Mk 55h

[roxin2002]

If you took a fishing mk 55 powerhead and changed the reedcage, intake manifold and carbs off of of a mk58a would you have the same thing as a factory 55h powerhead??
Also, do all mk 55;s have padded cylinders and is there any truth to the fact that the dark block early 1955 mk 55 fishing motors are more powerfull than the later ones with the intake port covers two on one casting vs the 4 indivual 4 intake covers.
One more question, how did merc get the extra 5 hp for the 50 hp inline 4 that I believe came out in 1961.
Thankyou for reading this and for your answers what ever they are.

[Mark75H]

No, the porting and pads are different on the fishing 55 and 55H. The ports are all .060" higher on the H and the compression is 20.5cc vs 24cc.

Dark blocks are just an anodized finish, it was test marketed to see if it would be stiffer and if any advantage would come of it. None did.

The extra 5 hp comes from exhaust tuning & more accurate advertising; they didn't want to sell a 50 hp 4 and a 60 hp 6 at the same time, so the 4 was called a 45 while they were pushing the 60 hp 6. When the smallest 6 was a 70 and then 85, the 4 became the 50.

[mk30h]

The Merc 400 had 43.9 ci while the KF7- Mk 55's had 39.9 ci, doesn't that account for the 5 hp difference? I would be interested to know however how the 61 Merc 400 45hp and 500 50hp differed as the only difference I can see is the cowl.

[lilabner]

The 44 ci 40 hp motors had small carbs
The 44 ci 45 hp motors had big carbs
The 44 ci 50 hp motors added an exhaust stack in the lower unit..
This was the basic difference, without getting technical about porting and compression..
Some of the 44 ci 45 hp blocks had the bosses for the stack.
Going from a Merc 400 to a Merc 500 added about 4 mph on my Raveau..with the same prop..I held the first 50hp kilo record in OPC set at Lake X.

[Mark75H]

The Merc 400 had 43.9 ci while the KF7- Mk 55's had 39.9 ci, doesn't that account for the 5 hp difference? I would be interested to know however how the 61 Merc 400 45hp and 500 50hp differed as the only difference I can see is the cowl.

You are jumping around between the stone age and iron age and later

I'm pretty sure you meant KF9, not 7

Yes, the 44ci Mk58 accounts for 5hp increase over the 40ci Mk55.
I'm pretty sure the 44ci 500 has a different combustion chamber and exhaust tuner that the 44ci 450 doesn't have

[25ss]

I just got a 55h mid and lower what would be the best powerhead.
I have a 55 fishing and 58 reeds and carbs.
I have a friend with a outboard graveyard what would be the biggest best to look for that would fit the mid?
Dale!

[Mark75H]

Merc 500's have a slightly different bolt pattern, the back 2 studs were moved back a little with the combustion chamber change, so you are looking for a 44 that has the early pattern ... a Mk58 or Merc 450 (or as a last resort a Mk55A with a front from a higher hp 44).

If the mid you got is actually a Mk40H mid it will have a boss in it that holds the water tube ... to make a Mk50-Merc450 fit on it you have to saw and file the water tube boss off the lower cowl so it fits flush on the mid; if it is really a Mk55H mid, it all bolts right up.


With a fishing throttle hookup you can use a regular throttle cable on the std connectors on the motor and saw the front of the cable off to fit in the throttle ... just make sure you pull the inner wire back while cutting so enough will stick out to connect inside the throttle later; a foot of extra inner wire can be trimmed later ... cut short is junk. Don't forget to remove all the shift limit/interlock stuff so you have full swing.

[velox]

The 44 ci 40 hp motors had small carbs
The 44 ci 45 hp motors had big carbs
The 44 ci 50 hp motors added an exhaust stack in the lower unit..
This was the basic difference, without getting technical about porting and compression..
Some of the 44 ci 45 hp blocks had the bosses for the stack.
Going from a Merc 400 to a Merc 500 added about 4 mph on my Raveau..with the same prop..I held the first 50hp kilo record in OPC set at Lake X.

If anyone knows these engines, its Liabner.
There was one more slight difference between the 450 and 500 and that is the 500 had small shocks for tiltup and the 450 did not. I ran a 450 with the 500 exhaust stack on my Raveau and it seemed to be equal to the 500's in power.

[lilabner]

Sam, You must be thinking about some other Merc 500..The ones built in the early 60's had the same block patterns as a Mark 55..we even put them on Wizard 25 lower units with a Mark 30 crank to build a sleeper..I'm sure the lower pans were the same..Mercury built modular motors back then to save on production costs I assume..I just don't remember any difference at all..I worked on them full time till 1964..

Butch

[Mark75H]

Its not much of a difference and maybe the first year or two of 500's used the old pattern ... but it is well known to current D mod racers ... most of the millions of 500's don't exactly fit a D Quickie tower

[25ss]

Sam
Thanks for the imfo it is the later 55h mid.
I will be looking for 58 or at least a 58 front.
If not i guess you change the bolt pattern and open up the intake.
Also where can i get some shims for the lower the gears are ok but i dont think its setup rite.
I got some for a 30H several years ago but forgot where.
Dale!

[Mark75H]

The bolts are too close to do anything but make a mess. Most guys just leave them out ... 6 seems to be enough. Any 44 ci front (except for later ADI motors) will be set up for KA's and contrary to popular belief ... you can swap crankcase halves (usually)

I don't know where to fins shims either

[lilabner]

Thanks for the info Sam..I thought they were all the same..I remember putting a 500 and a 58A on 15 inch D quickys on a cat once, just don't remember any issues..

Butch

[mk30h]

Sam, oops I did mean to type KF9.
I have a couple of Mk 55's, a 60 Merc 400 and a 69 Merc 500. I blew a piston on my 55H a few years back and plan to use the block from the 400 as replacement, it is good to know the later 500's can be used without much effort.
The question I had regarding the differences between the Merc 400 and 500 referred to the units that did not have jet prop exhaust. Was there and exhaust stack in these units?
My 400 and our orginal family 1960 400 both have the shocks while my 69 500 doesn't. I don't think the 63 450 had the shocks, but it was a jet prop unit.

[Mark75H]

Thanks for the info Sam..I thought they were all the same..I remember putting a 500 and a 58A on 15 inch D quickys on a cat once, just don't remember any issues.. Butch

If it was one of the first year(s) of the 500, no problem, its the same as the 58, your memory is correct

[lilabner]

[quote= your memory is correct[/quote]


Bless you..that's a first lately..

[2us70]

There were a lot of parts from the old Mark series motors that could be used in the later 500s. I found that the old fuel pump check valves and diaphrams worked better than the newer parts. Also the timing belts from the older motors were tighter and fit better giving better timing control. OPC inspectors would look for those parts and DQ you if they caught you with parts from the earlier motors in a late model one.

[Mark75H]

A lot of the parts have the same part number

[Xcelerator]

Merc 500's have a slightly different bolt pattern, the back 2 studs were moved back a little with the combustion chamber change, so you are looking for a 44 that has the early pattern ... a Mk58 or Merc 450 (or as a last resort a Mk55A with a front from a higher hp 44).

If the mid you got is actually a Mk40H mid it will have a boss in it that holds the water tube ... to make a Mk50-Merc450 fit on it you have to saw and file the water tube boss off the lower cowl so it fits flush on the mid; if it is really a Mk55H mid, it all bolts right up.


With a fishing throttle hookup you can use a regular throttle cable on the std connectors on the motor and saw the front of the cable off to fit in the throttle ... just make sure you pull the inner wire back while cutting so enough will stick out to connect inside the throttle later; a foot of extra inner wire can be trimmed later ... cut short is junk. Don't forget to remove all the shift limit/interlock stuff so you have full swing.

Will a mk40 powerhead bolt to both 40h and 55h towers?

[Mark75H]

The bolt pattern is the same, but you would have to make a base gasket shaped adapter from a Mk55 bottom pan to receive the water tube to use the Mk55H tower. That might cause other problems like stud length too short and driveshaft spline engagement too short.

If you are building a sawhorse queen that will never see water or run, the pieces will fit together ...

If you have a boat and want to run something, you'd be better off trading the Mk40 powerhead for an early 44ci or later than Mk40 40ci that fits the Mk55H tower