CUNO TIMER

By Staff
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Figure C
Figure C
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Figure B
Figure B
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Figure A
Figure A
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Figure D
Figure D

40021 Ben Morgan Road, Leonardtown, Maryland 20650-2521,
Copyright Retained.

One of the most, if not the most, popular marine engine timers
was made by Cuno of Meriden, Connecticut. I have not been able to
determine the production life span, but it does appear it began
about WWI and came to an end about the beginning of WWII. This
simple but reliable timer was used by a large number of engine
makers on one through four cylinder marine engines. The Cuno timer
was also often used as a replacement timer for marine engines that
had faulty or unreliable timers. See Figures A and B.

The Cuno timer is really very simple once one understands how it
is assembled. It is also apparent that a lot of marine engine buffs
don’t understand how it is supposed to be assembled. Hopefully
this article will be of help to those having Cuno timer
problems.

The Cuno timer has four principal external parts:

(1) The timer case with handle that contains the lower (angular
contact) ball bearing and an internal fiber ring mounting one to
four contact segments which are insulated from the case. The
contact segments would be connected to individual spark coils. (One
coil per cylinder.) See Figure C ‘A.’

(2)The cover containing the upper (angular contact) ball
bearing. See Figure C ‘F.’

(3) The top cap which contains a captive
?’-10/32 thread brass round head machine
screw. It is not uncommon for the captive screw to be missing and
it may be replaced without being made captive. It should be noted
the top cap rotates when the timer is in operation but the COVER
DOES NOT! REPEAT, DOES NOT ROTATE! If the cover rotates there is a
serious problem that must be corrected if the timer is not to be
destroyed. See Figure C ‘C’ for the top cap.

(4) The timer drive shaft. See Figure C ‘D.’

The ‘sector’ is ‘G’ and it is critical to proper
timer operation but it is not part of the timer itself. The radius
of the sector must be maintained if the timer is to be held in any
chosen position from ‘Full Advance to Full Retard.’ The
teeth of the sector can become badly worn over long term use, but
it is relatively easy to repair them with a three-cornered file.
Close examination of the detent shown on the underside of the
handle in Figure B or D shows the effect of years of wear on the
detent. In this case one can see the wear on the underside of the
handle on each side of the detent. The detent on this timer is so
worn down that the handle itself rather than the detent has been
resting on the sector. One should note the two brass rivet heads
showing that attach the handle to the timer case. Also note the
clearance provided around the bearing in the bottom of the case.
The rivets and clearance permit bending the timer handle down
slightly to maintain pressure on the sector. The whole timer will
move around slightly when the engine is running, and this perfectly
normal.

It is difficult but not impossible to rebuild the detent with
braze and then file a new edge to meet the sector.

The timer drive shaft ‘D’ will be found with either a
?’ or 5/8‘ bore to meet up with the
engine timer drive shaft. The timer drive shaft will be found even
in NOS with either 8/16,’ or ?’ long
?’/20 Thd set screw. Do not use the ?’ set screw, as it
will interfere with the sector.

Figure C shows the parts of the timer. Part B is the rotor which
slips onto the drive shaft part D. Part B is found in either steel
or brass, but the rotor is always steel. Either works well as long
as the rotor receives an occasional drop of oil. Some old time
watermen would fill the timer with grease to keep down sea water
corrosion problems. Immediately after filling the timer with grease
the engine might run rough for a few minutes until the rotor
cleared the grease from the stationary contact(s). My own
experience has been that the practice of filling the timer case
with grease doesn’t work very well, so I just keep the rotor
shaft and the angular contact bearings oiled with SAE 30 weight
motor oil.

One should note the flat on the timer drive shaft part D; its
set screw and the rotor all line up. This is helpful when one wants
to know where the rotor is without removing the timer cover.

The bushing and spring part E is installed between the cover and
the cap. The bushing has a radius that mates with the inner race of
the angular contact ball bearing pressed into the cover part F. It
should be noted that there is a radius on the timer drive shaft
which mates with the inner race of the angular contact ball bearing
pressed into the timer case. It is important that there be
clearance between the bottom of the timer case and the flat under
the radius on the drive shaft. The precise amount isn’t
important; probably not more the 1/64‘ is
adequate. What is important is the outer race of the angular
contact ball bearing not be pressed in so far that it causes the
rotor to wear on the underside of the cover. The same holds true
for the bearing in the cover not being pressed in so far that it
forces the rotor against the timer case. If one examines Fig. D
part F slight wear is present on the underside of the cover. This
wear is probably due to the rotor moving up and down on the drive
shaft rather than the lower bearing being pressed in too far.

