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 Converter 1 Inch Quill Steerer To 1 1

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Description

🚴‍♂️ Upgrade Your Ride, Elevate Your Style!

UNISEX DESIGN - Perfect for all riders, making it a versatile addition to any bike enthusiast's toolkit.
EASY INSTALLATION - Quick and hassle-free setup, so you can hit the road faster.
TRANSFORM YOUR RIDE - Easily convert your 1 inch quill steerer to a modern 1-1/8 inch threadless stem.
DURABLE CONSTRUCTION - Crafted from high-quality materials for long-lasting performance and reliability.
UNIVERSAL COMPATIBILITY - Designed to fit a wide range of bikes, ensuring you can upgrade without limits.

The Profile Design Converter allows you to seamlessly transition from a 1 inch quill steerer to a 1-1/8 inch threadless stem, making it an essential upgrade for any cyclist looking to modernize their bike setup. With universal compatibility and easy installation, this converter is perfect for both casual and serious riders.

Specifications

ASIN	B0028N14GQ
Best Sellers Rank	70,880 in Sports & Outdoors ( See Top 100 in Sports & Outdoors ) 99 in Cycling Stems
Customer reviews	4.5 4.5 out of 5 stars (1,225)
Date First Available	12 Aug. 2010
Department	Unisex
Item model number	ACTHRCONKT18
Language	English
Manufacturer	Profile Design
Product Dimensions	14.61 x 3.18 x 3.81 cm; 113.4 g
Release date	6 Nov. 2018

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Desertcart purchases this item on your behalf and handles shipping, customs, and support to Croatia.

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Profile Design Converter: 1 inch Quill steerer to 1-1/8 inch threadless stem One Size,ACTHRCONKT18

Description

Specifications

Common Questions