New Holland 451 Sickle Bar Mower Parts Diagram

Working with a New Holland 451 Sickle Bar Mower Parts Diagram

The sickle bar on these machines is one of the simplest cutting mechanisms ever put on farm equipment, which is exactly why it also fails in the most annoying ways. A parts diagram helps because when something breaks at 3 PM on a Saturday and you are standing in front of a barn full of random metal shapes, you need to know whether the part you need is called a guard, a finger, or a spacer. I pulled diagrams from a few different sources over the years and found that New Holland service manuals are generally more reliable than aftermarket reproductions, but even those have quirks. The most straightforward place to get an authoritative diagram is through New Holland's official parts portal or an authorized dealership. They list exploded views with part numbers keyed to each component. Third-party sites like PartsTree, Ag Parts Direct, and TractorPartsWithPrayers also host diagrams for this model, though you should cross-reference part numbers between sources because discrepancies show up occasionally. eBay sellers and generic PDF compilations sometimes recycle old diagrams without confirming they match your serial range, which is a real problem since New Holland made changes to the 451 over its production run. I keep a printed copy from a 1998 New Holland service manual tucked in the tool cabinet, and it has served me better than any digital version because paper does not crash and the resolution is consistent. If you are working from a screen, zoom in enough to read the smallest dash numbers on the drawing. Missing a dash number means ordering the wrong shim or the wrong washer, and those cost about as much per unit as your patience is worth at that point.

Major Components on the Sickle Bar Assembly

Walking through the main parts from the cutter bar out, here is what you will see on the diagram and what each piece actually does in practice. Cutter bar section — This is the structural base that holds everything. It is a rectangular tube or channel member that runs the full length of the cutting assembly. The bar itself does not move during operation. What matters here is that the bar gets dinged, bent, or corroded over time, especially along the bottom edge where it contacts the ground. A bent cutter bar will make the sickle bind regardless of how sharp the fingers are, which is the first thing I check before anything else. Sickle section (knife section) — The reciprocating blade made up of alternating dies and fingers. The dies are the hard steel pieces with the cutting edges. The fingers are the softer, curved pieces that slide between the dies. The diagram will show these as individual units, but they are sold and replaced as complete sickle sections. You do not typically swap individual fingers unless you are doing a repair in the field with limited parts on hand. A worn finger profile causes material to wrap instead of cut, which looks like a dull blade problem but is actually a finger geometry problem.

Guide shoes or sliding bearings — These support the bottom of the sickle section and allow it to slide back and forth. On the 451, these are typically bronze or plastic-lined shoes bolted to the cutter bar. They wear into an elongated slot pattern over time. When the clearance between the shoe and the sickle section exceeds about .030 inches, you will feel chatter in the cut and get uneven stubble heights. Replacing guide shoes is cheaper than replacing the cutter bar itself, and it fixes most of the vibration issues I see in the field. Wear plate or backing plate — This sits between the sickle section and the cutter bar and takes the lateral load. It is a flat steel plate with slots cut into it to align with the fingers. Wear plates get scored and grooved. A deeply grooved wear plate will accelerate sickle section wear because the fingers ride in the grooves rather than sliding on a flat surface. I have seen operators run wear plates until they were down to the bolt holes. Replacing the wear plate before it reaches that point saves the sickle section, which is the more expensive part. Pitman arm — This connects the driveline to the sickle section and converts rotational motion into reciprocating motion. The pitman arm has a crankpin at one end and a bearing at the other. The bearing ends typically wear out before the arm itself cracks. When the bearing is loose, you get a pronounced knock at operating speed and the sickle stroke becomes inconsistent. A worn pitman bearing also throws off the timing between the sickle and the reel, which causes material to be thrown ahead of the cut rather than fed into it. Checking pitman endplay takes about five minutes and should be part of every pre-season inspection.

Guard assembly — The guards sit over the sickle fingers and direct crop into the cutting zone. They are indexed by height and spacing. Incorrect guard height is the single most common cause of poor cutting performance on sickle bar mowers. The gap between the guard tip and the sickle dies should be between .010 and .030 inches. Most operators I have worked with set it too loose, which gives them a clean cut today but accelerates wear on both the guards and the sickle. Setting it tighter than you think you need to usually results in a longer useful life for both components. Reel — The reel positions the crop against the sickle. The 451 typically uses a four-slat or six-slat reel with rubber or metal tines. Reel height matters. The tines should be positioned just above the cutter bar, approximately one to two inches higher than the top of the sickle dies at their highest point in the cycle. Too low and the tines hit the sickle. Too high and you lose crop control, especially in heavy or lodging conditions.

