I'm currently designing a steel structure for a gym machine. I want to assemble two rectangular tubes, one being able to slide inside the other, for height adjusting. I intend to use a spring ball pop pin to sustain the inner tube.

I have not decided which fit and tolerance value to use yet, however, when looking for different dimensions of rectangular tubes, I only find gauges catalogues. These dimensions don't comply with the fits and tolerance dimensions mentioned in handbooks, so my question:

Can tubes for fits and tolerance be purchased? if yes, where can I find them? If no, how can I get those? At a local workshop? Should they be machined?

  • 1
    $\begingroup$ My squat rack has loose tolerances by machining standards and works just fine. You do not need linear rail tolerances. $\endgroup$
    – DKNguyen
    Sep 7, 2021 at 15:10

3 Answers 3


There may be multiple sources for your requirements, off-the-shelf purchases, not manufactured on spec.

I have constructed cylindrical telescoping assemblies by using 4130 cromoly steel with appropriate wall thickness. If you consider that a common wall thickness is 0.058 inches, a tube with an outside diameter of 3.00 inches will have an inside diameter of 3.00 - 0.058 - 0.058 or 2.884 inches. This is quite close to 2.75 inches, another common diameter. This provides a clearance of 0.13 inches, allowing for sliding without slop.

The cylindrical stock can be found at Aircraft Spruce and Specialties, but I was unable to find the rectangular versions I recall from the past. On the flip side, McMaster-Carr is also a reliable source for materials and this link points specifically to telescoping steel tubing.

telescoping steel tubing with holes

Image courtesy of linked site.

The site also contains other useful items such as the pip pin you referenced.


Before my answer, I need to point out what IMHO is the most significant difference between fits and tolerances.

  • fits usually refer to a mating of two components (an assembly).

In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or, at the other end, are temporarily or permanently joined together. (source wikipedia).

  • Torelance can refer to many things, but more usually refers to the permissible limit or limits of variation in a physical dimension.

Metallic bars and HSS.

In the above context, when you buy a metallic bar/HSS section there is usually no fitbe no fit. E.g. only tolerances of the diameter of the bar, or other types of tolerances (orientation, location, form etc), are relevant.

So (to my mind) when you buy a product a steel bar from a shop/vendor you accept the tolerances that this particular batch has. If you can order a much larger quantity (economies of scale), you can contact directly the manufacturer and have them produce the product in your specific tolerances. So in that respect they are manufactured.

interchangeable products

There is a certain class of products (again in my mind - this is very much an opinionated answer), that I call interchangeable. In that category fall e.g. bolts or bearings. It is very common products (of high engineering design, competence and expertise) that on their own don't perform much, but are essential parts of other machines (most if not all).

This particular type of products, are so common and yet specialized that you can buy them into classes of tolerances. e.g. SKF has for its bearings a class tolerance Normal, P6, P5 and it goes to P4 and below in what it calls "super-precision" bearings. Similar to bolts.

My point is that in that particular context (interchangeable products) it is possible to buy different classes of precision/tolerances (and not just manufacture them)


For rectangular or square you are pretty much limited to the manufacturer and his reps. Some distributor may happen to have an inventory containing a size you want, but it would be chance. Round tubing is different because some distributors will be able draw down or expand inventory to produce custom sizes; for a price. Also , with round , some distributers can swage down an end to fit into another round. It would be most practical if you could work with standard products.


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