Ask The Mechanic

Plow Bolts And Blind Nuts

Plow bolt and a blind, acorn nut, Image by Steve Thompson

Q: I have a couple of questions about nuts and bolts. I have “plow” bolts that I can’t keep tight, and I have a tech manual that is calling for a “blind” nut that is needed to secure an accessory I bought for my tractor. The question about the plow bolt is how to keep it tight, and the question about the blind nut is what in the heck is a blind nut (other than me)? The plow bolt holding my stationary blade on my square baler gets loose, and the knife on the plunger will then hit the stationary knife. What is a plow bolt doing on a hay baler?

A: Don’t worry; you are not a blind nut. These two fasteners do fall into the area of “farmer jargon solutions and questions.” The plow bolt is a bolt that has a circular head and shoulders that allow the round head to fit just under the item that it secures, which may not always be associated with a plow.

The plow bolt allows material (like dirt on a plow) to slide over it as it makes its way across the soil-engaging portion of the disc plow, moldboard plow or chisel point. The head of the plow bolt wears along with the ground engaging the plowshare, disc or point.

The key to keeping a plow bolt tight is to first tighten the nut, then hit the plow bolt head with a hammer a couple of times, then retighten the nut. Do this a few times, and the plow bolt will stay tight. As far as a “blind” nut goes--that’s a nut that you can’t see through. The blind nut has other names, such as the acorn nut. In technical jargon, if you can’t see through it, it’s “blind.”

Q: I have an older Echo line trimmer that I have enjoyed for years, but last year, toward the end of the season, it developed a problem that would not let it rev up. It would start and run at slower speeds. But, when I hit the throttle, it would try to run faster but would begin missing and smoking. I put a carb kit in it, but that didn’t help. The lawn mower shop told me to change the air filter. I did that, but I still had the problem.

A: More than likely your muffler or spark arrester screen is stopped up. This little screen can be at the entrance of your exhaust gas into the muffler. Some states require this spark arrester to be in the exhaust flow to cut down on the possibility of starting a fire from a piece of hot carbon.

If the muffler or screen (many times caused by excessive oil mixed in the gas) gets partially plugged, the exhaust can’t flow out of the engine. Pappy Thompson once said, “No exhaust out, no air in.”

Clean or replace the plugged muffler and/or spark arrester screen. With the muffler off, take a look at the exhaust ports in your engine. If they are partially plugged, clean them with a wooden dowel. Plugged ports can cause the same problem.

Q: My lawn mower shop told me over the phone that I might want to bring my mower in for a spring cleaning, which included removing the head and cleaning the carbon off the piston and head. He said that carbon built up in that area can cause the valves to burn. How can carbon make my valves burn?

A: Excessive carbon on the piston and head will decrease the area for the explosion in the fire ring area. A decreased area on the compression stroke means the compression ratio of your engine will go up--changing time in which the explosion occurs in your engine. Simply put, your engine timing will shift.

When this happens, the explosion on compression happens before it was designed to occur, which, in turn, changes the valve action with the piston location.

If carbon becomes excessive, when you start your mower, you could feel a “kickback.” This effect comes from the fact that the explosion is happening so advanced because of the compression ratio. That causes the piston to be blown back down before it normally should after it goes over top dead center and enters the power stroke.

Safety Tip of the Month:

Be careful when checking the spark on an engine by grounding the spark plug near the spark plug hole. If the engine floods, it will spray gas out the spark plug hole and set everything in its path on fire.

I can personally tell you that it can happen before you have time to think about it. It set my pants on fire, and on the way to the water hose, I set the side of the gas can on fire. The guys in the shop laughed, and I got a lesson.

Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email


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