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.

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

Product ID: 61538198

🔩Durable aluminium & steel build

⚖️Lightweight 160g design

⚙️Precision 1" to 1-1/8" conversion

¥10610

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Sold byAmazon JapanDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

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Description

🚴‍♂️ Upgrade your ride, join the modern cycling elite!

LIGHT AS A FEATHER - At just 160 grams, it adds minimal weight while maximizing performance
ROBUST BUILD QUALITY - Aluminium body paired with steel bolt ensures lasting durability
OPTIMIZED FIT LENGTH - 135mm total length with precise insert marks for perfect installation
SEAMLESS MODERN UPGRADE - Transform your vintage bike with a sleek threadless stem fit
UNIVERSAL COMPATIBILITY - Designed for 1 inch quill steerers with 22.2mm internal diameter

The Profile Design Converter effortlessly adapts 1 inch quill steerer bikes to accept contemporary 1-1/8 inch threadless stems. Crafted from lightweight aluminium with a steel bolt, it offers a durable, precise fit with a total length of 135mm, making it the perfect upgrade for cyclists seeking modern performance on classic frames.

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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¥10610

Duties & taxes included

In stock

Sold byAmazon JapanDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

Buyer Protection · Full refund if your order doesn't arrive as described.

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

Secure transaction

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TrustScore 4.5 | 7,300+ reviews

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