What was “wrong” with it?
The rear bushing that holds the mag shaft in place was damaged allowing the mag shaft to contact the needle cup, which spun it and broke the hairspring.
I had a parts 85mph speedo so I stole the spring and bushing from it and installed on this one. Seems to working great now, or at least reading in the ballpark of my other 140mph speedos. (I have 4-5 of them)
Bottom one was the damaged speedo.
Btw there is a subtle difference betwwwn the standard 140mph and the motorsport/certified 140mph. The standard one has a stop pin while the motorsport doesn’t. You align the needle by pulling the stop pin and pointing the needle at the alignment tick. You’ll see on the motorsport speedo that the alignment tick is 0mph and there is no stop pin. 10mph aligns on both. No idea why ford did it this way.
I’m gonna borrow a rheostat from work to use a corded drill on, use a handheld tach and spare speedo cable and make sure it’s actually accurate.
If it’s not accurate there’s not much I can do. Believe it or not, calibration of the head involves using two coil to “charge” or “discharge” the magnet in the head, which will drag or grab the needle cup more or less. Obviously I don’t have access to this, so no speedometer calibration video.
Here’s the busted motorsport speedo and a spare 85 head. You can see the broken spring. The part numbers are exactly the same. So really if you had access to the machine you could just calibrate an 85mph unit to 140mph of anything else you wanted as long as you had a face. Pretty sure this is what
BBK did when they recalibrated speedometers back in the early 2000s for 140, 170 and 200MPH.