Starrett micrometer 436 repair

[WobblyHand]

I recently found a micrometer that my father had. It may (or may not) have been used by him when he worked at the MIT Radiation Laborotory around the end of WWII. I do know he was a technician there and he had to do practically everything from machining, to making hydrogen furnaces to assemble those new fangled radar magnetrons. As I recall, they had to braze copper assemblies together. He was a jack of all trades by necessity, and a pretty clever guy. A professor noticed him, and recommended he attend MIT. He did, and spent the rest of his working career at MIT (more than 65 years).

The micrometer had gummed up a bit and was stiff. Opening beyond 0.5" opening was a bear. I tried some alcohol to try to dissolve some of the gummed oil which got me a little further. Finally out of desperation, I tried some Kroil to get into the mechanism. It's better but is quite stiff after 0.8". I don't feel any periodic stiffness, like a bent shaft, it's more like its still gummed up. In a fit of stupidity, I pulled the ratchet assembly and inadvertently lost the ratchet pin for a few hours. I didn't even know it was missing for a while. It was there, and then it wasn't! Finally found it after sweeping the floor with some powerful rare earth magnets. Got that together again and the ratchet is once again functional. Phew!

Anyways, what else can I do to get this in to better shape? Anything I can soak it in that won't take off the remains of the black paint? Alcohol? I know I could send it back to Starrett for repair, but is there something that I could do? This is more sentimental than anything. I realize that 436's are not top of the line and can probably be picked up used for less than the Starrett repair cost. On the other hand, the repair location is not all that far away from my home.

I have seen some stuff online on disassembly, but I have yet to see enough detail to actually know what to do. (Feel like they were skipping some steps.) Simply don't know if there are any hidden gotchas that need to be done (yet). If someone has done this, or knows of a decent video that shows the actual disassembly, I'd appreciate it. Some mics just unscrew all the way, but this one is very stiff and I don't want to force anything. It was so stiff, I could barely turn it. Thought some mechanical empathy was in order, and I ought to ask.

 

[Bone Head]

Try injecting some acetone into threads. Repeat until it moves freely. You will have to oil it afterwards.
Or toss it into a jar of acetone. But be aware that will in all likelihood eat the black out of the thimble and barrel. I used to run a black sharpie over where those markings disappeared then wipe them. Use no solvent doing that; it'll remove all the sharpie.

 

[WobblyHand]

The Kroil seemed to loosened things. I was able to remove the screw from the assembly, there was a little bit of gunk inside. Now I need to use a qtip or something to clean things a bit. Still waiting for my Starrett account to activate so I can access the exploded views and stuff. This model of 436 appears to be older than what is shown online, my lock is in the middle of the frame, not the outside edge like it is now, but I'm hoping it will give me some more insight for more disassembly.

I think I can remove the ratchet, pin and spring. Somehow you are supposed to use a pin to start unscrewing the ratchet assembly.

 

[WobblyHand]

Try injecting some acetone into threads. Repeat until it moves freely. You will have to oil it afterwards.
Or toss it into a jar of acetone. But be aware that will in all likelihood eat the black out of the thimble and barrel. I used to run a black sharpie over where those markings disappeared then wipe them. Use no solvent doing that; it'll remove all the sharpie.

Hoping you would chime in. I just managed to get it apart, so I did avoid using acetone. I'll start out with 99% isopropyl, as it is a little less forgiving than acetone. I'll use acetone if I need to though. Just have to avoid getting the acetone on various surfaces. I know exactly what it will do... BTDT.

 

[mmcmdl]

I have a few pairs of the identical mics downstairs Wobbly . My uncles and his dads . I have about 10 pairs of old time mics that need to be loosened up . I'm following along with you and Bone Head on this one .

 

[benmychree]

Insert a pin into the cross hole on the ratchet assembly and rotate it counterclockwise, after removing it, the thimble can be removed, it is on a fairly steep taper and should break loose without too much trouble, If it resists, you may be able to thread the ratchet assembly most of the way back in and give it a rap on some surface softer than steel to break it loose. I would avoid removing the lock assembly if possible due to the possibility of damaging it. To clean the ID threads, I'd find a soft fiber or plastic twisted brush and just use regular paint thinner or stoddard solvent. all my mikes from 1" and up to 6" are 436, I bought them all new in the 1960s, they should last a couple of lifetimes more if treated with care.

 

[WobblyHand]

There was some grease on the threads of the screw and nut. I was able to clean some of it off with alcohol and that made the action a lot smoother. Now it isn't fighting me and I can use my finger tips on the thimble to turn it. But it's still too tight to turn with the ratchet, so I will take off the ratchet assembly for better access to the screw. I used a #52 drill to stick in the hole and tried tapping it with a small hammer. This of course did nothing. Then a drop of Kroil was applied and I sort of wedged the drill in a small vise grips without actually grabbing the ratchet and the ratchet assembly broke free. Took a bit of torque and it popped free. I then removed the screw from the thimble? Still don't have access to the exploded view, sorry I don't know the proper names yet.

Everything looks to be in good shape so far, which is very encouraging.

 

[WobblyHand]

Cleaned it all up, with 99% isopropyl, and put it all back together again. Smooth as can be. I heeded @benmychree 's advice and left the locking mechanism in place. (I also have no idea how it comes out!) When the ratchet clicks, I'm exactly lined up at zero. Can't complain about that! The lock isn't quite as smooth as I'd like, but it is very functional.

All in all, pretty simple. Happy that it turned out this well.

Edit: I know you all like pictures, but the completed mic looks exactly like the first picture! There's no external difference whatsoever. But it works like a newer mic now.

 

[WobblyHand]

Well nuts, spoke too soon. Alcohol is not that great for cleaning out the threads. It's gotten noticeably harder to turn just over a 20 minute time frame. I'll have to do it again, but use a better solvent and brush. Off to the garage to see if I have some clean mineral spirits and some kind of jar.

 

[woodchucker]

I use mineral spirits to degunk the threads. It eats the gunk, then it lubricates it. Acetone and alcohol do not.
I don't use q tips (or I avoid it if I can). I have some fine round brushes for inside. Think spray gun cleaning, but don't use them for spray guns after, they are now cleaning brushes.
On my Starretts there is a collar that controls the tension. I'm not seeing that on yours. That collar at the end does not look like mine.
I've cleaned a bunch, but I also messed one up permanently. It just got worse and worse, felt like a cross threaded screw and just kept getting tighter. They are not hard to work on, just requires patience and realizing you don't want to introduce anything inside (like cotton from a qtip).

As far as oil, air tool oil (quality oil) 3 in one, Starrett oil or M1, or even plain mineral oil. The air tool oil has rust inhibitors, so does the M1 and plain tool oil.