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Forstner Bit Extender

Item #
88% of 100

Get an extra 2-1/2" of reach into deep holes with this Forstner bit extender. Great for rustic furniture and joining wide timbers. Also works with any other style of 3/8" shank drill bit greater than 1" in diameter. Forstner bit sold separately.

  • Works with 1" diameter bits or larger, smaller bits will not allow extender to enter the hole
  • 3/8" shank with triangular flats for positive grip
  • 2 hex-head screws securely grip drill bit
  • Socket for drill bit accepts 3/8" shank bit, and extends 1-1/2" into extender
  • Total shank measures: 2-3/16" long - flattened triangular section: 1-1/2" long
(1) Forstner Bit Extender
More Information
Brand Insty-Bit
Manufacturer Part Number 4" EXTENDER
Weight 0.2600
Tech Spec
  • Length: 4"
  • Width: 3/4" at Widest Section
4.4 / 5.0
57 Reviews
5 Stars
4 Stars
3 Stars
2 Stars
1 Star
I got the Forster 7 piece kit it makes things a lot easier & the extender is great thanks
March 25, 2020
1 month ago
Woodworking Experience:
Easy to use.
Easily attach Forstner bits for a deeper depth. Two set screws hold tight.
February 22, 2020
2 months ago
Woodworking Experience:
bought this item but have yet to use it. will have it when needed.
January 30, 2020
3 months ago
Came in handy
So far it's working good
January 6, 2020
3 months ago
Woodworking Experience:
Works as it should.
A necessity when doing salt and pepper grinders.
December 22, 2019
4 months ago
Woodworking Experience:
Just enough length
Exactly what I needed to add the length to my Ryobi (in this case) forstner bit. Zero issues.
December 18, 2019
4 months ago
Woodworking Experience:
Looks solid enough
Have not had a chance to use it yet, but it appears to be cleanly and solidly built.
November 11, 2019
5 months ago
Worked great
Was making a peppermill and needed to get my forstner all the way through the wood. worked great
September 15, 2019
7 months ago
Woodworking Experience:
Forstner extinction
Works like a charm lol works great just what I expected from Rockler nothing but the best
June 11, 2019
10 months ago
Woodworking Experience:
The shaft!
Got this awhile ago to run some romax and one run was so close I couldn't use the bit I had. Used this to go through the next joist and had just enough to make it through. Perfect length!
May 20, 2019
over 5 years ago
Woodworking Experience:
Start typing your question and we'll check if it was already asked and answered. Learn More
Browse 9 questions Browse 9 questions and 68 answers
Why did you choose this?
Rockler Store
Needed to add a 1" more to my hole saws.
Gregory B on Mar 21, 2020
JULIE E on Dec 20, 2019
Needed to add a 1" more to my hole saws.
Gregory B on Mar 21, 2020
to extend reach of forstner bit
John D on Jan 23, 2020
JULIE E on Dec 20, 2019
Priced right with good reviews!
MIKE LANE R on Dec 12, 2019
donald w on Nov 19, 2019
Trying to extend holes in acrylic
Robert O on Nov 17, 2019
To add to my woodworking collections.
Zack C on Nov 1, 2019
woodworking kits requiring 4 to 5 inch drills.
james p on Sep 30, 2019
Need to drill 4 consecutive absolutely straight 7" holes to mount a heavy slab mantel.
Doug H on Aug 29, 2019
Didn’t have one
Robert B on Aug 10, 2019
Larry B on Jul 27, 2019
There are times this product is useful and necessary.
George C on Jul 18, 2019
Sometimes I need to drill further into the 4x4 treated wood post than the normal bit can reach. Hoping this will help with this issue.
MIKE H on Jul 8, 2019
Only place I could find this item
Thomas W on Jul 6, 2019
I need it.
E T on May 10, 2019
Need an extension for deep holes
Daniel B on Apr 25, 2019
For deeper holes.
J I on Apr 22, 2019
Why not?
Madison D on Apr 6, 2019
hard to find short extension elsewhere
John B on Feb 18, 2019
It should fit my drill bits, and that is what I need.
Susan S on Feb 10, 2019
Extend reach of bit.
Donald V on Feb 4, 2019
needed it
Scott M on Jan 23, 2019
