FGS to SSM Conversion
Danny had asked if I would put together a little tutorial of how to convert an
early 60's Merc inline 6 to use the "Super Speedmaster" Lower unit.
The best candidates for conversion are the '62 and later Merc 700, 850, and 1000.
Second choices would be the '61 700 and 800 FGS versions due to a cosmetic mis-fit
at the front of the lower unit to tower mating surface.   The '61 800 DR can be
made to work but again is not a perfect fit.

The difference between the "Speedmaster" and "Super Speedmaster" is that the Super
  is a dual pinion, much stronger than it's predecessor. The Super evolved because of
  reliability issues once the motors got over 80H.P. The Supers were introduced in 1965.

There are a couple of issues to consider before deciding to make the switch to any type
of "quickie" lower unit.   In most cases you will no longer have a neutral or reverse gear
and once the motor starts you are in forward only and underway..  These are designed for
racing applications primarily and will only show a speed increase on a very lightweight
and high performance hull design.

Jeff Bowman, 1Mar05


All Super Speedmasters require short shaft (15") mid sections. They are designed to be run on 20"
or slightly taller transoms. Height of the cavitation plate is of little consequence; the units are able to run with the centerline
of the prop shaft even with the keel. On the front lower section of the nose cone is the water pickup.



I took the long road to the conversion as it was decided that the motors should have
solid motor mounts, so a complete teardown of the engine assembly was needed. Thanks to Dick Davis for
fabricating these beautiful mounts, it’s to bad they are hidden.






The short route would have been to simply:

  1. Remove FGS lower Unit.
  2. Replace the 2 rear cavitation plate mounting studs with new shorter studs.
  3. Cut off shift shaft in tower. Due to the DR style of water pump housing and the fact that the water pick up comes from the nosecone (forward section of the pump housing) the Shift shaft needs to be removed.
  4. Shorten the copper water pump supply tube by 1 ½" on the water pump end.
  5. Install lower Unit.


The Long route, including new motor mounts:

  1. Remove FGS lower Unit.
  2. Remove Powerhead from Pan.
  3. Pull Pan, Clean all gasket surfaces. Remove shift shaft assembly.
  4. Remove reverse lock rod and trigger assembly. (no longer used)
  5. Separate Tower from swivel assembly, replace motor mounts.
  6. Reinstall tower to swivel assembly.
  7. Reinstall Powerhead and pan assembly using new gaskets.
  8. Shorten the copper water pump supply tube by 1 ½" on the water pump end.
  9. Replace the 2 rear cavitation plate mounting studs with new shorter studs.
  10. Reinstall Lower Unit.









So what’s the difference?

  1. Depending on boat type, top speed can be improved by 10 to 15 MPH on a twin motor application.
  2. The gear ratio of the SSM and Speedmaster is 1:1, where the FGS lowers were approx. 1.85 :1
  3. Lower unit profile and Hydro dynamic differences. HUGE!! just a physical comparison tells the whole story.


  1. Weight: I was not able to do a weight comparison but I would estimate the SSM to be approx. 5 lbs. Lighter than an FGS, and by removing the shift shaft linkage and other miscellaneous hardware I would estimate the motor to be approx. 6 to 8 lbs. less in total. Add in the difference of the propellers (approx.9 lbs.) and the total comes up to approx. 15 to 17 lbs..




  1. The other major difference. The COOL Factor! There is nothing better looking than a pair of these on your White Merc!

And Yes, they are counter-rotating. :0)


<>A Million thanks goes out to Dick Davis and William Petrino
<>for all of their help in making this project such a success!

A little background history on Merc Speedmasters
by Sam Cullis
When the first Merc 6 cylinder motor was introduced in 1956, Merc's chief engineer promised that there
would eventually be 2 special high speed lower units for them. The stock lower units were very good up into the low 50 mph range.
One of the special lower units would be for boats that could run in the 50 to 80 mph range and the second special lower unit
would be for boats that could run more than 80 mph. Merc was already making racing lower units for their
very popular (and now famous) 2 and 4 cylinder racing motors that were running up to about 75 mph on small racing boats,
but these were not really big enough for the 6 cylinder motors.

Two years went by and Kiekhaefer turned away from producing the special lower units for the 6's for sale to racers.
Instead a side project came up that called for a special lower unit under a special short mid section for a world speed record attempt.
A lot of testing at Lake X lead to the Merc Mark 75H which raised the speed record to 107 mph in 1958, a full 7% increase over
 the previous record held by a one off special outboard motor based on a car motor. A very limited number of Mark 75H motors
with 1:1 gear ratio units were sold, probably 50, in addition to the 50 or so prototype pieces made before actual production.

