If you’re running a combine harvester operation, you know that threshing performance directly ties to your bottom line—too much grain left on straw or chaff means lost yield, while too much damage means downgraded grain quality that buyers won’t pay top dollar for. As a combine harvester supplier with 18 years of on-farm service experience, I’ve seen countless operators dial in settings wrong, costing them thousands in missed revenue. The good news? Adjusting the concave isn’t rocket science—if you follow the right steps tailored to your crop and conditions. Combine Harvester
First, let’s break down what the concave actually does, because you can’t adjust what you don’t understand. The concave is a curved metal grid plate that sits just above the rotor or cylinder of your combine harvester, creating the gap where crop moves through during threshing. That gap controls how much force the grain is exposed to as it’s beaten free from the head, while the bar spacing (the gaps between the concave bars) separates grain from straw chaff. The two key settings here are concave gap size and bar configuration—both change based on what you’re harvesting and the conditions in your field.
I want to emphasize this early: one size never fits all. Last season, a customer of mine in central Illinois was harvesting soft red winter wheat, and he kept adjusting his concave to run tighter because he thought more threshing force would mean more grain out. But instead, he ended up with 12% grain damage and too much fine chaff mixing with the good grain, which had to be cleaned twice—costing him 8 hours of extra time and $1,200 in rejected load fees. When we switched his concave gap to a wider setting, he cut damage to 2% and recovered 9% more grain per acre. That’s the difference between a little tweak and a costly mistake.
Let’s start with the basics of setting concave gap, which is the first adjustment you’ll make before starting a run. The general rule of thumb is: smaller gap = more threshing force, larger gap = gentler threshing. But how do you measure that gap accurately? Most combines have a gauge on the left and right sides of the concave—always check both, because if one side is off, you’ll get uneven threshing. For small grains like wheat, oats, or barley, start with a gap of ¾-inch to 1-inch at the beginning of the season. Wait, why start there? Small grains have brittle stalks and smaller kernels, so you don’t want to shatter them. For larger grains like corn or soybeans, start with a wider gap—1¼-inch to 1½-inch—because their kernels are bigger and need more space to move through without getting crushed.
But here’s where conditions change everything. If you’re harvesting standing, dry grain in mid-season, you can run a slightly tighter gap because the stalks are firm and the kernels are mature. If you’re harvesting lodged grain (that’s grain that’s fallen over and tangled in the field) or wet grain early in the season, you need a wider gap. Lodged grain gets pushed through the combine at a slower speed, so more time near the concave means you don’t want to over-thresh. Wet grain is softer, so tight gaps will turn kernels to mush and clog the concave bars. I once worked with a farmer in Iowa who harvested soybeans at 18% moisture content and ran a 1-inch concave gap—he ended up with a clogged concave that shut down the combine for 2 hours in the middle of a rain shower. We adjusted the gap to 1¾-inch, ran the fan a bit faster, and he was back up without a single clog the rest of the run.
Next, bar configuration. Most combines come with interchangeable concave bars—there are three main types: spiral bars, rasp bars, and wire-mesh bars. Each is designed for a specific crop. Rasp bars are the most common, with sharp, raised ridges that grip the crop and pull kernels free. They’re ideal for small grains like wheat and barley. Spiral bars have a twist to them, which creates a rolling motion for the crop—great for corn, because it prevents the heavy corn ears from getting stuck. Wire-mesh concaves have small holes that let fine chaff pass through, which is perfect for hay or straw baling operations, since you want to keep the straw intact.
A common mistake I see is using the wrong bar type for the crop. Last year, a customer in Minnesota was harvesting corn and stuck with his wheat rasp bars—he got 15% more damaged kernels than necessary, and the combine had to work twice as hard to move the tangled corn stalks through. We switched him to spiral concave bars, and his damage dropped to 3.5%, and his combine fuel use went down by 11% because the crop flowed smoother. That’s a double win—better grain quality and lower operating costs.
Now, once you’ve got the initial settings, you need to fine-tune them as you move through the field. This is where testing a small strip first is non-negotiable. Don’t just dive into a 100-acre field and set it and forget it. Stop every 10 to 15 minutes, unload a small sample of grain, and check two things: grain damage and un-threshed grain left on the straw. You can do this easily by taking a handful of straw after it exits the walker and rubbing it between your fingers—if you feel whole kernels, you need a tighter concave gap. If you pull apart a kernel and see a broken interior, you need a wider gap.
