TECHNICAL OBSERVATIONS
The Lauson OB-410 (or A-410) Sport King outboard was the only model offered by The Lauson Company of
New Holstein Wisconsin, for the years 1940 through 1947.
The 4-cycle air-cooled motor utilized a cast iron block with an aluminum head. The block, after casting, was
painted with a red rust preventative then machined and assembled with no additional paint applied. This model
used a wet-sump oil system with a plunger type oil pump driven by an eccentric on the lower (submerged) end
of the cam. The cam, having timed oil ports around the eccentric made for a very simple system, one pulse of
oil for every two revolutions of the crank.
In 1940, pre-serial #1200* models had a tiller handle that would pivot from the "storage" to "forward" position
only and was held by wing-nut tension. The later model used a tiller handle that would lock in three positions-
"storage" (facing starboard in front of the motor), "forward", and a unique position facing "portside". This
"improved" handle also featured a "tip-up" action, but had no tension adjustment and would not stay vertical.
The 410 remained basically unchanged until 1947. Starting with serial #7OB2010** the Eisemann magneto was
replaced with the Wico magneto. Production ended for the 410 in 1947.
* It is not known at the time of this writing if the starting serial number was "0001" or possibly "1001", at any rate
there were, at the most, 1199 of these rare versions made.
**The production numbers from #7OB2010 to the end are not known at this time, the highest serial number
known at the time of this writing is #7OB3041. This motor has a unique reversed letter "B" in the serial number
and features a brass exhaust tube as opposed to the steel exhaust found on the previous models with the
Eisemann magneto.
TECH TIPS
Oil seals- The lower crank seal can be replaced with Chicago Rawhide (CR) #6541.
The prop shaft seal can be replaced with CR #4985. To replace- CAREFULLY remove the brass retaining
shield, then the two rubber seals. Chamfer the bore edge slightly to remove any burrs. The new seal should be
pressed in far enough so the brass retaining shield can be reinstalled to retain a stock appearance.
Oil leaks- Although replacing the seals and gaskets will control most of the leaks, there is one area in
particular that needs attention. The mating surface of the iron block and the aluminum lower crank case cover
are aligned with a dowel pin, during assembly the lower cover is distorted as it is pressed down over the pin. To
repair this leak- remove the dowel pin from the block, drill and tap a 1/4 - 20 thread in the block where the pin
was, and install a 3/4" long bolt with a lock washer, slowly drawing the surfaces back together. There is a
recess in the oil sump surface for the bolt head so no further modification is needed.
Another common oil leak is at the crank case breather near the carb. The inner body of the breather has a
drain-back hole that needs to be facing down, also verify this hole is not plugged. Replace both the breather
body and cover gaskets with a slightly thicker material.
The Lauson OB-410 (or A-410) Sport King outboard was the only model offered by The Lauson Company of
New Holstein Wisconsin, for the years 1940 through 1947.
The 4-cycle air-cooled motor utilized a cast iron block with an aluminum head. The block, after casting, was
painted with a red rust preventative then machined and assembled with no additional paint applied. This model
used a wet-sump oil system with a plunger type oil pump driven by an eccentric on the lower (submerged) end
of the cam. The cam, having timed oil ports around the eccentric made for a very simple system, one pulse of
oil for every two revolutions of the crank.
In 1940, pre-serial #1200* models had a tiller handle that would pivot from the "storage" to "forward" position
only and was held by wing-nut tension. The later model used a tiller handle that would lock in three positions-
"storage" (facing starboard in front of the motor), "forward", and a unique position facing "portside". This
"improved" handle also featured a "tip-up" action, but had no tension adjustment and would not stay vertical.
The 410 remained basically unchanged until 1947. Starting with serial #7OB2010** the Eisemann magneto was
replaced with the Wico magneto. Production ended for the 410 in 1947.
* It is not known at the time of this writing if the starting serial number was "0001" or possibly "1001", at any rate
there were, at the most, 1199 of these rare versions made.
**The production numbers from #7OB2010 to the end are not known at this time, the highest serial number
known at the time of this writing is #7OB3041. This motor has a unique reversed letter "B" in the serial number
and features a brass exhaust tube as opposed to the steel exhaust found on the previous models with the
Eisemann magneto.
TECH TIPS
Oil seals- The lower crank seal can be replaced with Chicago Rawhide (CR) #6541.
The prop shaft seal can be replaced with CR #4985. To replace- CAREFULLY remove the brass retaining
shield, then the two rubber seals. Chamfer the bore edge slightly to remove any burrs. The new seal should be
pressed in far enough so the brass retaining shield can be reinstalled to retain a stock appearance.
Oil leaks- Although replacing the seals and gaskets will control most of the leaks, there is one area in
particular that needs attention. The mating surface of the iron block and the aluminum lower crank case cover
are aligned with a dowel pin, during assembly the lower cover is distorted as it is pressed down over the pin. To
repair this leak- remove the dowel pin from the block, drill and tap a 1/4 - 20 thread in the block where the pin
was, and install a 3/4" long bolt with a lock washer, slowly drawing the surfaces back together. There is a
recess in the oil sump surface for the bolt head so no further modification is needed.
Another common oil leak is at the crank case breather near the carb. The inner body of the breather has a
drain-back hole that needs to be facing down, also verify this hole is not plugged. Replace both the breather
body and cover gaskets with a slightly thicker material.
| Serial number location |
| Early style tiller mount |
| Later style tiller mount |
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