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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Description

⚡ Upgrade your vintage ride with precision and style!

BUILT TO LAST - Engineered with tight tolerances and premium materials for reliable performance
SLEEK SILVER SHINE - Polished finish that elevates your bike’s aesthetic and resists corrosion
FEATHERLIGHT UPGRADE - Adds minimal weight—just 0.8 ounces—keeping your ride nimble
SEAMLESS COMPATIBILITY - Effortlessly converts quill forks to modern threadless stems
ADJUSTABLE RISE POTENTIAL - Supports up to 25mm rise increase with spacers for personalized fit

The Profile Design Threadless Size Converter is a lightweight, polished silver adapter that converts 1" quill-style forks to 1-1/8" threadless stems. Designed with tight tolerances and premium materials, it supports up to a 25mm rise increase when used with spacers, making it the perfect upgrade for modernizing vintage bikes while maintaining a sleek, durable finish.

Specifications

ASIN	B0028N14GQ
Best Sellers Rank	#342,534 in Sports & Outdoors ( See Top 100 in Sports & Outdoors ) #403 in Bike Stems
Brand Name	Profile Design
Color	Silver
Customer Reviews	4.5 4.5 out of 5 stars (1,269)
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 kg
Part Number	15920002
Style	Stem Column Adaptor
Suggested Users	All Ages/Unisex
Warranty Description	Limited Warranty

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

Description

Specifications

Common Questions