Conical Silicone Plugs Medium Size Ø 20 to 53 mm

Medium Tapered Silicone Pipe Plugs for 20 to 53 mm

A seat tube coated with powder on the inner wall will no longer accept a seatpost; a coated bearing seat will no longer hold a bearing cup. These medium-sized tapered silicone plugs are engineered specifically for such precision fits. They seal pipes, frame openings, and caliper piston bores between 20 and 53 mm inner diameter, maintaining their shape in the curing oven and pulling out residue-free afterward. Because the taper offers several millimeters of range, 10 sizes cover virtually any round tube in this spectrum.

For diameters under 20 mm, choose our small tapered silicone plugs from 0.8 mm; for larger openings, select our large plugs from 48 mm upward. You can find caps, thread plugs, and pull-through plugs under silicone plugs for powder coating.

Masking Bicycle Frames, Brake Calipers, and Tubing

Bicycle Frames: Seat Tube, Bottom Bracket, and Head Tube

Bicycle frames feature three critical areas that require exact tolerances post-coating. The seat tube matches the outer diameter of the seatpost, commonly 27.2, 30.9, and 31.6 mm. The bottom bracket shell measures approximately 34 mm for BSA threads, 42 mm for BB30, and 46 mm for Pressfit PF30. For the head tube, dimensions depend on the headset standard: 34 mm for EC34, 44 mm for ZS44, and 49 mm for EC49 or ZS49. Headset cups and Pressfit bearings are press-fitted with interference tolerances. If a powder layer adds 60 to 120 µm per side internally, the diameter shrinks by up to 0.24 mm, making it impossible to press in cups without excessive force.

In a BSA shell, the plug keeps internal threads clean. If powder still deposits on the outer threads, have the threads chased at a bicycle shop using a specialized bottom bracket thread tap (standard taps will not fit). Head tubes measuring 56 mm (ZS56) can be masked using the smallest size of our large silicone plugs. Matching shades for frames can be found under Candy powder coating.

Coating Brake Calipers: Protecting Piston Bores

Before a brake caliper enters the curing oven, pistons, seals, and dust boots must be removed. The piston bore and seal groove must remain completely free of coating; otherwise, the new seal will not seat properly and the piston will bind. Because calipers are cast metal and brake fluid resides in the metal pores, degrease the caliper thoroughly and outgas it before masking to prevent bubbles and pinholes during baking. Automotive brake pistons typically range between 38 and 57 mm, while motorcycle pistons run between 25 and 34 mm. For pistons 54 mm and larger, use our large plugs.

Bleeder ports and hydraulic line connections on the caliper feature threads (typically M10). Seal these using silicone thread plugs. Protect pad sliding surfaces and mounting faces on the steering knuckle using strips of heat-resistant tape.

Railings, Garden Furniture, and Bearing Seats

Railing tubes with a 42.4 mm outer diameter feature an inner clearance of 36 to 38.4 mm depending on wall thickness, while 33.7 mm tubes measure roughly 28.5 mm. The plug keeps tube ends clean so end caps or connectors fit smoothly without post-processing. Tolerances are even tighter on bearing seats: a seat for a 6203 deep groove ball bearing with a 40 mm outer diameter typically carries an H7 tolerance, offering just 25 µm of margin. Even a thin layer of coating renders it unusable.

Which Plug Fits Your Tube?

Measure the inside diameter using internal caliper jaws (or the core diameter on threads). The closer your measurement is to the thick end of the plug's range, the deeper the plug seats into the tube. If it sits at the absolute top of the range, little material remains to grab for removal, in this case, select the next size up. If it sits at the bottom edge, only the tip engages, risking displacement inside the oven.

The specifications describe standard commercial dimensions. Please measure your component prior to ordering.

Practical Tips for Tubes and Hollow Components

• Leave one opening clear. If you seal a tube at both ends, trapped air pressure expands by roughly 60 percent at 200 °C. The pressure can push out the plug or cause bubbling along weld seams. Always leave a vent hole or one tube end open.
• Designed for round tubing only. In square and rectangular tubing, a round cone seals at only four contact points. Heat-resistant tape across the opening provides a cleaner solution for non-round profiles.
• Plan hanging points in advance. When masking head tubes and seat tubes, suspend the frame by the dropout or another unmasked point. Suitable hooks and hanging systems ensure proper grounding contact.
• Twist to remove. After curing, remove the plugs with a twisting motion while the workpiece is still lukewarm. Score any thin powder film along the rim beforehand with a knife to maintain a smooth edge.

Learn how to prepare frames and add-on parts from blasting through to the final finish in our step-by-step powder coating guide.

Features at a Glance

• 10 sizes for inner diameters from 19.8 to 53 mm
• Uniform length of 25.4 mm (1 inch)
• Solid silicone rubber construction, temperature stable up to 315 °C
• Suitable for liquid paint, e-coating, anodizing, and electroplating according to manufacturer specs
• Color-coded by size to eliminate re-measuring in the workshop
• Reusable until the silicone becomes brittle or cracks
• Available in packs of 5 or 10 depending on size, with tiered discounts up to 50 or 100 units

Questions from the Coating Workshop

Do I really need to mask the inside of the tube?

Deep inside a tube, minimal powder penetrates because the tube acts as a Faraday cage, shielding the electrostatic field. However, the first few centimeters do receive coating, and that is precisely where press fits for seatposts, bearing cups, or end caps sit. Plugs are well worth using in those areas.

Do I need to cover the face surfaces separately?

Yes, if components must sit flush against them later. This applies particularly to bottom bracket shells and head tubes, where bearing cup shoulders seat face-to-face. A tapered plug seals only the internal bore, leaving the face exposed. Mask the faces with tape or have them faced at a bicycle shop post-coating.

How do these plugs differ from plastic end caps?

Plastic caps made of PE or PVC are designed for shipping and deform under oven curing temperatures. Silicone maintains its elasticity and seal throughout high-temperature bake cycles.