Mercruiser 120

[Ghg]

I have 16 ft boat with a 1967 Mercruiser 120, I’m running 23 inch pitch prop and was told it’s too much pitch, I have a Mercruiser service manual and the chart recommends a 23 inch prop. What other pitch prop would be recommended?

 

[makomark]

those charts are for starting points.

The correct prop allows the engine to operate in the WOT operating band (RPM range) with a full load on board.

If you are BELOW the recommended RPM value, try another prop with less pitch.

A good local prop shop will work with you to get the correct wheel and do so economically...

 

[Droid]

Those old charts were made in the old days of non cupped props. By the mid 70s most props were cupped which effectively is like adding 1" of pitch.

Mid 80s Mercury switch to the Black Max series of prop which effectively added another 1" of pitch.
I'll bet you run a 21P if lightly loaded or 19P if heavy. Top speed 35 to 38 MPH.

Next and most important problem....
The Black Max series props used a different style thrust hub. You have the square shoulder prop shaft & they never made a replacement thrust hub for that "pre 1970" shaft. I have the service bulletin & will find it & post a bit later. Do not try to put a Black Max style (regardless if Mercury, Michigan Wheel, Solas or whatever) without the proper mating hardware or you will cause damage to gearcase or if lucky only loose the prop depending on what thrust hub combination you mistakenly use.

Michigan Wheel used to make the replacement early style (pre Black Max) props. Not sure if they still do.

 

[Droid]

Here are the service bulletins.

1st one talks about MC1 prop to Black Max. Prop #s listed will be the latest generation (supersession) of MC1 props, not what is in your chart above. All earlier MC1 props are included.

2nd one references the pre 1970 square shoulder prop shafts (yours).

I don't know why they made the change but I saw several damaged gear cases & lost props back in the mid 80s.

 

[Droid]

Square shoulder

 

[Droid]

MC1 style on left. Rubber hub is visible & recessed. Prop body pushes against the step of hub.

Black Max on right. Rubber is not visible. If you try to install with original stepped thrust hub, "castle hub" for tab washer/nut will not engage splines of shaft. Nut will not thread on enough & will back off & say goodbye.

Also a close-up of MC1 prop.

 

[Droid]

Original square shoulder thrust hub on left.

Original tapered shoulder thrust hub in middle.

Hardware required to install Black Max prop onto square shoulder shaft on right.

 

[Droid]

I have a pair of never been rebuilt early 70s vintage props. 19P & 21P
Also the hardware new as seen in picture the picture above.
Will sell if you want it.

 

[Ghg]

Ok, thanks for the input, I’ll have to sort this info out