Fig. D shows how the rotor fits on the drive shaft. One should
note that the flat on the drive shaft stops the rotor from sliding
all the way to the bottom of the drive shaft. Occasionally a rotor
will be found that does not slide down to the end of the flat on
the drive shaft. Generally this will be caused by a burr in the
hole in the rotor. A few passes with a file should cure the
problem.

Occasionally a drive shaft that has had long service will be
found with two slots worn where the rotor mates with the shaft.
While this may throw off the timing, one can easily compensate
simply by moving the handle a little further on a four-cycle engine
but it may make it difficult to control a two-cycle engine easily
when ‘reversing on the spark’.

It is relatively easy to build up the worn areas with weld,
grind the shaft round and mill a new flat on the shaft. This is
probably much easier than attempting to make a new drive shaft
because the distances and radius for the ball bearing must be
maintained. The weld, grind and mill approach can be done with
little regard for precision measurements, as sufficient reference
surfaces can be maintained as the weld need only be two small areas
on the drive shaft.

In the single cylinder timer there is a small brass screw
opposite the contact segment. Occasionally this will be found to
project beyond the face of the insulating fiber ring. This screw
can be seen in Fig. C part A. This screw should not project beyond
the surface of the fiber ring. If it does it will cause two sparks
per revolution of the rotor which, while of itself may not cause
engine operational problems, can cause the rotor to bounce and wear
the surface of the fiber ring badly.

One should also check the fiber ring adjacent to the contact, as
it will wear faster than the contact and in turn cause a ‘ski
jump’ effect on the rotor. This can result in a very weak
spark, particularly at higher engine speeds.

When assembling the timer the cap should not be screwed down so
tight it binds the drive shaft and cap from rotating easily.

The angular contact ball bearings are NICE 505. These may be
hard to find today but any bearing dealer should be able to find an
essentially identical bearing made by another maker. Just give him
the NICE 505 part number.

For a single cylinder engine, any of the one to four cylinder
timers may be used. Just use one contact. For a two cylinder
two-cycle engine a timer with two contacts at 180 degrees is
required. A four contact timer may be used by selecting two
contacts at 180 degrees. For a two cylinder four-cycle engine with
the cranks on opposite sides of the crankshaft, a timer with two
contacts at 90 degrees or a four contact timer may be used by
selecting two contacts at 90 degrees. For a two cylinder four-cycle
engine with the cranks on the same side of the crankshaft, a two
contact timer may be used with the contacts at 180 degrees or a
four contact timer may be used by selecting two contacts at 180
degrees. In a pinch, running a two cylinder four-cycle engine with
the cranks on opposite sides of the crankshaft at low speeds at a
show, a three contact timer will sometimes work satisfactorily. For
a three cylinder engine one needs a three contact timer with the
contacts at 120 degrees. For a four cylinder engine one needs a
four contact timer with the contacts at 90 degrees. One should note
that some early four cylinder marine engines had a firing order of
1234 rather than the later more common firing order of 1243.

One can easily determine the firing order by examining the
intake and exhaust valve operating sequences. In the typical old
time marine engine the exhaust valve is closed at top dead center,
and the intake valve opens 8-10 degrees after the exhaust valve
closes. Further, most old time four-cycle marine engines are left
handed, that is the engine rotates counterclockwise when one faces
the flywheel.

Most Cuno timers are brass, but some were made of steel; most of
the steel units that turn up that were used in marine applications
are rusted beyond use. They are directly interchangeable.

In conclusion any of the typical vibrator type spark coil work
well with the Cuno timer. Six to twelve volt AC or DC may be used
depending on the coil requirements. Model T Ford spark coils work
well with the Cuno timer on six volts DC.

One coil is required for each cylinder. Coil paint erosion can
be reduced by starting on six volt DC and switching to 8-16 volts
AC for running. Early so-called ‘Jump Spark’ magnetos
typically put out low voltage AC that depended on how fast the
engine was turning. The advantage of AC being the points don’t
wear as fast as on DC. One trick the watermen used was to have two
batteries and with a simple three position switch (Plus-OFF-Minus).
They could run on a Plus to the coil for perhaps ? hour, then Minus
for the next half hour. This not only reduced point erosion, it
also lengthened battery life.

Remember, rotating the tier handle in the opposite direction to
rotation of the time top cap ADVANCES the spark.

  • Published on Mar 1, 2000
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