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M Head Repair

John E

Cast Iron
Mar 10, 2007
MD Eastern Shore
I'm new to the PM forums, but read everything I could find on this subject.
I just got an M Head Round Ram BP (sn 6148, 1946 vintage). The upper bearings sound terrible, although the spindle spins easily. Based on what I read, these bearings are just basic bearings. After MUCH searching on the part numbers listed in other posts (verbatim search produced no hits except on the PM forums) I finally found enough details to be confidant I understand what they are, and the differences between the 3 pair. Based on that, I plan to get a pair of 6206 abec-3 bearings from Mcmaster.
Now is where I get stuck. Just how do you go about taking the spindle apart? It looks like the 'spindle pulley hub' should unscrew from the bearing housing, but since it rotates, where is the appropriate grip location to prevent it from spinning? Based on the other posts, open bearings are an absolute must, but since these are up inside the pulley, how does the lubricant get 'up' there? Can these be sealed bearings?
Although I don't plan to take it apart (yet...), does the balance of the spindle come out through the bottom of the quill by removing the nose piece as is described in J Head posts? If so, there there a special tool required that grips into the hole in the nole piece, just below the quill set screw? Would this be very tight, related to the loading of the lower bearings?
Any insite into the procedures would be greatly appreciated.

If you had a parts breakdown you could get a good
idea of what's going on.

Oil gets *down* into the bearings, not up.

It goes from the oil cup at the upper left
side of the aluminum housing, to the
pulley bearings, to the upper two spindle
bearings, to the lower two bearings, and then
out onto your shirt.

Hmm. How do you lock the spindle so the
nut at the top of the pulley can be undone? I
forgot how that works, maybe a strap wrench?

Once the pulley comes off the aluminum housing
comes off, then the quill comes out the bottom.

You do need a special wrench to undo the locknut
at the top of the spindle though. Bend up the
tabs so you can loosen it.

There are two pulley bearings, and four spindle
bearings. The upper two can be electric motor
grade but the bottom two have to have preload
set by the spacer so you either need to grind
the spacers to set the clearance, or purchase
a matched set of bearings for the lower ones.

The spindle comes out the quill from the bottom
once the nosepiece is off, but the nut at the
*top* of the spindle has to be off because
the entire stack of four bearings is held on the
spindle by that nut, along with several spacers.

The top two bearings stay in the upper part of
teh quill when the spindle is taken out the
bottom. The lower two will try to come with.


Send me a PM with your email address if you need a BP manual with the parts breakdown. I have a PDF which describes the disassembly. -Mike
Thanks for the details. I do have 2 different diagrams, an expoded view and a cross section. Shows the relationship of the parts, but not the exact details on how it goes together/comes apart.

The pulley bearings are inside the pulley and both above the lowest pully groove, so it is not obvious how the oil gets to them. The oil from the cup cannot gravity feed 'up', no matter how much you want it to, hence the reason for the question.

I tried using the belt to grip the pully, but could not break the nut free. Maybe a strap wrench will work.

Hopefully I won't have to touch the rest of the spindle and quill.

Any secrets on the spindle spring? Mine is in place, and winds/unwinds but offers no asistance on return. I'm thinking it might need a few more turns to tighten in up. Any possibility that I might be able to free the outer end and wind it a couple more turns?

Thanks again,

The upper pulley and bearing housing/bearings and all come off by simply loosening the pinch bolt in the aluminum housing--this will expose the bearing locknut when you turn the assembly over. (remove the motor first to lighten the whole thing)

The above is true only if age has not yet claimed my memory!!!
Assuming I understand what you've said, this is quite a significant piece of information.
Let me see if I truly do understand this...
The pinch bolt that holds the entire belt/pulley/motor assy to the head casting will release the entire thing from the head?? This infers that the pulley, pulley bearings, and bearing housing simply drop down onto/slip off the splined portion of the spindle, and 'floats' on the spindle? Sure wish I was in my shop to test this theory...
Hopefully I won't test your memory too much here, is the bearing housing screwed down to (up from?) the bottom of the motor/pulley/belt housing? Does the spindle pully hub (nut&top pulley) screw into the lock nut under the bearing housing?
Just hunted down a third M Head drawing, which is better that the one I've been using, and I think I've got it now. It looks like the nut, with locking washer (as you noted), under the motor/pulley assy is the key to getting it all apart. Can't wait to try it!
Thanks for this critical piece of info!
I think the pulley has to come off first, yes?

The largest pulley diameter is, I think, larger
than the hole in the bottom of the aluminum

I *think*.

The aluminum housing is however usually a really
snug fit on the iron casting underneath. One
trick is to put the pinch bolt in from the
other side and close off the slot with a thin
piece of stock, to turn it into a pusher. Go
easy as the aluminum casting is not too flexible.

After studying every M head drawing I've found, so far 4 different ones, I believe Bob has hit it right. The pulley is an assembly that is bolted to the base of the motor/belt/pulley housing, from below. Removing this entire housing from the head will expose the lock nut/washer that holds the whole pulley/hub assy together, and the bolts that hold it into the housing.
Hopefully I can get to it tonight and find out for sure.

Mmm, so the pulley slides off the spline,
as the entire aluminum housing comes up.
I do recall that at some point you do need
to disassemble the pulley - the hex at the
top is how you do that - the topmost pulley
is steel and that unthreads to allow the
rest of them to come off.

I do recall there is a gasket in there with
a single hole for the oil passage, that has
to go in with the oil hole lined up - if it
is off the gasket blocks the oil flow. But
the oil goes through the outer aluminum housing
and into the part that bolts onto there that
holds the bearings.

When you get it apart that far be careful when
you remove the thin sheet metal shield that
covers the slot in front. That shield is
vital to keep crud out of the lower bearings
and it can go in a couple of different ways. It
took me a while to get it all back together
after realizing that!

That shield also helps to funnel oil down into
the quill and into the lower bearings.

Once you get in that far you need to borrow or
make a wrench to undo the castellated nut that
holds the bearing stack on the spindle.


Pulley won't come out till you've removed the casting. The upper big hex, top step of pulley, and spline sleeve are all one piece and pass thru the bearings and are retained by an N-06 locknut and it's toothed washer. It helps to have an N-06 wrench, but you can proceed with the bfi method if you don't. (brute force and ignorance) Whatever you do, after you get the locknut off (you can hold the hex end in the vise), really support the casting well while drifting out the spline sleeve/pulley/hex, cuz Jim's right--that casting is delicate!

good luck
Well seems it has all come together (or apart actually) as expected. Pulled the motor/pulley housing off no problem. Unfortunately that's where 'no problem' ended. The noisy bearings are the top spindle pair, not in the pulley hub. And, part of the 'brass quill skirt' was smashed between the mill head and top housing. I was able to get the skirt out, and about 20% of it was totally destroyed, but the rest was fine, and I was able to slip it back in place. Should it have tabs that fit in the grooves on sides of the quill to keep it moving up and down with the quill? I can make out the remnants of such a tab on one side. Any likelihood that I can successfully fabricate a new one from a brass sheet (not a question of my ability!)? This could have been causing some of the noise I was hearing, as it may have been rubbing on the bearing races.
Oiled up the bearings while it was open, and put the top housing back on, and test ran it again. The inverter is a piece of crap, but suffices for testing. Lowest speed is not too noisy, but gets progressively worse with higher speed. At 4250 it can barely get started (did I mention the piece of crap inverter??), and -very- noisy. Power off to spindle stop at 4250 - about 10 seconds. Not good. I suspect the skirt damage caused the bearing failure. High likelihood of metal remnants in the bearings.
With the cost of bearing at $200 - $300 per pair, it's not cost effective to rebuild this one. What's the worst case scenario if I use cheaper (relative to $400-$600) bearings to make it usable till I find a good deal on a J Head, and adapter (it's an early round ram)? I'm guessing they won't last as long, and won't be as accurate. Neither would be an issue for my needs.
So much for the excitement of a 'new' mill. Now my Jones & Lamson Lathe CNC conversion will be delayed more....

