Bridgeport M Head Rebuild (for the Benchmaster Mill) – Pt. 1

This post will chronicle the rebuilding of my Bridgeport M Head milling head that will be put onto my Benchmaster mill. I’ve heard of the “Benchport” but I’ve never seen photos anywhere online. Most people seem to say it won’t work, but none of them have said they tried or have seen it done and failed. So with that, my plan is to rebuild this M Head that I bought on Craigslist and put it on my 1948 Benchmaster mill. I may post about the BM mill later – that was a worthwhile restoration and I enjoy using the mill often.

Benchmaster Mill (BM)

(A few other things I may post about later are my Light timpani project, new welding hobby, and the ring roller I built to make timpani hoops and repair existing ones. Be prepared for a technical geek-out.)

The Bridgeport M Head was original built in the mid-20th century and featured a 1/2 HP motor and several tapers for holding tools in the spindle. Mine has a Morse Taper no. 2. This is nice because so is my BM. No additional tooling is needed to get started. The spindle (lower) section of the head is made of cast iron and the upper portion is (usually polished) aluminum. Mine came painted and gunked up. The gunk also hid several broken portions which must be fixed.

Pulley casting as received:

Under the badge was the original finish:

Cracked motor mounting area:

Stripped thread on other motor mount:

Oiler port cracked and stripper (and Gits oiler missing):

The first part of the project involved checking the motor to make sure it was still functioning. I bought a Variable Frequency Drive (VFD) to run the 3 phase motor and it tested fine, but it was obvious it needed new bearings. So the whole thing came apart and was cleaned using paint stripper, Purple Industrial Cleaner, and a fine wire wheel. It will get new sealed bearings when put back together.

The castings have been primed, but I’ll be sanding them later, re-priming, and then putting a coat or few of Rust-oleum Charcoal Grey on them. Once the paint has hardened (I’ll give it a week or more), I’ll reassemble the motor with new bearings and hook it up to the VFD again to test it. Once done, it will be sitting on the shelf waiting for when the whole head is rebuilt.

The next (concurrent) part of the project is the aluminum pulley casting. When I bought this on CL, the seller sent me a picture of ANOTHER head, leading me to believe I was getting something a (good) bit different. The picture had a nicely kept appearance, no missing parts, and no stripped bolts head with a mounting bracket. The package I received contained a grimy mess, missing parts, stripped and missing bolts, and pieces missing from the casting. Oh, and no mounting bracket. It was so bad that the casting couldn’t have held a motor or oiler. So… a quick run down of what I did:

Removed all the paint and cleaned the casting in the Purple cleaner. This was dangerous since the cleaner is not meant for aluminum. However, it was worth it to get the 1/16″ of oily grime off in addition to some paint. Removing all the paint and grime allowed me to see any additional cracks or missing parts. Luckily, no more surprises were found.

Cracked motor mount:

Extra holes (some will be filled, some may be used…)

Oiler port hole:

All cleaned up and ready to fix:

I milled the cracked and stripped oiler port down. I messed it up a little by moving the wrong handle on the mill. Not a huge deal. (The irony of working on one mill using another one isn’t lost on me…)

Milled a 3/4″ flat hole, drilled a 7/16″ hole farther down:

Made a 1/8-27 NPT threaded aluminum piece to be press fit into the casting:


Side view:

Next, the motor mounting “threaded” holes were milled out and new aluminum press fit and Loctited in.

The first side was simple – bore out the stripped thread to 3/4″, press fit and Loctite a piece of aluminum stock. I peened it in a bit with a ball-peen hammer to mushroom the top a little. It was a tight fit but there was a tiny wobble in the cut when starting. This covered that up nicely.

Filed and drilled to 3/8″, later threaded to 7/16-14.

Using a 3/4″ endmill to cut straight down into the cracked hole.

After the 3/4″ endmill, I used a 1″ Forstner bit (really meant for wood) to cut a 1″ hole above the 3/4″ hole. It worked really (surprisingly) well and cut slightly undersize, which made it easy to make a -1″ press in bushing. 

