1981 Mercury 7.5 2 stroke

[Mc Tool]

The hole in the end of the crankshaft is usually there to enable the crankshaft to be pressed apart . A drift is placed in the hole and rested against the end of the middle main journal , wjth the crank properly supported the crank can be separated in to two halves to replace bearings and seals . My Tohatsu has an aluminium plug that is tapped out to allow the drift to be inserted . The plug must be refitted ( preferably a new one ) before the crank is pressed back together ..... I have always called them welsh plugs , you fit them in a similar manner to a frost plug . An air leak in this area could cause major issues in the bottom cyl.
In this case , with the one piece crank its probly there for ease of manufacture , probly easier the form the internal splines in a full length hole than a blind hole .

 

[jimmbo]

Are both Engines the Same Displacement? Were the Camshafts, the Heads, the Pistons, Intake Manifolds and Carbs, all the same Numbers on both engines. Something is different to move the HP and Torque Peaks to different rpms.

When it comes to Outboards, I am more familiar with 2 strokes, and both. OMC and Mercury, often had 3 engines that had the same displacement, but in 3 HP Ratings 90,115,and 140

Differences in the Carbs, Reeds, Porting, Exhaust Tuning, Heads(OMC), even the Block, were what made the Differences.
It was relatively easy to turn an OMC 90 into a 115, and with a bit more Work, and a few Parts, both the 90 and the 115 could be turned into a 140
Mercs were not so easy. The 90 could be made into a 115, but neither could be turned into a 140

 

[Mc Tool]

Are both Engines the Same Displacement? Were the Camshafts, the Heads, the Pistons, Intake Manifolds and Carbs, all the same Numbers on both engines.

yes , all the same , 808cc

Something is different to move the HP and Torque Peaks to different rpms.

Nah ,the torque and power are the same . With the 45hp they may well measure peak hp and torque ( I doubt it as there is a 50hp the same size ) but with the 35 they measure it at the revs it produces 35 hp and then say " oh , this one is rated at 35 hp at 5000rpm, when they measured the 45hp they took it to 6000rpm " .
Dunno but I suspect that in some markets larger motors attract more tax , and I do know some places limit hp on smaller waters .
I guess Im lucky that they decided to de-tune a 45-50 hp donkey rather than trying to wring another 10-15 hp out of a smaller capacity 35hp.

 

[jimmbo]

Something is Different for the Peak Torque to be in a higher Rpm Range

 

[Texasmark]

Point taken guys. I will ensure I run it in a larger bucket with more water whenever I fire it up at home.

I thought I read somewhere that it will sputter until the thermostat opens. This one has a thermostat.

On my engines, pee stream has nothing to do with the thermostat. Its plumbed off the exhaust cooling water jacket, Port (left side looking from behind the engine) side of engine and obviously this water supply can't be restricted if you expect it to cool the exhaust while the engine is running.

 

[racerone]

On his 7.5 / 9.8 models the thermostat does have something to do with the pee stream.

 

[keithb7]

I’d like to discuss 3 ring vs. 2 ring pistons in the 7.5 and 9.8 engines.

The ‘74 9.8 I have, has 3 rings. Same bore and stroke as my ‘81 7.5 HP. Yet the 7.5 has 2 piston rings. Is this possibly required on the 9.8 to seal a wider section of the cylinder walls? The section of the cylinder walls with ports? The 9.8 using larger diameter ports. Larger holes in the cylinder walls need 3 rings to fill the larger ports?

The “Comet” designation on the 7.5 engine: I assume names the 2” bore 7.5 engines over a period of years. All having similar design and specs.

In the same era, the 4.5 HP has the same bore & stroke as the 7.5 but just one cylinder. What is it’s designed name?

Both the ‘74 9.8 and ‘79 9.8 engines I have here, the blocks are branded as “Lightening”. I assume indicating just a line of higher HP version of the 2” bore twin engines? Thx.

 

[jimmbo]

Older Motors had 3 Rings, improvements in Ring Technology, enabled the 3rd Ring to be removed, reducing Friction.
The Blocks, while both are Lightning, as mentioned above, have different Porting, and thus are different Part #s

 

[Mc Tool]

Something is Different for the Peak Torque to be in a higher Rpm Range

OK ,Im having trouble trying to get an image of the specs but the poxy computer wont let me cut a page off the pdf so I took a pic on my ph and emailed it to myself ( yeah , probly just me )and here tiz

If you look at max torque , it is measured a grand higher for the 45 , but Im sure that if torque was measured at the same speed as the 35 it would show the same torque as the 35 .( I would love to see the graph )
The idea for an outboard is to have as flat a torque curve as possible and what Im seeing here is that at least from 3500 to 4500rpm the curve is flat . This engine will happily spin up to 6500 rpm. Peak hp is usually higher up the rev range that peak torque so any way you look at these figures this engine has a little more to give than the figures would suggest.
I believe that when rating the hp of an engine if they are looking for 35hp they stop testing as soon as they get that figure and produce the specs we see here . If they want 45hp they just wind it out a bit more and then claim that figure . I also note that they do not give a max rpm for the engine .

 

[jimmbo]

There is some difference in the Motor for the Difference in Horsepower

 

[Mc Tool]

There is some difference in the Motor for the Difference in Horsepower

FFS ! THERE IS NO DIFFERENCE, they just measure it at the engine speeds that provides the figures they are looking for. Anybody with half a brain and who doesnt feel the need to keep stirring the pot could (would ) see that, even the friggin Honda dealer said they are the same powerhead .
You may recall 60's and 70's muscle cars , in some instances the rated hp was well under the actual hp . Factories do what they have to to get past legislation.
I aint interested in squabbling with you over this .

 

[jimmbo]

They can't label and market 2 two engines as different HP when there is no Difference.
There is a difference between the two

 

[Texasmark]

On his 7.5 / 9.8 models the thermostat does have something to do with the pee stream.

OK, How's that. Doesn't make sense to me for the reason given.

 

[racerone]

These Mercury models have the thermostat at the top of the exhaust cover.----A spring loaded cap for easy inspection / cleaning.-----Hose for pee stream comes off that cap.

 

[Mc Tool]

They can't label and market 2 two engines as different HP when there is no Difference.
There is a difference between the two

OK Jimmbo I give up . What are the differences

 

[keithb7]

Re pisser and thermostat on 7.5 and 9.8 engines.

Thermostat housing is seen here circled in red.
Hose to pisser, red arrow indicating water flow direction.


Thermostat housing open in next photo. When thermostat is closed water only spits through by-pass port, seen within red circle below. When thermostat is open full-flow water moves through passage circled in green.

 

[Texasmark]

Well it looks to me like the stat has to open to get pee. If its sitting on top of the exhaust manifold cover then there is water pressure against it from the impeller. So, the exhaust heats the cover plate which has cooling water on the reverse side with its cover. The whole shebang gets hot enough, the stat opens a little at first meaning minimum pee and as the engine runs longer the circuit gets hotter, the stat opens more and you get more pee.

I don't like the idea but "its not for me to say".....song I once heard, forget the origin.