times to satisfactorily do the fine job that this combine is capable of doing. Extreme high, extreme low or varying speeds all lead to an inferior job of combining.

The basic speed of the combine can be checked at the left hand end of the feeder canvas drive roller. It should turn at 540 R.P.M. when tractor is operating at fast idle. Underload basic speed is 520 R.P.M. It is very difficult to check speeds with combine working under load. It is for that reason we must recommend that basic speed be checked at fast idle.

Regular Separator Drive Sheave

AP17049 H, 7-7/8” Separator drive sheave on gear case cross shaft is considered standard equipment. This sheave will provide a basic speed of 540 R. P. M. when take-off shaft on tractor is operating at the average take-off shaft speeds of 540 to 550 R.P.M. If speed is above or below 540 R.P.M. governor on tractor or motor should be readjusted. This is important!

Special Separator Drive Sheaves

14099, (PK694 H), 8-7/16” Separator drive sheave.

This sheave should be used in connection with any tractor having a slow speed take-off shaft. It is furnished with the hook-up equipment for the McCormick-Deering Farmall F-20 tractor, which has a take-off shaft speed of 505 R.P.M.

14101, (PK696 H), 6-3/4” Separator drive sheave.

This sheave should be used in connection with any tractor having a take- off shaft speed of around 600 R.P.M. The Minneapolis-Moline Model “Z” which has a 615 R.P.M. take-off shaft speed is an example. This sheave may also be used on older tractors that are a little short on power. Speed of tractor should be increased so that feeder roller operates at 540 R.P.M.

14100, (PK695 H), 9-1/2” Separator drive sheave.

This sheave can be used with larger tractors of a power rating of the John Deere Models “D”, “G” or “GM”, where sufficient power is available for throttling the tractor engine to obtain slower ground travel when handling a heavy crop. This sheave should never be used with smaller tractors where throttling down results in an excessive sacrifice of power.


John Deere No 12A Combine

Study This Illustration Carefully

This cutaway view of the John Deere No. 12-A Straight-Through Combine shows how the grain and straw are handled from the cutter bar straight through the machine.

The four-slat, ground-driven reel, “A”, divides the grain and holds it to the cutter bar, “B”. The cut grain is gently elevated by platform canvas, “C”, which together with feeder canvas, “D”, delivers grain in a thin, even stream to the extra-wide, rasp-bar cylinder, “E”.

As the grain travels between cylinder, “E”, and concave and perforated grate, “F”, and back against beater behind cylinder, “G”, the grater part of separation takes place. The grain falls though perforated grate to shoe pan, “K”, and is moved back to shoe chaffer—grain is not remixed with straw to overload straw rack. Beater, “G”, deflects grain down through the chaffer section at the front end of the straw rack, and passes the straw onto full-width straw rack, “I”. During its outward movement, the remaining grain falls through cells in rack onto grain conveyor, “J”, and is delivered back to shoe pan, “K”, which moves it to front end of chaffer. Straw is then tossed out on the ground in a wide, even spread.

A blast of air from fan “N”, is directed by deflector, “O”, against shoe chaffer, “L, and shoe sieve, “M”. This blast with the aid of chaffer and sieve agitation, blows chaff away and moves the tailings to tailings auger, “P”. This auger carries them to tailings elevator, “Q”, which conveys them to distributing auger, “R”, where they are delivered to the center of the cylinder for rethreshing.

Clean grain, after dropping through shoe chaffer, “L”, and shoe sieve, “M”, is carried by clean grain auger, “S”, to elevator, “T”, on opposite side of combine and elevated into grain tank.


Simple and positive adjustments for handling a wide variety of crops under varying conditions have been built into this machine. These adjustments, even though simple, have an important bearing on the quality of work.

It must be realized that it is impossible to lay down hard, fast rules for any particular crop or condition because there are no such rules that will apply in all cases. The same crop under different conditions may require entirely unrelated adjustments. We can only explain how the adjustments are made and in a brief way tell what a change in adjustment will accomplish and leave it to the operator to use his best judgment in applying them.

John Deere No 12A Combine


(See Figure 1)

The reel slats gather in the straw, hold it until it has been cut by the sickle, then lay it back onto the platform canvas. Reel must be set to keep grain moving steadily and uniformly to the cylinder.

Reel plates accommodate three, four, six or eight slats. Regular equipment consists of four slats.

Reel Speeds

The reel is ground driven from right hand wheel. Fast ground travel often necessitates slowing down the reel. PK2 H, 12-tooth Special Sprocket (Shipping Package 10992) for right hand end of reel jack shaft through grain tank (see “F”, Figure 2) is available for that purpose. Slowing down the reel prevents excessive agitation of the standing grain and carrying of straw over top of reel.

In a heavy crop, when ground travel is too slow to allow reel to lay grain back onto platform rapidly enough, a faster reel speed is necessary.

Order P2943 H, 7-tooth Sprocket (Shipping Package 10902).