Drilling the new aluminum to 3/8″ for the threaded stud:

All but threaded!

I had to be creative to fit such a large work piece on my BM’s table. I made some 5/16″ threaded rod into some 1/4″ threaded clamping bolts. (They thread into 5/16″ t-nuts in the table.) This was level enough to trust for drilling for the threaded studs.

New mounting holes and threads were put into the casting. I’m not sure what the original mounting bolt size was, but the ones it came with were not that. I put in 7/16-14 threads and will eventually thread in and Loctite some studs to mount the motor.

Next up will be to polish the aluminum head and paint were needed. I plan to paint the inside and lowered sections of the logo’s area, and polish the outside. More on that later. Also, next step will be stripping down the cast iron lower section and determining what work is needed there.

Sharing is Caring!


11 responses to “Bridgeport M Head Rebuild (for the Benchmaster Mill) – Pt. 1”

  1. Nelson Timken Avatar

    Great job. I’ve been looking for a decent M head for a while.

  2. Steven Villegas Avatar
    Steven Villegas

    I just got a benchmaster base that was set up for a M-head, but didn’t come with one. It has the risers and mounting plate and all.

    Now I just need a head!

    Steven Villegas

    1. Daniel K Avatar

      If you’re not in a rush and patient (unlike I was), you should be able to find something pretty decent! A lot of machinists dislike these heads these days simply because bigger and better ones have been built. But for this size machine, it’s a nice fit. Personally I think the Rusnoks may fit this machine perfectly, but the head’s capabilities are too small.

  3. Steen Kruse Avatar
    Steen Kruse

    I have an Adcock & Shipley 1ES and a M head. I will build them together.
    I do not know which size collet there is in it.
    It is possible to change it out to a MT2 if it is a B&S 7.
    Krgds Steen

    1. Daniel K Avatar

      From what I understand the only way to do that is to switch out the entire spindle. (Maybe not the quill, though? Those are honed specially for each head.) If you’re replacing bearings, you’d have just the spindle in hand at some point. That part could be interchangeable with other M-heads. If it is the B&S7, I’d just find an endmill holder or keep watching eBay for collets. You may even be able to have the spindle reground, but I’m not sure.
      Check out this link:

  4. Ron Straub Avatar
    Ron Straub

    If your M head has a MT2 you are in good shape, BS7 still relatively available, but if your spindle has the B3 get ready to shell out some bucks if you can’t find what you need on ebay. You can still get the collets but they are over 150.00 each. Still trying to figure out what i have but it’s looking like the B3. No tooling holders with it so I am on the hunt also. Not having too much luck though. The M head was made mainly as an attachment to mount on the back end of the turret.
    Good luck guys.
    Ron Straub

  5. Ron Kellis Avatar
    Ron Kellis

    I just picked up a Clausing 8511 with an M head. I was told it was an 8520 with a low use factory M head, wrong on all accounts. In all my research, I’ve only seen references to the MT2 and the B&S#7. I’d try marking up a B&S#7 collet with a Sharpie and see what the rubs show. Very good to see how you managed what you were handed.

    1. Daniel K Avatar

      Thanks! Good luck with your head.

  6. Fernando Gonzalez Avatar
    Fernando Gonzalez

    I love how your restoration of your M head came out-just excellent. I’ve owned an M head for years, self taught and still learning. I’m thinking of rebuilding mine which you’ve given me some inspiration.

    1. Daniel K Avatar

      Just saw this – I’m bad at keeping up with this blog! (Started a YouTube channel, instead: I’m glad I may have helped you. I enjoy using my mill, even though a lot of folks will say disparaging things about the M heads. (It’s not large, so if used in its envelope, it’s great.)

  7. Bryan Avatar

    This is great information for the few of us who own the mighty M head 🙂

Leave a Reply

%d bloggers like this: