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@fuse.net>
To: <triumphs@autox.team.net>
Subject: [TR] Tachometer calibration
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
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