Clausing 8520 Mill

Technical Reference on the Clausing 8520 / 8530 Mill

The Clausing 8520 is a bench top mill, and is highly sought after due to its size. It is early broken down into lighter parts which make it easy to move. It is very rigid for its size, well designed and easy to work on and maintain.


The main mass produced models of this mill were the 8520, wich is the most common model.

The 8520 and the later 8530 have a Morris Taper 2 nose spindle, while the 8525 and the 8535 have the less common Brown and Sharp #7 taper. The Morse Taper 2 spindle is much more desirable as it is much easier to find tooling for it.

The mill was offered in many other variants during its life:

  • Model 8512 that was sold without a head, or custom configured to other applications
  • There was a model specially made and fitted with a Bridgeport M head
  • There were some early models that had hand weels only on the right side.
  • There were some early models that apparently left the factory painted tan instead of gray.

If you are interested in more history on these machines please visit Tony Griffiths wonderful site. Tony has a great deal of historical information on not only these machines but just about every metalworking machine ever made. The 8520 page is available here.

There are some other pages on the net with information on this milling machine.

  • There is a Yahoo Groups Page here
  • Ron Thibault’s has some information here
  • Machine Manuals has some information here
  • Igor Chudov 8530 has pages of photos  1 2 3


Parts and Tooling:

Some parts and manuals for this mill are still available from Clausing.

Clausings contract information:

Clausing Service Center

811 Eisenhower Drive South Goshen, IN 46526

Phone: 800-535-6553


My archive of documentation



New Morse Taper 2 tooling is readily available, due to some of the newer import milling machines also taking MT2 tooling. Depending on your draw bar configuration (See Below) all the tooling should be 3/8-16 threaded.

Besides eBay I have purchased tooling from the following.

  • has lots of MT2 tooling and arbors available.
  • CDCO Machinery also has some MT2 tooling.
  • has Brown and Sharp and MT2 tooling.
    • I purchased a 3/4 inch endmill holder from them.
    • The only source that I could find.

One of the questions that come up from time to time is what size vice and what size rotary table works best on the mill. I have a Enco Model #425-7240 with out the swivel base that I purchased which is a good fit for the mill.

 I have a small 4 inch rotary table, but a 6 inch fits nicly also. A 8 inch is really too big for the table.


I have installed a DRO on my mill. Here is my info on my DRO install.

Replacement Belts

The Mill uses standard automotive type 4L belts.
Part Numbers are 4L-220 and 4L-480


The mill table uses 9/16 T nuts.
I find that the 1/2 inch T Nuts work fine also.


Over the life of the 8520, there have apparently been several variations on the drawbar design. The earliest ones had a ring brazed onto the 3/8-16 drawbar, which fit under a large knurled nut on the top of the spindle. The end of the drawbar was ground 1/4 inch square, and they supplied a small right-angle wrench to loosen or tighten the drawbar. The ring would either push or pull, so you could eject the collet by simply loosening the turning the drawbar. This is a nice set up and no hammer is needed. My mill has this set up, and I like the way it works. I built a wrench to fit it from 1/4 inch socket and a 1/4 inch hex wrench, works great! (See my projects page)

Later drawbars used two nuts on the top of the drawbar, still leaving the big knurled nut on top of the spindle. This required the use of a brass hammer to dislodge collets.

The last batch of machines they made didn’t have threads on the top of the quill for the knurled nut, and the bottom nut on the drawbar rode directly on the top of the spindle. You still have to use a brass hammer on these.

Restoration Information

Reported Paint Color Matches

One user found this was a match on his mid-60’s 8520.

The paint was color matched by a local paint dealer to a small part that I brought in to the store.

Benjamin Moore paint, labeled “Urethane Alkyd Gloss Enamel Base 4 M22 4A”
The color code is:

Another user color matched the paint on his mill and found found
Benjamin Moore & Co.,Urethane Alkyd Gloss Enamel “1567 Night Train”
They use a Deep Base M22 3B to start with, and then add the following tints. 
UTC Gallon Formula:
OY 0x21.00
BK 1×28.00
TG 0x10.00
WH 1×4.00


Quill Drive Plate Spring Plungers:

Apparently not all mills came from the factory with these installed. Some mills have been reported to have nylon plungers installed instead of the brass. The plungers are adjusted for quiet running and for added drag to hold the quill from dropping easily. 
These are 1/4-20 screws 180 deg apart drilled/tapped into the plate edge.
The brass/nylon plungers bear on the high parts of two splines.