One more thing, what exactly is the N-06 wrench? I can see the nuts it's used on, but that nomenclature is new to me.

The top pair of bearings are electric motor
grade, not high accuracy ones. Only the
bottom pair of bearings does the real work on
that spindle.

Thanks for yet another key point. The Series 1 BP manual (no date, but included DRO stuff) shows that the M Head one as a single fafnir RMM 205 KCR bearing, but it looks like that number is obsolete. The 1966 BP Manual shows this as a pair of SAE 205 bearings. They changed the design at some point?
More than an ample supply of other '205' bearings to consider, but not sure which one is acceptable. Most are sealed/shielded and general purpose. I'm thinking that an annular load style from McMaster ($48 from my favorite supplier) would be good choice. Next lower quality is quite a bit lower (abec1 vs abec3). For $48 it's not a big deal. If mine has a pair, I'll mount them opposing, otherwise I'll make sure I install it in the correct orientation, as there are 'wavy washers' under it.

Things are looking up again...

And for anyone that might be reading this, a few more questions.

Are the 'lock nuts' (re N-06 in earlier post, N-05 in BP manual) used with a spanner wrench? I'd prefer a socket to grip it more positivly, like a spanner socket, but with the spindle shaft sticking out, that won't work, even if I found such a socket. I did find a style with a square shank (for a wrench) and a shaft clearance, but I suspect it's big bucks.

Also got my VFD inverter fired up this morning, and it seems to work fine, with the belt off.... Any idea how I can find out how many poles the motor has? It's a 'US Motor' brand. The inverter needs to be set for this, and the choices are 2, 4, 6 and 8. 4 is the default.

I also noticed that in the Series 1 BP manual, the M Head was promoted to 3/4 HP, and a higher RPM (assuming the same pulleys, around 1400rpm). Anyone know anything more about this?

OK, *somebody* did this on the board within the
past year or two (replacing m head bearings) and
I want to say they got them from Alpine Bearing
in the boston area. I *think*.

The lower two bearings could be abec 5 or 7
or so, but the way b'port set those things up
(unless somebody's been in there) was they
made the OD and ID spacers exactly the same

I do recall the person who recently went through
this did kiss off the spacers to make them
exactly the same height, and clean them up.
Using a surface grinder that is.

When I did my M head I did not buy matched pair
angular contacts but used abec 7 radial
bearings which meant I had to re-work the spacers
to get the right preload/clearance. Buying
a matched pair would be a better idea.

The originals were indeed called out as "SAE205"
bearings but there's a cheat there, bridgeport
ground the races on those to set preload so they
were really radial bearings being run in an
approximation of angular contacts. I've
had folks tell me "no way they did that" but
I've still got the original bearings I took out
and it's pretty obvious that's what they were
doing. Maybe the bearing manufacturer was OEMing
them for bridgeport that way at the time.

But if the fellow who re-did his M head could
chime in here, that would be a help.
Alternatively one could use the search feature
to go back and find that thread.

But the folks at Alpine Bearing are pretty good
so you could also give them a call and see whta
they suggest: www.alpinebearing.com, but their
web site seems to be down at the moment....

Thanks again for your insite.
I've since re-read numerous posts on the M Head, and I'm pretty confidant I've got the details on the bearings figured out. Regerences to GEM in the 1 post of bearing numbers really through me off. Seems it is spelled JEM.
The posts make a lot more sense now, and I understand bearings better as well. Found a nice doc on the Timken site that give lots of details, including preload.

Now I just need to figure out the locknuts and spanner wrenches....can't be too big of a deal. A couple 'adjustable' ones ought to do it.

hope this helps, i used these instructions when i changed the bearings in my mhead and had no prblems.

To be used with Drawing No. C-442

Caution: Bearings and spacers are ground absolutely flat and parallel, and must
Remain in this condition for the proper performance of the Attachment. Keep
Spacers in pairs as they are removed. All parts must be protected from dirt and
moisture. It is not necessary to remove the slushing grease in which the bearings
are packed.

