You would think that bike seat posts are a standard size, sorry they’re not.  While upgrading my wife’s bike to have a suspension seat post, I found that the seat post didn’t fit so I had to get a seat post shim to get everything to work.

Let me show you how to measure and install a bike seat shim.

A bike seat shim allows you to adapt different seat posts by using a shim.  The important thing is to correctly measure and buy the correct shim. The replacement seat post MUST be smaller than the bike seat tube.

Measuring for bike seat post
Measuring for bike seat post

When To Get a Seat Post Shim

There are two reasons for you to get bike seat post shims:

  • when you want to reduce your bike’s weight
  • when you want to increase your bike’s comfort

But before ordering a new seat post, make sure to measure the diameter of your old one. This way, you will have the time to purchase a shim and have it ready once your upgrade seat post arrives.

DIY MTB Pro Tip: Is your seat flopping and dropping? Check out this article 👉 How to Fix a Seat That Won’t Stay Up

Get a New Seat Post to Reduce Your Bike’s Weight

One reason to get a new seat post is to shed some weight off your bike. If you are going to use your bike very often, making your bike lighter means making it faster. So, make sure to replace a seat post that is lighter than your old one.

If you worry that a lighter seat post may not fit your bike’s seat tube, a seat post shim can solve your problem.

In addition, it is ideal for replacing your regular steel bolts and screws for titanium as it is lighter than steel. While the screw and bolt replacement may be minimal, every gram counts when replacing a seat post.

Get a New Seat Post for An Added Comfort

If you want to make your bike more comfortable, the ideal thing is to replace your old seat post with a suspension seat post. A suspension seat post is perfect for mountain bikes as it absorbs bigger hits. It also reduces the vibrations that come off the road.

As a result, your ride will be smoother than before. You will even feel less fatigue at the end of your ride.

When To Get Your Seat Post Shim

Once you have a new and upgraded seat post, you need a seat post shim to ensure that the new seat post will fit your bike frame. Bike seat post shims usually consist of steel, aluminum, or PVC. It allows you to increase the diameter of your seat post. This way, you can install a narrow seat post into a broad seat tube.

So, in an instance where your new seat post does not fit your seat tube, you need to purchase a shim and use it as an adapter. (source)

How to Measure for a Bike Seat Post

Bike Seat Post
Bike Seat Post

Chances are you have experienced making the mistake of purchasing a new seat post, only to find out that it does not fit your mountain bike. When I changed my seat post, I learned that you should only deal with the diameter. The reason is that the length of the seat post depends on the geometry and size of your frame.

That said, you only need to measure the seat post’s diameter correctly when replacing it.

Measuring The Seat Post’s Diameter

Getting the diameter of your seat post is essential if you use bike seat post shims on your mountain bike.

Using calipers is the most convenient and accurate method to get your seat post’s diameter. For instance, you measured the outside diameter of your seat post, and you got 27.2mm. On the other hand, the diameter inside the seat tube and you got 31.8mm. Meaning your seat post is too narrow for your seat tube.

For this reason, you need to install a seat post shim to fit the seat post on the seat tube.

Measuring The Seat Tube’s Diameter

You can use calipers to measure the seat tube’s diameter. But you can also use seat post sizing rods if you want the measurement to be accurate. You only have to insert the rod into the seat tube. The correct measurement will be the first number on the rod above the seat tube’s end.

How To Choose a Seat Post Shim?

How to select a Seat Post Shim
How to select a Seat Post Shim

Bike seat post shims are adequate for fitting your small seat post to your larger seat tube. However, you cannot simply decide that you need a seat post shim and proceed to buy one.

To choose the suitable seat post shim for your bike, you first need to measure your seat post’s diameter. Then, measure the internal diameter of your seat tube. Once you have these measurements, the next thing you need to do is to purchase a seat post shim that fits your bike.

For instance, if your seat post’s outer diameter is 29.8mm or the seat tube’s internal diameter is 27.2mm, then look for bike seat post shims that fit these measurements.

