My kid lost that small base piece to his "20" Thruster Dirt DS Boys' Mountain Bike" Will this work as a replacement?

It depends on the size of the steer tube and stem sizes. I'd the bike has a one inch threaded steer tube it'll work. But, you may have to get a new stem with a 1 1/8 inch collar.

how many spacers could be fit under this adapter?

You don't use spacers for this. You only use spacers for a real threadless setup. This just emulates a threadless and allows you to use a 1-1/8 stem. You just set the height you want and tighten the allen bolt

what stem size does it fit? 1" or 1 1/8"

1 1/8".It inserts into forks with 1 1/8" threaded headsets (actual diameter measurement is 1"). It fits 1 1/8" threadless stems (1 1/8" actual measurement).

How high can you raise the stem above the headset?

I have lifted my stem by 25mm from the top of the threaded steerer into which the extender inserts. Done roughly 2500km, including a 500km bike tour over gravel terrain - no issues whatsoever.

Profile Design Threadless Size Converter 1 1 1 8

Fully operational. Shipping worldwide, on schedule.Customs & duties included in the price

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Fully operational. Shipping worldwide, on schedule.Customs & duties included in the price

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Description

⚡ Upgrade your ride, ditch the old tech, and join the threadless revolution!

FEATHERLIGHT BUILD - At just 0.8 ounces, it adds zero bulk while boosting your bike’s performance.
POLISHED SILVER FINISH - A premium, corrosion-resistant sheen that elevates your bike’s aesthetic and durability.
VERSATILE RISE ADJUSTMENT - Gain up to 25mm rise with spacers for a customized, comfortable riding posture.
PRECISION FIT FOR 1 1 1 8 TUBES - Engineered to perfectly convert between standard quill sizes, ensuring a rock-solid fit.
SEAMLESS QUILL TO THREADLESS UPGRADE - Effortlessly modernize vintage forks with threadless stems for sleek compatibility.

The Profile Design Threadless Size Converter is a precision-engineered adapter that converts traditional 1" quill forks to modern 1-1/8" threadless stems. Weighing only 0.8 ounces and finished in polished silver, it offers a lightweight, durable solution for vintage bike upgrades or custom builds. Compatible with standard steerer diameters and allowing up to 25mm rise adjustment with spacers, it’s the perfect component for riders seeking seamless integration and enhanced bike ergonomics.

Specifications

ASIN	B0028N14GQ
Best Sellers Rank	#32,783 in Sports & Outdoors ( See Top 100 in Sports & Outdoors ) #19 in Bike Stems
Brand Name	Profile Design
Color	Silver
Customer Reviews	4.5 4.5 out of 5 stars (1,261)
Date First Available	September 24, 2009
Included Components	Stem Column Adaptor
Item Dimensions LxWxH	5.75 x 1.25 x 1.5 inches
Item Package Dimensions L x W x H	7.44 x 3.23 x 2.24 inches
Item Weight	4 ounces
Manufacturer	Profile Design
Material	Multi
Model Name	Threadless Converter
Model Year	2016
Number of Items	1
Package Weight	0.19 Kilograms
Part Number	15920002
Style	Stem Column Adaptor
Suggested Users	All Ages/Unisex
Warranty Description	Limited Warranty

Reviews

R**R

3.04.2026

L**U

8.08.2024

F**N

11.11.2019

G**.

18.03.2026

F**E

19.06.2023

I**B

24.11.2020

A**O

25.12.2024

C**.

5.11.2025

S**H

6.06.2025

A**U

2.08.2024

E**G

24.03.2022

M**A

15.03.2025

B**N

29.04.2015

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Profile Design Threadless Size Converter (1" - 1 1/8")

Description

Specifications

Reviews

Common Questions