Introduction: How to Fix a Sticky Dial Indicator Plunger (bent Plunger Rod)

So you got a new-to-you old dial indicator and found out the plunger is sticking instead of depressing smoothly. Of course, the first thing you do is take apart the indicator and figure out what's up. And here's how you do it.

Note: I performed this operation on a Starrett No. 25-441 Unless you have the exact same model things will look a little different inside. You should be able to open the back up without destroying anything, but if you get the case open and can't see how everything works, just close it up. There are shops that will fix pretty much any dial indicator for you for 1/3-1/2 the price of a new indicator.  If you are like me, and don't mind risking breaking the indicator, have a go and post your results!

In this instructable I did not disassemble any of the gear train which drives the needles. These gears frequently use jewelled bearings and should only be disassembled with the correct tools to pull the bearings apart. I don't have those tools so I left things alone.

latex gloves (Non-powdererd)
fine screw drivers
fine tipped needlenose pliers
arbour press
fine toothed file suitable for aluminum
Flat surface (I used a granite machinists plate, but a glass tabletop would work too)
industrial solvent (I used lacquer thinner)

Step 1: Examine Your Indicator

Take a good look at the indicator, sometimes you can see some of the problem from outside the case.  For me, I could see a very slight bend in the plunger rod. It was hard to tell, and didn't show up on film so I don't have a picture here. There was also a sticky residue on both ends of the plunger. These things indicated to me that the case had to come open so I could flatten the plunger and clean off the sticky mess.

Step 2: Remove the Back Plate

The back of my indicator is held on by 4 screws. using a sharp screw driver (don't use one with a chewed up blade) remove the screws and pull of the back plate.

Stop and look at the indicator. I have labelled the parts you can see in my indicator.
1) The plunger rod has a rack gear machined into it, and a pinion gear rides along this which drives the hands on the face.
2) to the left side of the rod is attached a pin, which carries the tensioning spring.
3) The pin rides in a slot. This pin-in-slot arrangement keeps the plunger aligned to the pinion gear.
4) to the top right and bottom left are a pair of screws. These pass through the outer housing and screw into the plate which carries the clockwork and hands and pinion gear. If you look closely you can see that these screws ride in slots inside the outer housing. This allows you to adjust the position of the pinion gear relative to the plunger.

My indicator also has a dovetail machined into the back with a  threaded hole that passes into the indicator. Oddly the only thing keeping dust out of the inside was a piece of cellophane tape.

Its hard to see in the photos, but there was some dark coating on a few surfaces that looks like it might be residue from clockmaker's oil

Step 3: Remove the Dial Face

Keep the indicator face down on your work surface. Carefully loosen the two screws holding the indicator body to the dial face and the pinion gear carrier. Next rotate the outer housing against the dial face so that the pinion gear moves away from the rack. When you do this, the coil spring under the pinion gear carrier plate will spin the gear out rapidly. I did this accidentally and I suspect that it was not very good for the instrument.

My recommendation would be to try putting on a clean latex glove and using your finger. I didn't this, so I can't really attest to how well it works, but it has to be better than just letting the pinion gear spring free.

Once you have relieved the tension in the pinion gear, go ahead and finish removing the screws holding the housing to the pinion gear carrier. Then simply lift the housing and plunger assembly off of the dial face and pinion gear carrier assembly.

Step 4: Remove the Plunger From the Case

Take the body-plunger assembly and remove the flange piece with the slot for the plunger tension spring. the plunger alignment pin is threaded into the plunger rod and has a very small hex nut on it. with a set of fine tipped needle nose pliers loosen the pin by grabbing the nut and gently turning. once loose, use your fingers to turn it all the way out. Finally, remove the caps on either end of the plunger, and carefully slide the plunger out of the body. take care not to scrape the rack gears on the housing as the rack passes through the body hole.

Step 5: Examine the Plunger

Now you need to find out if your plunger is really bent. I did this by placing the plunger on a granite machinists surface plate and looking along the surface of the plate to see if I could see light slipping under the rod. While looking you need to slowly rotate the rod. This will give you an idea where any bends in the plunger are. I tried to photograph the process, but my camera wanted to focus everywhere except the plunger. In the image the peak bend is actually just to the right of the pinion gear.

I used a sharpie to mark the peak convexity point. on the plunger so I wouldn't lose track of it.

Step 6: Debending Tools

Now that we have identified a bend in the plunger we need to make a few tools to bend it back to straight. We could just throw it in the arbour press, but I think that would likely deform the plunger while it straightened. As a consequence I made a set of bending props out of a few pieces of aluminum rod in order to make a simple 3-point bending jig. The point of using aluminum is that it should be softer than the steel of the plunger. Consequently the aluminum parts will deform and hopefully leave the surface of the plunger unscathed.

I used a scrap of what I think was 1/4inch aluminum rod and cut 3 pieces. two were approximately 2 inches long, the third was only one inch long. The exact dimension is not important. Take a file and flatten one edge of each piece. You only need to flatten enough that the piece will not roll on you if left on a flat surface. On the opposing edge file a notch

Step 7: Debending the Plunger

I apologise for the blurry photos, but you should get the idea.