However, it is essential to note that a shim only lets you increase your seat post’s diameter. If you want to install a more extensive seat post and a smaller seat tube, you will need to get a new post. (source)

DIY Mountain Bike Tip: Bike seat post shims are inexpensive. Don’t skimp, get the right size. Having a bike seat slip can be dangerous. Amazon delivers fast, check out this short cut link the brand I bought. – BIKE SEAT SHIM

Common Seat Post Diameters

A bike’s seat post has an essential role in providing you with a comfortable ride. For this reason, you need to ensure that your seat post perfectly fits your bike frame.

Moreover, most of the standard seat posts that you can find on the market have a similar form and function. Still, there are several differences that you need to know when replacing your seat post.

In my years of biking, I learned that the most significant factor to consider when choosing a new seat post is its diameter. The reason is that your seat post’s diameter needs to fit snuggly with your seat tube, or else it will slide. That said, you need to know what the most common seat post diameters are.

Moreover, the most common seat post diameter for mountain bikes that cost more than $500 is 27.2mm.

On the other hand, Huffy, Mongoose, and other big-box bikes often have a standard diameter of 25.4mm. From my experience, using a bike with a narrow post provides a comfortable ride even on rough, bumpy roads.

There are also oversized bikes with a standard diameter of 30.9mm or 31.6mm. Such bikes have extra strength and stiffness, which guarantee ideal power transfer. For this reason, it can resist failure and bending.

If you have an oversize post and you want to install a light and standard 27.2mm seat post, you can use bike seat post shims to do so. (source)

How To Stop a Seat Post from Slipping?

One way that I made my bike comfortable and efficient to ride is to adjust the seat post to a height that fits me best. But over time, my seat post began swiveling left and ride and slipping down.

If you are experiencing the same dilemma with your seat post, here are five easy ways to fix them:

  • Reduce the grease on your seat post. Too much grease can cause your seat post to slip, so wiping some off can make it less slippery.
  • Use carbon paste– a gel that consists of tiny beads which reduce the friction between the seat post and seat tube.
  • If you do not have a carbon paste, hair spray will help you stick your seat post with your seat tube.
  • Tighten your seat post clamp by rotating the seat post collar 180 degrees. This solution will help you tighten your seat post if it is bent.
  • If nothing works, it is time to replace the seat post clamp. (source)

Are Seat Post Shims Safe?

Bike seat post shims do an excellent job of holding your seat post and seat tube together when they do not fit snuggly. They feature an insertion length that prevents the bike frame from having any crack. This tool is generally safe as long as the shim has the same length or is longer than your seat post’s minimum insertion length.

However, I found out that there is one problem that bikers may encounter when using bike seat post shims – corrosion.

If you use a metal seat post shim, it will become rusty over time. For this reason, you need to grease your bike shim if you will install it in a metal frame. If you have a carbon frame, you need to treat the shim with an assembly paste to prevent rust.

Installing Bike Seat Post Shims

Another essential factor that makes your seat post shim safe is its installation. If you installed your shim yourself, it would be best to ensure that using a seat post shim is safe by testing its secureness before hitting the trail.

To install a seat post shim, the first thing that you need to do is to grease or treat your shim with a carbon paste. Then, place the seat post into the frame, ensuring it aligns properly with the seat tube.

Finally, tighten the collar to the manufacturer’s required setting. This way, you can ensure that the bike seat post shims remain in position. (source)

DIY Bike Seat Post Shims

One of the simplest yet effective hacks for making a do-it-yourself seat post shim is to use an aluminum can. For this hack, you will need:

  • aluminum can
  • a pair of thick gloves
  • a sharp pair of scissors
  • grease

The steps to making your seat post shim are simple. But before anything else, wear a pair of thick gloves to ensure that the aluminum can’s edges will not cut your skin.