Another key indicator: threshing cylinder speed. The concave setting and cylinder speed work together. If you have a tight concave gap, you’ll need a slower cylinder speed to avoid damage. If you have a wide gap, a faster cylinder speed will help you thresh more grain without harming it. For small grains, cylinder speed is usually 800 to 1,200 RPM; for corn, it’s 500 to 800 RPM. If you’re combining at higher ground speed (more acres per hour), you’ll also need a tighter gap to keep up with the volume of crop moving through. But here’s the catch: don’t push ground speed so high that the combine can’t process the crop—too much volume will cause clogs and poor separation, no matter your concave settings.
I also want to talk about seasonal adjustments, because what works in August for wheat isn’t the same as what works in October for soybeans. Early in the season, when crops are greener and have higher moisture, run the widest possible gap to prevent clogs and grain damage. Mid-season, when crops are dry and mature, you can tighten the gap a little to get every last kernel. Late in the season, if stalks are dry and brittle, run a slightly wider gap to avoid shattering kernels as they move through the concave. One of my long-term customers in Kansas harvests wheat in two passes each year—first, he uses a wide gap to take most of the grain, then a second pass with a tight gap to recover any missed kernels. That little extra pass gives him an average of 7 bushels more per acre, which adds up to thousands of dollars over a 2,000-acre farm.
Now, what about common issues that come from wrong concave settings? If you’re seeing excessive un-threshed grain (this is called “tailings” in combine terms), your concave gap is too wide, or your bars are worn out. Rasp bars wear down over time—after 500 hours of use, their sharp ridges get smooth, so you need to replace them or adjust the concave gap a little tighter to compensate. If you’re seeing too much grain damage, your gap is too tight, or your cylinder speed is too high. Another issue: clogs. Clogs happen when crop gets stuck in the concave bars—usually from wet crop, too tight a gap, or too high ground speed. If you get a clog, never reach into the concave while the combine is running—shut off the engine, engage the brake, and clear it with a long stick. Trying to clear a clog while the machine is on is one of the most common combine injuries, and it’s 100% preventable with good settings.
I’ve talked to so many operators who think the concave is a “set it and forget it” part of the combine, but that’s the biggest mistake you can make. Every field is different—even two adjacent fields of the same crop can have different moisture levels, stalk thickness, and lodged crop that needs slight adjustments. Last month, I was at a farm in Ohio where two neighboring fields of soybeans looked identical, but one had 2% moisture and the other had 12% moisture. The operator ran the same concave gap for both, and he got 18% damage on the wet field and 3% on the dry. A ½-inch gap adjustment would have fixed that easily.
As a combine harvester supplier, I always tell operators that the concave is the heart of the threshing system. Get that right, and the rest of the combine (cleaning, separation, ground speed) will work so much smoother. I’ve seen operators upgrade to newer combines thinking that will fix poor performance, when all they needed was a ¼-inch concave gap tweak and new rasp bars. It’s not about spending more money—it’s about understanding how the settings interact with your crop and conditions.
If you’re still unsure about your concave settings, or you’ve noticed consistent issues with grain damage or lost yield, don’t guess. Reach out to our team to discuss your specific operation, crop type, and field conditions. We can provide on-site support to adjust your settings, recommend the right concave bars for your harvest, and answer any questions you have about optimizing threshing performance. Every farm is unique, and we take the time to tailor our guidance to your needs, because your success is our success.
Rice Transplanter References:
- ASABE Standards. (2022). Test Procedure for Measuring Threshing Performance of Combine Harvesters. American Society of Agricultural and Biological Engineers.
- Smith, J. (2021). Combine Harvester Adjustment and Maintenance Guide. Farm Equipment Association Press.
- Purdue Extension. (2022). Harvesting Small Grains: Concave Gap and Threshing Tips. Purdue University Department of Agricultural and Biological Engineering.
- USDA Agricultural Marketing Service. (2023). Grain Quality Guidelines for Combine Harvesting Operations. U.S. Department of Agriculture.
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