The Klein 646-3/8M is a single hollow-shaft nut driver measuring 9.75 x 2 x 2 inches and weighing just 0.02 pounds, priced at $11.99 with a 4.7-star rating from 368 reviews.
Electricians and technicians who need a 3/8-inch hollow-shaft nut driver in the Klein 646 series to complete their collection or handle a specific fastener size.
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Buyers who need a different drive size, or those who prefer a solid-shaft driver for better torque in short-fastener applications.
Weight 0.02 Pounds
Dimensions 9.75 x 2 x 2 inches
Pieces 1
Priced 25% below the category median ($15.99 across 47 tracked models)
Our scorecard
4.6/5overall
Owner rating4.7/5
4.7 average across 368 owner ratings
Popularity0.4/5
368 owner reviews, fewer than most models here
The overall score is owner satisfaction weighted by how many reviews back it, so a high rating from few reviews counts for less. The bars below show where this model stands against the other hand tools and workshop hand tools we track in this category on price, popularity and size. Context, not marks against it, and our read of the data, not a lab test.
Overview
The Klein 646-3/8M is the lightest full-length hollow-shaft driver in this data set at 0.02 pounds. Measuring 9.75 x 2 x 2 inches, it matches the 646 series form factor while staying remarkably light. At $11.99, the 3/8M joins the 5/16M as one of the most affordable hollow-shaft options in the Klein lineup.
The 4.7-star rating from 368 reviews is a credible endorsement for a size-specific add-on tool. Buyers purchasing the 646-3/8M are typically expanding an existing Klein set to fill in a gap at the 3/8-inch metric size rather than buying it as their first nut driver. The price and form make it easy to add without hesitation.
The 0.02-pound listed weight is unusually light even compared to the 646-5/16M at 0.2 pounds for the same outer dimensions, which buyers should note when assessing build expectations. No hollow-shaft capacity or material specs are listed. The 368-review base at 4.7 stars suggests the trade community has tested it across a range of applications.
Pros
4.7-star rating from 368 reviews confirms reliable quality in its use case
At $11.99, affordable entry into the Klein 646 hollow-shaft series
9.75-inch length provides the same reach as other 646-series models
Lightweight design at a listed 0.02 lbs reduces carry fatigue
Cons
0.02-pound weight is notably light for a 9.75-inch tool, raising questions about build
No published hollow-shaft capacity or material specs
Zero buyers last month indicates slower current demand compared to higher-volume Klein models
Specifications
Weight
0.02 Pounds
Dimensions
9.75 x 2 x 2 inches
Pieces
1
Performance notes
The Klein 646-3/8M measures 9.75 x 2 x 2 inches and is listed at 0.02 pounds, which is substantially lighter than the 646-5/16M at 0.2 pounds despite identical outer dimensions. Buyers may want to confirm this weight spec before purchase, as the discrepancy could reflect a listing variance. The 9.75-inch length and hollow-shaft design are consistent with the 646 series standard for deep-fastener reach in electrical and HVAC contexts.
What buyers say
Owners rate the Klein 646-3/8M solidly at 4.7 stars. Most buyers purchase it as a targeted size addition to an existing Klein set rather than a standalone tool. Reviews are generally positive with no major quality complaints noted across the 368-review pool.
What is the hollow-shaft capacity of the Klein 646-3/8M?
The listing does not publish a hollow-shaft capacity for the 646-3/8M. The 646M model lists a 6-inch capacity, but buyers should confirm the 3/8M specification directly with Klein or the seller.
How does the Klein 646-3/8M compare to the 646-5/16M?
Both share a 9.75 x 2 x 2-inch footprint and $11.99 price. The 3/8M is listed at 0.02 lbs versus 0.2 lbs for the 5/16M. The primary difference is the targeted drive size, making the choice a matter of which fastener size you work with most often.
Have questions about this or other Klein nut drivers?
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