  1. Then, cut the aluminum can to make a thin roll of aluminum.
  2. Now that your DIY seat post shim is ready, get your bike and slide the post into the frame without any bike seat post shims installed. Measure your desired saddle height and mark the area where you will clamp the post in the frame.
  3. Grease the inside of your frame and the seat post.
  4. Wrap your DIY aluminum seat post shim tightly around the seat post, making sure that all the edges are flat. The upper part of your aluminum shim needs to sit right at the marked area. This way, you will entirely clamp your shim between the seat post and the tube. You need to ensure that the shim is as close to the seat tube’s edge as possible so it can fill the gaps that make your seat post slip.
  5. Tighten the clamp, and it’s all done. (source)

YouTube Video:

Where To Get Shims for Bike Seat Posts

It is easy to find bike seat post shims. In fact, just a few clicks on the internet will lead you to online bike shops that sell different bike accessories. You can also use Amazon to purchase bike seat post shims.

Keep Rolling

From many years of biking, I learned that a snug fit seat post is the key to a comfortable ride, especially on bumpy trails. For this reason, you need to use a seat post shim to ensure that your seat post fits perfectly to your bike’s seat tube. However, you need first to measure your seat post’s outer diameter and the inner diameter of your seat tube. This way, you can ensure that the seat post shim you purchase will fit your bike.

You can also make a DIY seat post shim. Cut an aluminum can and wrap it tightly around the seat post, as if you are using a manufacture-made bike seat post shim. With this hack, you can prevent your seat post from slipping down as you ride it.

MTB Tools I Love and Recommend

Bike Hand Repair Stand
Bike Hand Repair Stand
Bike Hand 37 pcs Tool Box
Bike Hand 37 pcs Tool Box
Topeak Smartgauge D2 Air Pressure
Topeak Smartgauge D2 Air Pressure

I own each of these tools and only recommend things I own and use.

  • Bike Hand Bike Repair Stand.  Nice mountain bikes don’t have a kick stand so keeping your MTB safe but conveniently stored is essential.  I keep my bike on my stand whenever I’m not riding it.  This makes it easy to lube the chain, inflate the tires and adjust the derailleur.  Highly recommended – Bike Hand Bike Repair Stand (👈 Link to Amazon to see what thousands of others have said)
  • A basic MTB toolbox for replacing a chain, adjusting brakes and dialing in the fit.  Bike Hand has a 37-piece box that has most of the specialty bike tools to keep your MTB properly maintained.  The Bike Hand brand is value packed for the avid rider.  Check out the competitive prices with this link to Amazon 👉 Bike Hand 37 pcs Bike Repair Tool Kit
  • Get a good air pressure gauge, if you get just a tiny bit serious about MTBing you’re going to start playing with tire pressure.  A couple psi can make your tires sticking or not.  Get a good gauge, I highly recommend the Topeak Smartgauge D2, it’s accurate, flexible and easy to use.  An Amazon best seller, here’s a link 👉 Topeak Smartgauge D2
  • Carry a multitool with you on every ride.  I’m serious, most of the time you can MacGyver something to get back to the trailhead if you have a multitool.  I’ve got the Crank Brothers M19, it’s worn, rubbed and abused – but it still works.   Thousands sold on Amazon – check it out with this link 👉 Crank Brothers M19


David Humphries is the creator of DIY Mountain Bike. For me a relaxing day involves riding my mountain bike to decompress after a long day. When not on my bike I can be found wrenching on it or making YouTube videos at 👉 DIY Mountain Bike Read more about David HERE.


  1. Fred Milson, Complete Bike Maintenance New and Expanded Edition: For Road, Mountain, and Commuter Bicycles. MVP Books, 2011.
  2. Joe Lindsey, “DIY Aero: Convert Your Road Bike Into An Aerodynamic Speed Machine In Just A Few Simple Steps.” Bicycling. Vol. 44, No. 7. (August 2008) 76.
  3. Christopher Wiggins, Bike Repair and Maintenance. USA: Penguin Group Inc. 2014.
  4. Jennifer Sherry, “Owner’s Manual.” Mountain Bike. (November 2008). 83-84.
  5. Bike Magazine, “Bike Hack: Shim Sham,” Youtube Video 0:04, posted by Bike Magazine, June 5, 2019.