[TR] Tachometer calibration

[spamiam at comcast.net]

Dave,

I think oil is OK to lube the shaft, but a light grease MIGHT last longer. 
I use lithium grease, and 18 years later, it seems to still be fine.

As for moving the pointer,  The idea is to move the pointer on the spindle. 
USUALLY by holding the drag cup, you also have a good hold on the spindle 
and the pointer will turn on the spindle.  But with a tachometer, you have 
pretty easy access to the spindle itself, so you can gently grab it with a 
needle nose pliers and secure it, then turn the pointer hub.   Be careful. 
I have broken a spindle by twisting a pointer that was evidently corroded 
into place....

Heres how I set the calibration:  I run the tach at a certain known RPM (by 
driving the car at a known speed, or by a calibrated external motor drive to 
the tach.  Then I observe what the tach INDICATES, and I know what it SHOULD 
be.  Then turn the pointer to what was indicated, and hold the drag cup or 
spindle.  Then gently turn the hub of the pointer to what it SHOULD be, then 
test again, and re-adjust if needed.

I am not sure why you lost calibration when you opened it up!

If you have no ability to test, then you can go back to the original "zero 
point".  The rest pin at "zero" is spring loaded and cab be moved backward 
away from the face of the dial from the rear.  Then observe where the 
pointer rests.  There is a little white dot on the dial face in this 
vicinity.  This is true zero.

The way the factory adjusted the calibration was to set zero, then run it at 
a specified RPM, and adjust the magnetism of the magnet bar with an 
externally applied electromagentic field.  I guess they assumed that the 
spring constant of the hair spring was reliable and they only needed to set 
the pointer and magnet in relation to a fixed hair spring's constant.

-Tony


-----Original Message----- 
Date: Sun, 5 Jul 2015 11:42:23 -0400
From: "Dave Connitt" <dconnitt at fuse.net>
To: <triumphs at autox.team.net>
Subject: [TR] Tachometer calibration
Message-ID: <F190249B860D40A2B04B1AEC78A4B305 at DaveLaptop>
Content-Type: text/plain; charset="windows-1252"

I had a problem a week ago or so where the tachometer in my TR4A was making 
a lot of screaching noises and figured it was a dry bushing from 50+ years 
of life. I figured I would try fixing it myself. What could happen??
As I was taking the tach apart, I noticed there was a black paint mark on 
the drag cup that was connected to the needle so I just noted where the mark 
was with the needle "at rest" against a point on the frame and continued 
taking it apart. From reading information on the topic, I knew that that 
little spring attached to the drag cup was very delicate and was very 
careful to hold the tach upside down and lift the "drive" end off the drag 
cup, not the other way around. Once I had separated the frame, I just set 
the drag cup half on the bench in that same orientation.
The other half of the "frame" contains the magnet, shaft and bushing which 
connects to the tach cable. Sure enough the little shaft that the magnet was 
attached to was dry as a bone.  The bushing and the shaft looked OK so I 
figured it just needed cleaning and lubrication. After careful cleaning of 
the shaft and it's bushing with some carburetor cleaner, I lubricated it 
with a few drops of 3-1 oil and put the magnet and shaft back into the 
bushing. MUCH BETTER.. The shaft would spin very smoothly by hand.
The assembly went well. Keeping the frame half with the drag cup upside 
down, I dropped the "drive" half back on the drag cup half and reinstalled 
the 5 little screws. It took a little trial and error to get the two pieces 
together where the drag cup would move smoothly around the magnet. The two 
pieces can only go together one way so all the screw holes line up and there 
are two keying pins to help hold the thing in alignment but there still is a 
slight amount of movement so it's not a quick assemlby.  Next, I aligned the 
mark on the drag wheel to the spot on the frame I found before taking it 
apart at pushed the needle back on the shaft. By the way, I didn't put any 
lubrication on the drag cup bearings.
The tach is now running smoothly and the best of all, no noise.
The only problem is that somehow the tach is running about 500 RPM below 
what I think it should be. At idle, the needle just floats off the pin from 
"0". I think I can get a pretty accurate reading of the actual engine RPM 
using a Harbor Freight digital tachometer ($39.00). My problem is that I am 
wondering which way I should turn the needle on the drag cup? I have to take 
the tach out of it's case to move the needle as I have to hold the drag cup 
while I twist the needle on it's shaft and don't want to end up taking it 
apart 10 times..
One way I thought about doing the "calibration" was to pick a RPM, say 
2,500. With the tach out of it's case and the cable out of the firewall, 
have someone hold the throttle at 2,500 RPM and paint a new mark on the drag 
cup. Stop the engine and move the drag cup to the new mark and install the 
needle pointing at 2,500 RPM. At least it would be accurate at mid-range.
What do you guys think??
Dave Connitt
Thanks,
Dave Connitt