PA SPEEDO REBUILD

I had a Smith PA 90 degree Speedo that was in poor condition, with the needle moving a bit too easily and the odometer not working at all. The speedo rattled and was clearly loose inside. It would also appear that someone had a go at it in the past, as some of the case screws were missing and the bezel and glass would not sit properly to the case.  So it appeared to be a good idea to take it apart and see what was wrong. This article only covers the parts of the speedo I needed to fix to get it working, a full speedo rebuild would probably examine the components in more detail.  The first point to note is the components are very small, as you will see later.  I had two magnifying glasses, desk lamp, tweezers and very small screwdrivers.  Like most people, I have a set of precision screwdrivers, the smallest being 1.4mm, but these were too big and I had to buy another set, as some screws needed a 0.8mm screwdriver.  By the way, I can not find a detailed description of the components so I have made up some names for items. See photo of main parts once dismantled.

The first stage is to remove bezel and the speedo mechanism from the case.  The bezel is attached by three 7BA screws and will then need to be gently prised off.  When I removed mine I noted that there were some small bits (like grains of sand) floating around in the speedo - keep these, they are the ball bearings! To remove the mechanism there are three 2BA screws in the back. If you are lucky the mechanism should just come out, if not (like me) I had to put the base of the mechanism in the vice (soft jaws), sprayed some penetrating fluid and then gently moved the case to free it.  Be very careful as the main body is made of MAZAK and liable to break.  Record the needle location at rest on the face and how far it fits onto the spindle.  Remove the needle, this should just prise off, or in the case of mine, needed some persuasion and ended up flying across the room. I found a tack removal tool good for this, but put some cloth etc between the tool and the face.

There are then three very small screws to remove the face and then three screws to remove the mechanism to expose the main shaft, these are the steel screws holding the brass plate. Photo below shows the face removed and screws for the mechanism. Once the mechanism has been removed, be aware that the odometer gear on the back can and will fall off.  At this stage I worked out that the small grains of sand in the speedo were the ball bearings. The speedo has two ball bearing cups, one at the top and one at the bottom of the main shaft.  It appears to be a common fault for the ball bearings to fall out.  Missing ball bearings cause the main shaft to move around and not engage with the odometer and to allow the spindle to jump, moving the needle too much.  I believe at this stage it would be possible to loosen the top bearing cup and add any missing ball bearings to the top or bottom, then reassemble, but mine needed a good clean.

The next stage is to remove the speedo gear for the main needle, which just unhooks from the main speedo shaft at the top of the speedo and then the odometer gear on the diagonal shaft, by removing the brass retainer and sliding out the odometer shaft.  This will then leave you with the speedo body and main shaft held in place with the top and bottom ball bearing cups.

Then remove the main speedo shaft.  To do this, undo the retaining screw for the top bearing cup and then the brass ring holding the top cup can be unscrewed.  I found it easier to completely remove this brass ring, as it made the cleaning easier.  The main shaft can then be lifted out.

The next decision is what to do with the main mechanism and odometer dials, they can be left alone and refitted or dismantled and cleaned. They only go back in the one order, due to the gears the way they overlap - it is useful to take a photo so you can remember later. The red dial is held in with a small retaining clip, once removed you can then either remove the springs &slide off the dials, or gently unhook the springs from the gears and slide the dials off. 

Check at this stage that the needle will fit, my needle shaft had corrosion and needed cleaning to ensure the needle fitted - don’t over do it or the needle will fall off!  Once the parts have been cleaned, it is then a case of reversing the process and reassembling the speedo.

The speedo will need 30 1/16 ball bearings, 15 in each cup. They are very small and as I discovered, once dropped on a hard floor bounce and disappear. Luckily they are easy to obtain new, so I replaced all the bearings with new.  Applying a small amount of grease, the bearing can be held in place before the main spindle is installed.  The photos show the lower and upper ball bearing cup, the cotton bud gives an idea of the size.

The main shaft is then installed and the top ball bearing cup brass ring is screwed in place to hold the main spindle, check that the spindle spins freely.  Screw in the locking screw and then add the odometer spindle with the brass retainer.  You will then need to clip the speedometer gear onto the main shaft and locate onto the main body of the speedometer.  Once the main odometer dials have been added and have been adjusted to the desired reading, you will need to add the odometer gear onto the back of the brass plate holding the dials, this can be a pain as the gear will fall off.

Then attach the brass mechanism to the MAZAK main body with the three screws. You will need to ensure that the speedomer indicator arm gear is engaged with the spindle for the needle and that there is no run out, or the needle will bounce. Reattach the face, new faces are available, and inset the speedo back in the case.  The needle can then be fitted to it’s correct location.  When fitting the bezel, make sure that the needle is not touching the glass.

All the photos show the speedo after I had cleaned it. 

Before undertaking the rebuild, I looked for some time to try and find a detailed explanation of how to repair the PA 90 degree speedo, but could not find a detailed explanation with photos of all the stages.  The process is relatively straight forward and this is what I did to rebuild the speedo - there are a number of companies who will undertake for you. 

Douglas Alderson DA7C