Introduction: How to Fix/make Sinking Office Chair Reusable

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I had an unusable, sinking office chair and wanted to fix it without spending any money, so I only used what I already had in my apartment (not house/garage/workshop, so anyone should have what I do).
To make the seat stand fully up at all time I needed a cylinder (excellent compression resistance when the force is perpendicular to its circular bases) that I could slide along the strut, ideally without play –so a perfect fit. And I found just the thing I was looking for: a piece of PP water pipe, 32mm exterior diameter and 1.6mm wall thickness; it fits so well, it might as well have been designed for this purpose.
To prevent the edges (bases of the cylinder) from splitting, I tensioned them using zip-tie loops. I found the zip-ties in a Corsair computer power supply box, so if you kept everything that came with the power supply of your old computer you might find some there.

Materials:
--Polypropylene(PP) water pipe: 32*mm ext. diameter, 1.6mm thickness, length bigger than 130mm but anything you have laying around the house is ok if it fits your optimum seat height;
* different chairs may use different thickness gas struts so make sure the outer diameter of the shiny part of the strut exactly matches the inner diameter of the PP tube !
--zip-ties, small

Tools :
--rubber head hammer or regular hammer wrapped in a cloth so as not to damage the chair
--measuring tape or other measuring tool for measuring the optimal length of the tube
--a marker for marking where to cut
--a cutting tool for cutting the PP pipe with;
--optionally, a 4*mm hexagonal key or 10*mm hexagonal socket for removing the seat and backrest ensemble in order to make handling easier.*my seat uses hexagonal screws that require the 4mm key, yours may require a different size so take a look at it first.

Step 1: Optional Step – Remove Seat and Backrest Ensemble

0-unscrew the 4(in my case) screws connecting the seat to the metal support, using a 4mm hexagonal key (if you have ever assembled a furniture kit from Ikea or another company then use the hex key supplied in that kit, see if it fits) or a 10mm hexagonal socket wrench; this operation is not mandatory but it makes easier the handling of the parts we are working on: the strut and its housing.

Step 2:

--holding chair upside-down, vigorously tap the metal support of the gas-strut on all sides until it falls off;

Step 3: Step 2

--having removed the black plastic, telescopic, cylindrical dust protections (see image from step 8), put the chair  upright,  put back the seat without pressing it on too much and measure the distance between the end of the seat and the beginning of the black gas cylinder(in my chair it was 127mm or about 5 inches);

Step 4: Step 3

-- measure 127mm (or whatever your value from the previous point was) starting from the thick end of the  PP tube .
* If you want a lower seating position then cut a smaller length piece; however, by cutting a piece smaller (say 90mm) than the length of the strut(127mm), when you sit on the chair it will collapse 127-90=37mm and ram the PP tube with the momentum of your weight times the speed it has gained in those 37milimeters, potentially reducing the tube’s life. You can increase the tube’s strength by adding zip-tie loops, tightly tied to its circumference; then, it should be fine.

Step 5:

--cut it perpendicularly to the cylinder's axis.
Voila! the part you need. I tightly tied zip-ties at the points where I thought it was most likely to fail in case of uneven loading;

Step 6: Step 5

--press the tube down on the strut (tight fit);

Step 7: Step 6

-- tie two zip-ties to make a loop and wrap it tightly around the tube (one loop in its lowest part, where the PP tube makes contact with the strut cylinder and another loop in its upmost part, where it meets the seat) using a pair of pliers if necessary but don't pull too hard as that might damage the plastic zip ties;

Step 8: Step 7

--put back the telescopic plastic dust protections;

Step 9: Step 8

--insert the tip of the strut in the seat's housing (red arrow) and press it so that the housing's ridge enters the gap between the strut and the surrounding PP pipe's flared end. It should be a perfect fit;

 You've done it!  The seat is usable once again.