 If you mill does not have them its an easy job to add them.
Each hole is directly opposite from the other (on diameter).
Drill thru 13/64 dia (oversize) to the center, centered on the edge of the drive plate disc. The holes are also centered on a gullet of the drive plate spline.
Tap the hole 1/4-20 about 1″ deep. Make a brass plug to be a slip fit in the hole 3/8″ length.  Fit a coil compression spring (.028 dia wire x .26 long x 3/16 dia) to sit on the plug. Adjust the set screws  to compress the spring and push the plug onto the spline.

Lead Screws

David Shaws Site (Now gone but was here Link)
Had an article on replaceing the leasscrew with standard 3/4 inch acme rod.
Here is a local PDF copy of his old site.


The gears in the quill can easily be broken. Here are some notes I collected on replacement gears:

One owner found one of the gears in his mill was marked Boston Gear S-3232.
Their online catalog shows this to be a 32 DP 14.5 degree PA 32 tooth

The last pinion that drives the last gear that slides on the spline is 24 DP instead of 32 DP.

It turns out that the small spur gear portion of the 8520 compound gear (Clausing part 990-065 – cost $66) is a close match to McMaster-Carr part no. 6325 K31. Cost $7.79. A bit of cutting to get the hub thickness down to 1/4 inch, some finish sanding and reassembly using 1/4″ dia. cold rolled stock as a shaft and a broken length of 1/16″ drill bit as a pin to lock the two gears – and everything works. Whole job took about an hour and a half.

When I purchased my mill the gears seemed a little crunchy. It turned out that they were just dirty with dried grease. Here are some photos I found on the net that show the gear placements.

Rebuilding the Mill Head

There are two nice articles on rebuilding the head on this mill.
Dennis Pantazi on changing the bearings – Link – Local PDF
Fred did a wonderfull job of documenting his rebuild – Local PDF


All the bearings with the exception of the lower head bearings are commonly available from many sources. The Lower Head Bearings are speced as a Class 3 which specified as having a runout of 0.0003, and were sold as a ground matched set . They are no longer available from Clausing.


Lower head bearings

    • Qty (2)
    • Clausing number 044-005
    • Size (25x52x15)
    • Use Fafnir 7205WNSU
    • These are 39 degree angular contact bearings.
    • They are flush ground for minimum preload when duplexed (2 bearings together)

Upper Head bearing
Qty 1
Clausing Part Number
Use Fafnir 304KDD or, sub 6304ZZ/ND 3304/KBC 6304
Size 20x52x15mm

Upper spindle bearing
Qty 1
Clausing PN 044-016
Size 20x47x14mm
Use MRC 204SFF, FAG equiv to
Use Fafnir 204KDD, 6204ZZ,304KDD

Spindle driven step pulley bearings
Qty 2
Clausing PN 044-007
Size 25x47x12mm
Use Fafnir 9105KDD, Fafnir 9105KDD, 6005ZZ

Tables and Upper idler pulley
Qty 6
Clausing PN 044-004
Size (12x32x10mm)
Use Fafnir 201KDD, Fafnir 201KDD, 6201ZZ

Lower Idler pulley
Qty 1
Clausing PN 044-003
Size 15x35x11mm
Use Fafnir 202KDD, Fafnir 202KDD, 6202ZZ

Knee screw upper trust bearing
Qty 1
Clausing PN 044-011
Use Andrews D-3, standard D-3 thrust bearing for replacement
Size A .625 – B 1.244 – C .563

Knee shaft bearings
Qty 2
Clausing PN 044-008
Use NICE 1607DS, 1607ZZ, 1607
NICE 160105 ( Part Number of bearings in my mill)
Size A 7/16 – B 29/32 – C 5/16

One thought on “Clausing 8520 Mill”

  1. The main question to ask is do you need tooling that large. I love my 8520, it is a nice size mill for my shop. Most of what I work on is on the smaller side. The largest mt holder I have is 3/4 inch, and in it I usually have a 1 inch reduced shank end mill, that I use as a fly cutter. 1/2 inch end mills are the largest I usually use for cutting, and most of the time I use 3/8 inch cutters. For the few larger jobs that I do I might have to make two passes to but a wide slot etc, but that is a rare job for me.

    I also have a Mini-Mill here, and there is a night and day difference between the mini mil and the Clausing. The bridgeport would not fit in my shop. (Low ceilings) and I have found that I have never had a job that I had to go the the bridgeport on that I could not do on my Clausing. If I had to list one thing that was a limitation on the Clausing it would have to be the table size not the MT2 spindle.

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