Why do my parts have mismatched bolt holes?
We’ve had a lot of questions lately about bolt patterns and why bolt holes don’t always match up, so here’s a brief overview of bolt layout.
Bolts holes are set up so that a bolt can pass through 2 parts and secure them together. The goal is for this connection to be tight and easy. Some ways we ensure that securing parts results in a tight seal are making sure that parts have the same standard and use the same measurements.
There are two units of measurement when it comes to bolt holes. They are either standard or metric. Standard and metric measurements will not match, so if you start with standard, do not deviate from standard!
There are a lot of standards that exist to maintain specifications that apply to aspects like external pressure and internal pressure. For example, the National Aerospace Standards Committee (NASC) provides standards used in aircraft while the American National Standards Institute (ANSI) is used on a lot of military sea crafts. If you are in the U.S. Coast Guard or the U.S. Navy, check your regulations before ordering a part and make sure that it complies to military specifications.
Most of the time, the easiest way to figure out what you need is to use a chart. Most flange dimension charts are going to be similar to the one on the right side of this page, so today, I’m going to walk you through how to read it.
Start by examining the part you already have. For example, if you have a 150lb flanged gate valve and need a flange connection to pair it with, use a chart for a 150lb flange. The top of the chart tells you what the chart is for. This chart is for use when we need a 1150lb flange meeting a specific military need (MIL-F-20042), so this is what we need.
Starting from the left column going right, definitions are as follows:
Size: This is the size of the pipe or tube going into the flange. If you are using a 1-4″ pipe, your flange will be a size 1-4.
Diameter: This is the distance from the outer edge of a flange to the opposite edge. This is important to know, because if your flange is too wide to fit into the intended space, you may need to rethink your layout before ordering flanges.
Thickness: This is how thick the flange is. It’s important to have the right flange to match the pressure it needs to hold. Some flanges may have the same bolt hole layout, but are not intended for the same purpose. The thickness may be the only differentiating factor that you notice, but it will save you a lot of hassle later on if you are paying attention.
Bolt Circle: This is the distance between the center of one hole to the center of the hole opposite to it.
Number of Holes: The number of holes is a great way to double-check whether or not you ordered the right part. If your valve has 16 holes but you are ordering a flange with 19 holes, chances are you are ordering the wrong part.
Hole Size: This measures the actual size of the hole. If you are ordering standard sizes, you will not see any fractions like the ones in this chart because metric measurements will all be in decimals. If you have mismatched hole sizes, you will either not be able pass a bolt through both parts or your fittings will be loose. Don’t try to monkey-rig parts together that don’t fit! If the parts aren’t secured tightly there will be more chances for shifting and vibration which ultimately leads to more expensive repairs.