Merc 4 Cylinder Cranks

[MTECHMARINE]

Reference

MARK 40
MARK 50
MK55 SMALL TAPER
MERC 300 - 500 THRU MID 60'S LARGER TAPER 40 and 44's

With correct end caps and flywheels will all these interchange??

And I have been told different cranks have different finish grinds on the crankpins. older are looser (smaller diameter) Which have what??

Been Evinrudin' for years, now back to 1st love "Sandcast" Merc 4 bangers! Need to refresh my knowledge base. Thank you. BILL

[Mark75H]

Not sure about the Mk40, but I'm pretty sure all the others interchange

[Danny Pigott]

Mark 55 were looser than the later cranks.

[Trident]

Reference

MARK 40
MARK 50
MK55 SMALL TAPER
MERC 300 - 500 THRU MID 60'S LARGER TAPER 40 and 44's

With correct end caps and flywheels will all these interchange??

And I have been told different cranks have different finish grinds on the crankpins. older are looser (smaller diameter) Which have what??

Been Evinrudin' for years, now back to 1st love "Sandcast" Merc 4 bangers! Need to refresh my knowledge base. Thank you. BILL

OK, here's the basics that I'm aware of...

Difference between 30 and 40/44 cranks is just driveshaft spline.

From there, there are two basic sizes to the rod journals. Early are .880" OD, later are .882" OD.

Early would be Mk40, Mk50 and early Mk30 and Mk55. Early 30 and 55 have a forging number 400-2 on #4 and the area where the bottom ball bearing runs did not clean up all the way down, the forging steps down. These are all small taper for the flywheel

Late had various numbers, but were machined all the way to the bottom, allowing room for double seals. Those were mostly big taper for the flywheel.

The rub is, I have seen a few 'transition' models (manufacturer reserves the right to change specs at any time) where the big journals were on a small taper... or was it other way around?

So, best to measure the journals.

All these will fit a 40 or early 44 with appropriate parts. Be careful though, with the later 'solid' top bearing cap... there are two lengths. The short is for the later-yet 44 cranks with the thick (approx. .100") top counterweight. You need the long, same length as the slotted cap version, for these cranks.

Hope this helps.

Jerry Wienandt
Trident Racing

[MTECHMARINE]

For adding to my knowledge base without my making big mistakes.

This forum is great! I might get some of this stuff running yet! Still have my Merc service manual CA: 1957 so I'll get down to work!

I have a whole series of Merc racers to go thru as well as a group of Wizards.

Here's a shot of some of the ones I have that are done. The KE4A is original as is the KG9 on H tower. WK7 is a restoration. Boat in the background is my Formula 100 rig from early 80's. 2 carb S Strangler on a MOD 50 boat!!

[Ron Hill]

Trident said, "Mark 40 is the same...as???" Or 40 Cubic inch.. I know the KG-9 is different, but thought the Mark 40 was too...


MtechMarine.. I have four 44 cranks, with the top taper......Giving them to Lonnie Morris, but could give you one! (How'd those props work out???)

Thanks, for posting GREAT FACTS!!!

[Aeroliner]

I have my Murcury MIDAS CD and will check part numbers. Do know that my 44SX Merc has a crank this is slightly different in that it has larger counterwights.

Alan

[Trident]

Trident said, "Mark 40 is the same...as???" Or 40 Cubic inch.. I know the KG-9 is different, but thought the Mark 40 was too...


MtechMarine.. I have four 44 cranks, with the top taper......Giving them to Lonnie Morris, but could give you one! (How'd those props work out???)

Thanks, for posting GREAT FACTS!!!

Mk40 is different from the KG9 in that the KG9 had a train of gears for the mag drive and a cam for a mechanical fuel pump. Mk40 was the first belt drive magneto and was equipped with a check valve (easily converted to a pump BTW, just like on 20H conversion) for a two hose pressure fuel system.

Upshot is the Mk40 crank works in a Mk55 or whatever...

The MK50 is similar...

One other characteristic to note on these cranks, is the shoulder on top, where the flywheel taper steps out to the seal / bearing OD is a sharp corner. It lacks the smooth rounded edge of the Mk30 / Mk55 early cranks, the 400-2's. That edge is nasty when installing the top cap in that it catches the seal. When I prep on of these, I put a tiny radius on that edge, lube it, and install the cap with great care. There's your free tip of the day!

Hope this helps.

Jerry Wienandt
Trident Racing

[MTECHMARINE]

Ron, the time is coming - - I have gone thru some real problems since I returned to the mainland from Hawaii - - Hurricane IVAN flooded my house and shop here in FL., separation and pending divorce, finances - your basic mess. Now we are much better off (letting fate take its course on the above) and I am taking time now to work on long awaited projects.

I have, to redo, these 4 cylinders
Merc 300 (This one just needs a good tune up.)
WA25
MK55 black block
MK55 sandcast powerhead
MK40H
MK50 "mod" motor that will be ported, KC6 carbs, etc similar to the motor that David just did, except I will run it on a light 13' glass runabout. Either a Q length tower or KG/MK40 std gearcase, for which you built me those 2 props.

SOON!

Pic is what we came back to in 2004.

[Trident]

I have my Murcury MIDAS CD and will check part numbers. Do know that my 44SX Merc has a crank this is slightly different in that it has larger counterwights.

Alan

Hi, Alan. Yup, all 44XS cranks have larger yet journals, approx. .883" (its stated as a range to the tenth on the print, not a nominal dimension with a tolerance), have a longer top 'shank' above the ball bearing and seal for the CDI, and have that approx. .100" thicker top throw I referenced in my earlier post. These also have the larger OD Center Main Bearing with the large rollers.

That thick top throw appeared way back on the Merc 500's, on the small CMB, belt drive ignition motors. I'd have to research what year. Those motors, even with the old style top cap, had a shorter top cap, by Duh! approx. .100" to make room for the extra crank length. These thick top throw cranks are way off in static balance, but its the only legal D stock 44XS crank. Key here is to ensure you have the right cap for your crank in whatever you're building.

Jerry Wienandt
Trident Racing