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How To Machine A Concave

The grinding and measuring was time consuming, but if you figure that spending six or eight hours grinding the concave saved $1,000 for a new one, you’re being paid over $100 an hour.

Our son Ben’s Massey Ferguson 750 combine needed a new concave. The old one had been rebuilt once and was now totally worn out, as were the rub bars. We checked around and found out the price of a new one. We didn’t like that too much. Used ones were about half the price of a new one, but we had to drive a long way to get one. Then we looked in the parts combine we had bought. It was burnt beyond repair in the engine compartment but the concave was usable. We yanked it out (which is a lengthy procedure as you will know if you’ve done the same job) and I thought it needed a tuning to bring it up to par.

Now, before you get too far into this article let me warn you: If you’re expecting us to use a huge boring machine that weighs seven tons and would fill a grain truck box, you’re going to be disappointed. Perhaps that’s what a person should use, but unfortunately we didn’t have one. However, we did have a side grinder and a straight edge. What more to you need for a farmer fix?

We also had a couple of good books by Ray Stueckle on combine performance, theory and upgrades. If you haven’t heard about Mr. Stueckle, he used to have a column in Grainews two or three decades back. (He has since passed on but one of his books are still in print. It’s available from Stueckle Publications at P. O. Box 1323 DR, Caldwell, Idaho, USA 83605 or 1-208-459-1507. You may also find some of his books used on eBay or through some other used book sources.) I’ve learned a lot from these books and they were just plain interesting to read. Some of the info is very specific to certain makes, and sections include Stueckle’s answers to problems readers had submitted. He has sections on the various combines, including Masseys, IH rotaries and New Holland rotaries. However, don’t blame Ray for the side grinder idea. That was just me.

Now back to that concave. New concaves and cylinder rub bars are supposed to be perfectly parallel, like this:

When they get worn they look like this, with the middles worn down more than the sides:

If they get too worn, then they are past repair and you’re stuck with replacing them. However if there is still 5/16-inch of height above the filler bars in the middle, they are still usable. The trick is to make both the concave and the rub bars perfectly flat so they can thrash properly. If you’re buying new rub bars they won’t need much attention, but the concave should be ground to make it work the best.

A note about filler bars: Filler bars are flat plates of steel that go over (or under) the wires in the front of the concave. These are necessary to thrash

the grain properly. According to Mr. Stueckle, the first third of the concave should be covered. (If you have nine spaces on the concave, cover the first three spaces. You must have 5/16-inch of concave bar above the fillers to thrash the grain properly. You could put the filler bars under the concave if you haven’t got enough height on the concave.

Basically, all we did was grind the outer edges down on the concave so the bars were flat and not dished out in the middle. It takes awhile and you have to make sure that the concave is ground evenly so that none of the bars are lower than the rest. I made a cardboard pattern in a quarter-circle shape to check the progress of my grinding.


Once the concave was done, we installed it and levelled it. Levelling is very important. Stueckle suggests that zero clearance at the back is the best, so it has to be perfectly level at the back and square with the cylinder.

Then we spun the cylinder to make sure all the rub bars were exactly the same distance from the concave. If the combine has put even one rock through it, the cylinder can be bent enough that the bars are uneven. Shim up rub bars where necessary so the clearance is exactly the same on each bar along its full length.

You can make a set of HD feeler gauges out of various thicknesses of flat iron (1/16-inch, 1/8-inch, etc.) so you can check

this easily. Even with a new set of rub bars, you may have to grind a few high spots down. On a worn set, you might have a lot of grinding to do to get your clearances correct.

Stueckle comments that, “The only time cylinder bars need to be changed is when they will no longer pull the material into the cylinder.

“The height on the ridges on the bars has very little to do with thresh. Remember, concaves and cylinders do not thresh the grain. They are only the agents to cause the material to rub against itself to do the threshing.”


On concave wires: Mr. Stueckle recommends that you take out every second concave wire and weld up the holes where the wires came out. As we were growing peas this was necessary so we wouldn’t crack the peas. Both the concaves we worked on needed the wires taken out. It’s necessary to weld up the holes or plug them with rivets so straw doesn’t get stuck in them.

A word about Stueckle’s modifications: While poking around on the Internet looking for the address for his books, I found a few items claiming his modifications didn’t work as well as advertised. This is kind of like a Ford or Chev debate. Some people love his modifications and others (especially engineers whose work he redesigned) don’t like his modifications. No matter what you think, I have still learned a lot from his books and they helped me to repair our concave and fine tune the cylinder.


The machined concave threshed quite well and should hopefully last a few more years. Was it a lot of work? The hardest part was taking out the concaves and juggling the feeder houses around. By the time we had taken out three concaves and put two back in, we were definitely looking for a break. The grinding and measuring was time consuming, but if you figure that spending six or eight hours grinding the concave saved $1,000 for a new one, you’re being paid over $100 an hour.

I hope this helps you to get a few more years out of your old concave and keep a bit of that hard earned money in your pocket.

Ron Settler, his wife, Sheila, and their sons Ben and Dan farm and run a repair and salvage business at Lucky Lake, Sask.

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With AutoSteer, the windrower maintains a continuously full header so you can cut more acres per hour. You get the job done faster and burn less fuel.

AutoSteer provides path to path accuracy in straight lines, curved paths or circles. The module is easily transferred to other tractors, sprayers or combines using optional switch kits. That way you can bale, chop or combine using the same headings that you set while windrowing.

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