Three more years went by and Kiekhaefer got wind of Scott McCulloch prepairing to introduce a racing lower unit for their
3 cylinder 60 hp motor. At about the same time Scott got their racing stuff to market in 1961, Mercury released the
2 special racing lower units they had promised 5 years earlier in 1956.

Right off the bat in 1961 Merc released 2 versions of the 1.5:1 Sportmaster and 4 versions of the 1:1 gear ratio Speedmaster.
The Sportmaster came in right and left hand rotation with a 2 piece drive shaft to accomdate the 2 different crankshaft
spline sizes in the 70 and 80 hp motors. Speedmasters came with full length driveshafts for either the 70 hp crank
spline or 80 hp crank spline, both types in right hand and left hand rotation. In general the internal works and hydrodynamics of
the first version Speedmaster was directly derived from the bottom end of the Mark 75 H lower unit, but altered to fit
directly on the bottom of the Merc 700 & 800 midsection. Water inlet is at the bottom front of the nose. Before the end of 1961
a heavy duty version of the 80 hp type came out with stronger bearings in the upper driveshaft bearing position.
With the introduction of the 100 hp six in 1962 an improved version of the Speedmaster was released with a slightly
different housing and different water inlet position.

The Sportmaster was not a commercial success, because in 1961 there were very few boats that ran between 50 and 80 mph.
You either had a pleasure boat that went something under 50 mph or a raceboat that went well over 75 mph;
only a very few boat racers used them with success. Cosmetically a 1961 Sportmaster looks very much like a
Merc 450 or 500 fishing lower unit. Almost all of them were set aside and lost or mistakenly trashed as unused 50 hp units.
Water inlet is the same place as fishing Mercs and many other brands of the 1960 era, from prop spray into
an inlet on the bottom of the cav plate.

By 1964 racers were really punishing the Speedmasters in marathon racing under the 100hp stock motors
and 110-115+ hp modified motors.   Some races were won by fast pit changes of worn out Speedmasters getting back
on the course before the other guy changed out his busted lower unit and got out. Merc's answer to this was the
"Super Speedmaster" introduced in 1965. Again the gear ratio was 1:1 and forward only ... no neutral or reverse.
The Super version also came in right and left hand rotations. The Super Speedmaster was stronger than the regular
Speedmaster because it had 2 driveshafts one behind the other turning opposite directions driving 2 gears on the prop shaft.
Equal loading (more or less) of the gear pairs was acheived by making the lower driveshaft sections very thin.
So thin that they flexed. The Super Speedmaster prop shaft was also supported by more bearings as were the dual driveshafts.

Super Speedmasters (SSM's) were sufficient until hp approached 150. At 150 hp right hand SSM's were still up to par,
but left hand SSM's could not take the load. Merc's solution for boats running 2 motors wanting one RH rotation prop
and one LH rotation prop was to run one motor backwards and make a few minor changes to a RH SSM to allow it to
work with CCW input and output. A second tier solution was to use the first MerCruiser sterndrive Super Speedmaster
which had turned out to be a little weak for sterndrive use. This SSM was called a MC-1 SSM for "MerCruiser type 1 Super Speedmaster"

The first V-6 racers used the same SSM's as the inline 6's. The only significant change was the introduction of 14:15
gear ratio instead of 14:14. As power dramatically increased with the V-6's 2 larger heavier SSM's were introduced:
the VI and the IV (6 and 4). The 6 is larger than an inline SSM, but slim and trim compared to a big fat type 4.
The VI and IV only bolt up to the special racing mid section, no longer an accessory fit to the std mid.

As far as putting one on your 150 the ideal one to find and use is a RH SSM or MC-1 SSM.
A few words of caution:
(1)start in gear forward only is a big fat pain in the butt
(2) props for all Speedmasters start at 15 inches of pitch and go up into the 20's.
These are the equivalent of 30 to 42+ inches of pitch at the 150's std 2:1 gear ratio. If your boat isn't already running 80 or so,
plan on spending $600 or more for a custom made prop to use a SSM at lower speeds
(3) SSM gears and bearings are tiny, wear is instant and continual.
Change your gear lube EVERY time you use the SSM
(4) exercise caution buying a used SSM. You are probably buying a used up piece that needs $1000 worth of rebuild
(5) don't expect much if any speed increase on a rig over 550 pounds with driver. SSM's were just not made for this use<>
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