My bits are too short and your extender is a lot less expensive than Woodcrafters (theirs is twice as long and without shipping, costs twice as much as yours with shipping).
Dave on Jan 3, 2019
Do not have one.
Richard R on Dec 28, 2018
To use with my lathe
Albert M on Dec 17, 2018
Needed for other projects
Gary L on Dec 13, 2018
Christmas Gift.
Mark S on Dec 2, 2018
Need for single humidor kit
David N on Nov 30, 2018
In case it is needed for deeper holes.
Al on Nov 29, 2018
Nice to have on hand. Filled in the order to qualify for free shipping.
Anthony on Nov 18, 2018
for longer blanks
Steven J on Nov 4, 2018
because they are so hare to find anywhere
Jim on Oct 24, 2018
To make sure there is enough length on my bits.
Darrel G on Sep 28, 2018
because I cant buy long forstner bits
John H on Aug 19, 2018
It was on sale and had more than 4.0 stars.
Dan H on Aug 1, 2018
I needed to drill deeper holes
Marion H on Jun 17, 2018
Bill B on May 9, 2018
drill deeper holes
JOSH J on Apr 20, 2018
can use it on some of my turning projects
richard m on Apr 4, 2018
for purchased tool
Brian R on Mar 17, 2018
Longer range
Randy C on Mar 8, 2018
To go with kit purchase
Rick H on Mar 1, 2018
Not sure the bit is quite long enough . Say's it doesn't work with bits smaller than an inch. Say's it's 3/4" at widest point. My bit is 7/8" Going to take my chances .
Bryan M on Mar 1, 2018
price and reviews
Jose M D on Feb 16, 2018
Need an extender for my bits
Robby on Feb 15, 2018
Good price
John W on Feb 12, 2018
I have a mortiser but the throat is too deep and my bits are too short to fit into the chuck.
Ivan G on Feb 7, 2018
I purchased the Forstner Bit Set and think I may need this for my projects.
Gerald B on Jan 31, 2018
Need this with forstner bit
John H on Jan 25, 2018
Need to drill deeper holes
DOUG A on Jan 5, 2018
to extend reach of forstner bit
John D on Jan 23, 2020
Priced right with good reviews!
MIKE LANE R on Dec 12, 2019
will this work with the Jig IT........?
Billy S on Dec 14, 2015
BEST ANSWER: Sorry, I don't know. I bought it to use with a forstner bit and Chuck in lathe tail stock.
can you connect two 2.5 " extenders to my bit to drill a deeper hole?
M G on Dec 14, 2015
BEST ANSWER: No you can not. The ID of the extender is .375 inches and the Shank OD is .403 (Too Big)
I was hoping to do this too but the shank is almost a 1/32 too big.
What is the ID of the set screw end?
Jay R on May 11, 2015
BEST ANSWER: I measure at 3/8". Forstner bit is a snug fit. I use Bosch and Porter Cable bits.
what is the diameter of the extender? in other words what is the smallest forstner bit i can use?
Troy M on Mar 7, 2017
BEST ANSWER: It's designed for 3/8", I would think anything smaller would be offset and not be true. The set screws are both on the same side
Can Forstner bit extender be used with Rockler drill guide? Or are long-shank bits required?
Neal G on Apr 2, 2018
Forstner Bit Extender
Forstner Bit Extender
Rockler JIG IT® Drill Guide
Rockler JIG IT® Drill Guide
BEST ANSWER: A long shank bit, such as Rockler item 10117, is required with this jig.
will this work with the portable drill guide with Ryobi 3 flat sided Forstner bits?
Elenita E on Aug 27, 2016
BEST ANSWER: I have round round w/flat and 6 sided bits that all work well. I use it in a chuck in the tail stock of a lathe. Quite a bit of pressure and it has held secure each time
What is the OD of the female end? Can I use a 7/8" bit in this extender?
Craig on Oct 16, 2018
BEST ANSWER: No......3/8" shanks only
I used the extender for three times, both set screws are now stripped. I need to replace them. What are the measurements of these set screws? Thank you.
Louise D on Jul 6, 2017
Prop 65 WarningWARNING: Cancer & Reproductive Harm -

Prop 65 WarningWARNING: Drilling, sawing, sanding or machining wood products can expose you to wood dust, a substance known to the State of California to cause cancer. Avoid inhaling wood dust or use a dust mask or other safeguards for personal protection. For more information go to