Common Problem I Ran Into With the 451 Sickle Bar

About four years ago, I was running a 451 on a hay field with dense clover and the cut quality deteriorated rapidly. The sickle was sharp, the guards were within spec, and the pitman bearing showed minimal play. I had replaced the sickle section twice in the previous season and was frustrated because the new sections were wearing out in under fifty hours. I pulled the cutter bar, removed the sickle section, and inspected the wear plate closely. The wear plate looked fine visually, but when I rotated it and checked the opposite side, there was a grooved track running the full length. The plate had been flipped once during a previous repair, and the new side was already developing a groove pattern. More importantly, the guide shoes had worn to about .050 inches of clearance on the outboard side. The sickle section was flexing laterally under load, and the fingers were rubbing against the grooved wear plate instead of riding flat. That lateral deflection was what was destroying the sickle sections. I replaced the guide shoes, installed a new wear plate on the unused side, and the sickle life jumped to over two hundred hours on the next cut. The diagram would have shown the guide shoe wear limits if the manual had included them, but New Holland did not publish those specifications in the standard parts book. I learned to measure shoe clearance with a feeler gauge and replace them proactively rather than reactively. This is where most people go wrong and where a parts diagram alone will not help you. The sickle must be timed so that the dies pass the center of each guard at the proper point in the pitman cycle. The correct timing position is when the sickle dies are centered over the guard fingers and moving in the same direction as the reel tines. At this point, the cut occurs. If the timing is off by even a fraction of a revolution, you will get poor cutting, excessive wear, and material pushing ahead of the bar. To set timing, you need to rotate the driveline until the pitman arm is at the top of its stroke and the sickle section is centered in its travel. Then check that the dies are aligned with the guard centers. The method varies slightly by model year on the 451. Some versions have timing marks on the crankshaft gear and the pitman gear. Other versions require you to measure the distance from a reference point on the cutter bar to the end of the sickle section at both extremes of travel. If the travel is not equal on both sides, the timing is off. This adjustment typically involves loosening the gearbox mounting bolts and rotating the entire sickle drive assembly slightly, then rechecking and re-tightening. It takes about twenty minutes once you know what you are looking for.

Part Number Verification

When ordering from a diagram, always verify the part number against your machine's serial number. New Holland changed several sickle bar components during the 451 production run, particularly around 1995 and again in 2001. A diagram from a 1993 manual may list a different guard style than what your machine actually has. The serial number decal is typically located on the left side of the cutter bar housing or on the driveline shield. If you cannot find it, check the transmission housing near the PTO input. Having the correct serial number before you order saves the return trip and the delay, which is significant when you are trying to finish a cutting window. Parts diagrams show you what the parts are and how they fit together. They do not tell you about fitment tolerances, wear limits, or the sequence in which components should be serviced. The sickle section replacement sequence, for example, is not obvious from the diagram. You need to remove the drive end first, then the idler end, then lift the sickle section out from the bottom. If you try to remove the idler end first while the sickle is still under tension from the drive spring, you will fight the assembly the entire time. The diagram will show you the spring and the tensioning bolt, but it will not show you the correct removal order. That comes from experience or from a service manual procedure section, which is a different document from the parts diagram. Similarly, the diagram will show you the guard bolts and the spacer washers, but it will not tell you that the spacer washers between adjacent guards are optional on some configurations and mandatory on others. Running a guard setup without the spacers on a model that requires them will change the guard spacing and cause material to be pushed rather than cut. I learned this the hard way after installing a replacement guard set that was missing the spacers. The first pass through tall alfalfa pushed a wide swath ahead of the mower instead of cutting it. Adding the spacers back in corrected the issue immediately.

Reliability of Aftermarket Diagrams

Free diagrams found on auction sites and general machinery forums are often low-resolution scans of original drawings with incorrect part numbers assigned. I have seen at least three instances where a generic diagram assigned a Part A number to a component that was actually Part B on the genuine New Holland listing. The visual appearance of the parts was similar enough that a casual comparison would not catch the error. If you are ordering expensive components like a complete sickle section or a replacement gearbox based on a diagram you found online, verify the part number through at least one other source before placing the order. The time saved by using a free diagram is usually lost in the return shipping costs when the part does not match. Lubrication points shown on the diagram are usually grease fittings on the pitman arm bearings, the guard bracket pivots, and the reel shaft bearings. The sickle section itself should never be greased along the cutting surface. Oil or grease on the dies and fingers will attract dust and crop residue, which turns into an abrasive paste that accelerates wear significantly. A light application of cutting oil on the sickle section before operation is acceptable and sometimes recommended in heavy or humid conditions. Commercial sickle bar lubricants are available but they are optional, not required for basic operation. Checking the tension on the sickle drive spring is another item the diagram hints at but does not quantify. The spring should provide enough tension to take up slack in the reciprocating motion without binding. If the spring is too loose, the sickle will chatter. If it is too tight, you will accelerate bearing wear and increase driveline load. The correct tension is achieved when you can move the sickle section by hand with moderate resistance and it returns smoothly to center position when released. This adjustment typically involves a turnbuckle or an adjustable eye bolt on the spring attachment point.

When to Upgrade Rather Than Replace

If your 451 cutter bar is severely corroded or bent beyond straightening capability, replacing individual components from the diagram may not be cost-effective. A cracked cutter bar cannot be reliably welded and re-machined to specification. In those cases, sourcing a used cutter bar assembly from a salvage source and rebuilding it with new guards, wear plates, and guide shoes from the parts diagram is often more economical than buying a new bar. Used bars in decent condition are available through tractor auctions and farm equipment sales at a fraction of the new cost. The key is inspecting the bar rails for wear before purchasing. If the bar rails are worn to a rounded profile, no amount of new guide shoes will fix the lateral play issue I described earlier.