Place the two long aluminum rods front to back in your arbour press with the flat side down. then place the plunger rod across them so that it rests in the two notches. Rotate the plunger so the mark for the convexity is on top. Place the third short aluminum piece so that the notch is resting on the sharpie mark. hold the short rod in place while carefully lowering the press so that it just rests on the flat side of the short aluminum rod, holding the assembly in place.

My bend appeared to be fairly uniform across the plunger, so I put the two supporting rods far apart. If you appear to have a sharper kink you can put them closer together. Either way the bending of this apparatus will be focused near the small rod pressing from the top.

Now you want to apply some pressure. I found that a little goes a long way here. My recommendation is to apply less force than you think you need. Ten take the rod back to your flat plate and check for straightness. If it still needs work, re-mark with the sharpie and repeat the process. Slowly increase the force till you see small changes in the straightness. Don't try to fix the bend all in one go, as this may move the bend to other points in the rod. Small iterations getting progressively better will yield a much better result.

I would be very wary of pressing directly on the rack gears. Deforming those would seriously impair the performance of the indicator. If you absolutely must press on the rack, I'd try a wooden top bar before aluminum just to lessen the chance of marring the rack.

Step 8: If You Want to Remove the Glass From the Dial for Polishing...

If you want to remove the glass face (crystal) from the needle assembly for polishing of the crystal or to remove dust from the face here is what you need to do.

The crystal and black metal bezel is held to the mechanism by a bent wire clip, which wraps around the edge of the mechanism, and has 'bulges' that protrude into the bezel. This wire has the two ends bent centerward and inserted into the mechanism plate. You need to carefully remove this wire clip. I used a fine tip flathead screwdriver and shimmied it under the wire near one of the 'bulges'. I then worked the wire up over the lip of the mechanism plate. Finally I continued to work the screwdriver around the circumference of the plate lifting the wire as I went. A screwdriver, while it works, is the wrong tool for this job. In retrospect I think a dentists pick would be a better choice if you have one.

Once you have the wire loose flip the assembly over and you can carefully lift the crystal off of the mechanism. take care as the zeroing ring actually sits under the primary needle and may come up with the crystal. If it does, you need to carefully work it off around the needle. The zeroing ring is pushed up into the crystal by a brass spring washer. Both the zeroing ring and the brass spring washer have a small notch in them which mates with a small tab in the bezel for the crystal.

Step 9: Clean Off the Gunk

I used lacquer thinner on q-tips to clean off all the bare metal surfaces in the indicator. Just remember to change q-tips frequently, its no good just smearing the dirt around. I tested on a bit of the face hidden by the zeroing ring, and the paint on the face is VERY susceptible to industrial solvents, so I left it alone. When cleaning the body be sure to get inside the holes for the plunger. This is an area where oils and grit from the machine shop frequently accumulate.

I also replaced the piece of cellophane tape with a new piece since the old one dissolved during the cleaning.

Step 10: Re-assembly

Everything goes back together pretty much the same way it came apart. The indicator mechanism in this particular tool uses jewelled bearings, and consequently requires no oil. The tolerances are so tight that most oil will actually gum things up and cause sticking. I have been told that clockmakers oil, which is designed for very fine watches is OK for these indicators, however, I have no experience with it, and my indicator works fine with no lubricant.

To re-assemble the crystal and needle face, put the wire retaining clip ends back into the holes in the indicator mechanism. Align the notches in the zero ring and spring washer to the notch in the crystal bezel. carefully insert the primary needle into the gap between the zero ring and the crystal and press the mechanism and crystal together. I used a small flat head screwdriver to press the wire retaining clip back into the gap between the bezel and the pointer mechanism. Work your way around the circumference pressing the wire back into place. This is tricky and it took me a few tries to get right. Have patience. You are done when the clip sits completely in the trough between the mechanism and bezel, and the 'bulges' press into the slot for them that is in the bezel.

The plunger and main body go back together in the opposite order that they came apart, with no special surprises.

When returning the dial assembly to the body, you need to pre-tension the dials, by winding up the pinion gear. I used my finger after putting on latex gloves. Insert the body over the main mechanism so that the pinion gear is not in contact with the rack. Insert the retaining screws but leave them loose so that the body can turn against the mechanism. Wind up the pinion gear, and hold in place with your finger while you rotate the mechanism against the body to bring the rack and pinion into mesh. Be careful not to force the two together- you may need to wiggle both the plunger and the pinion to encourage meshing. Once everything is meshed check where the dial is zeroed. You want the pinion gear rotations set so that the zero point of the indicator is approximately in the middle of its travel. This is a 1inch travel indicator, so this means the 10ths indicator will be near .5 at the ends of travel. This causes the small 10ths hand to hit zero when the gage is in it's prime operating zone. If your 10ths hand is off, put your finger on the pinion gear rotate the body to disengage the rack from the pinion. Then adjust the pinion rotation till it is where you want it and re-engage the rack and pinion.

When engaging the rack and pinion, you don't need to apply any pressure on the rack with the pinion, in fact the less pressure the better. You just need to align the two so that the gears mesh properly. If you have too much pressure the plunger will be pressed against the guide holes on the body and will be stiff to move. Test that the plunger moves freely and then tighten the screws holding the body to the mechanism. After tightening check that everything still works as it should and replace the back plate.

Congratulations, you have now 'refreshed' a sticky old dial indicator.