Removal of old bearings

Remove drawbar and nut. Loosen belt housing clamp screw (M-69)
And remove belt drive assembly.

Remove set screw (M-85) at lower end of quill and unscrew nosepiece
(M-5) using spanner wrench in hole at A.

Bend down lockwasher tab which locks bearing lock nut (M-16) and
Remove nut.

Turn entire assembly upside down and tap threaded end of spindle on
bench or wood block until bearing set No. 1 clears quill.

Remove long spacer (M-10) which will remain in quill.

Drive out bearing set No. 2 by tapping with a length of tubing or hard wood.

Press set No. 1 off spindle in an arbor press.

Re-assembly with new bearings

Place a bearing, the lower spacers, (M-8, M-9), and another bearing on the
Spindle and press into position on an arbor press.

Slip the quill over the spindle held in an upright position and work bearings
into place. Bearings may be seated against quill shoulder by tapping spindle

Replace long spacer (M-10) on spindle from top.

4. Place a bearing, correct spacers (M11, M12) and another bearing on spindle.
After making certain that inner races will clear locknut thread, tap into place
with a length of tubing on inner race.

Replace bearing lock washer and nut; tighten, and bend back correct lock
washer tab.

Before replacing nosepiece, check runout of spindle nose as close to end as
possible. This should not be more than 0.0002. If the indicator reading is
higher, loosen lock nut slightly and tap threaded end of spindle. Spin spindle
several times, tap spindle nose lightly, and tighten lock nut. Check runout
again and if reading is still more than 0.0002, repeat above procedure until
this reading is attained.

Page 2

Replace nosepiece, tighten with spanner wrench, and replace set screw.

Replace belt drive assembly, tighten clamp screw, and replace drawbar and nut.

Instructions for replacing spindle pulley bearings Bearing Set No. 3.

To Remove Old Bearings:

Remove motor from belt drive housing.
Loosen drawbar nut, and withdraw drawbar. Loosen belt drive housing
clamp screw (M-69) and lift belt drive housing from quill housing.
Turn belt drive housing upside down, bend down bearing lock nut lock
washer tab, and remove lock nut (M-14).
Using a length of tubing, drive spline piece (M-17) through bearing and
pulley (M18).
Remove six screws (M-65) which hold bearing housing (M-20) in place.
Pulley and bearing housing may now be removed from belt drive housing.
Unscrew lock nut (M-21) and press out bearings.

Re-assembly with new bearings

Press new bearings into bearing housing and replace and tighten lock nut.
Place bearing housing in original position inside pulley. Return pulley and
Bearing housing to belt drive housing.
Line up bearing housing with six screw holes and oil as at B, replace and tighten six screws (M-65. Pull bearing forward to line up better (motor and belt pulls back).
Line up pulley and bearings and drive in spline piece.
Replace and tighten bearing lock nut (M-14) and bend back correct lock washer tab.
Return belt drive housing to quill housing, tighten clamp screw and replace motor drawbar.
Rod Builder,
Wow, it doesn't get much better than this!
These instructions even match the item numbers on one 1 of the 4 M Head drawings I have.

Other comments:

1) never press the bearings across the balls.

2) once the bearings go on, they should stay
on. Removing them pretty much requires pressing
across the balls and will degrade their accuracy
noticeably. So be sure everything is right
and clean before you start.

The tool to remove the locknuts is being ever elusive. The closest thing I found is made by the same company as the nuts (WH), but won't work on the spindle because of the shaft. It has a square hole for a socket wrench, and they have no idea where to find the right tool. 'Must have been custom made' they said.
With as many of these machines as there out there, there's gotta be a source for this tool.
Can anyone point me in the right direction, or have one I could borrow/rent? It is the N-05 size.
The backup plan would be to send the spindle to a shop that can do it, but the overall cost of the machine is climbing too much already. I did get lucky and found a pair of the exact right precision bearings on ebay for $125.
Hope someone can help!