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

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Fully operational. Shipping worldwide, on schedule.Duty & taxes paid at delivery

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E£ 3935

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Sold byAmazon EGYPTDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

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Description

⚡ Upgrade your vintage ride with modern precision — don’t get left behind!

FEATHERLIGHT BUILD - At just 0.8 ounces, it adds minimal weight while maximizing your bike’s handling agility.
POLISHED SILVER FINISH - A premium, corrosion-resistant look that complements any bike aesthetic with timeless style.
BOOST YOUR RIDES COMFORT - Enables use of suspension forks, reducing impact on wrists and shoulders for longer, smoother rides.
SEAMLESS THREADLESS UPGRADE - Effortlessly convert classic quill forks to modern threadless stems for sleek performance.
PRECISION FIT FOR VERSATILITY - Engineered to adapt 1" forks to 1-1/8" threadless stems with exacting 25.4mm and 22.2mm diameters.

The Profile Design Threadless Size Converter is a lightweight, polished silver adapter that converts 1" quill-style forks to fit 1-1/8" threadless stems. Designed for seamless compatibility and enhanced bike performance, it supports up to 25mm rise with spacers and is ideal for modernizing vintage bikes or custom builds.

Specifications

ASIN	B0028N14GQ
Best Sellers Rank	#175,467 in Sports & Outdoors ( See Top 100 in Sports & Outdoors ) #183 in Bike Stems
Brand Name	Profile Design
Color	Silver
Customer Reviews	4.5 4.5 out of 5 stars (1,266)
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	0.25 Pounds
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

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