Building a Super Mod 2 cylinder Mariner/Yamaha

[hydrospeed77]

After asking around and searching for info, I have a feeling that the 6F1 was unique in that the displacement was exactly 750cc - It had slightly smaller bore pistons than the very common 760cc 82mm bore engines. I can't fully confirm that but I have a strong feeling that was the case based on part numbers and measurements of pistons.

The 6F0 has 84mm pistons for 798cc.

My guess is that all of them share the same crank and are 72mm stroke.

I noticed in some photos, the 663 Cyl Head is different from the common 675 Cyl Head, and looks like the 663 Air Box is smaller than the late 70s' Mariner 60 and mid-90s' Yamaha 55.

I've now seen photos of 3 different flywheel designs. 4 if you count the 6F0, which has different Flywheel, Stator, CDI than the most common newer Yamaha 55hp and older Mariner 60hp.

I had a great person help out and measure a new old stock 6F1 piston, and as per speculation, the 60DES bores were 81.30mm for a displacement closer to 747 or 750cc..
Stroke and rods are the same as the 3cyl 85/90, but ports are slightly different. 55B is also slightly different from the 85/90hp. Not sure yet on the 55B/60DES port differences - might be none.

[hydrospeed77]

I recently re-read this whole thread and want to clarify a few things I've found more recently.

There are claims in this thread the 6F1/60DES tuner was dual horn - it is not - you can find pics online and in the most recent posts here of a single opening tuner. Both cyls feed in to the one pipe tuner at the adapter plate.

Some folks are thinking that the 55B is a 2cyl version of the 85/90hp. They are very close but they are not the same. Sleeve port map is different, but close. Combustion chamber shape and volume is different. Carbs on the 2cyl are different in terms of venturi size and overall design. What is the same and even use the same part numbers are the rods and pistons (and bearings) - both the 90hp and the 55B use the same pistons and rods. I believe all these 2cyls use the same rods. 6F0 had 84mm bores, 6F1 had 81.30mm bores. 90hp and related 3cyls,and the 55B have 82.00mm bores.

[Supremesoundzuk]

Indeed my tuner is a steel construction with a single cone I’ve recently had it repaired and ceramic coated

There is also much confusion as to the ratio of the gearbox
The 55b was 1.85 13t 24t
The 60de 6f1 is 2.0 13t 26t

Bores as your stated the 6f1 is smaller

My motor parts are now back from being cerakoted happy with the natural looking finish wanted them to loom retro just out the cast look

[Supremesoundzuk]

[hydrospeed77]

To clarify and correct for historical purposes, the 60D (6F1) came with 1.85 gears from the factory (13:24). I'm guessing some racers lobbied to have the 13:26 gears from the 2cyl 40hp (679) officially homologated in via supplemental homologation.

update:
I have older Yamaha docs saying 60D came with "675" gears, like the 70CES (1.71), and a Yamaha pamphlet from like 1984 or 1985 stating 1.85 gears for the 60DE.
So either a change at some point or one of them is incorrect.

[Supremesoundzuk]

To clarify and correct for historical purposes, the 60D (6F1) came with 1.85 gears from the factory (13:24). I'm guessing some racers lobbied to have the 13:26 gears from the 2cyl 40hp (679) officially homologated in via supplemental homologation.

No the 6f1 was homoglated with 13t 26t

[Supremesoundzuk]

Bit more progress

Dry fit of the parts to see what it would look like

Still fettling away at the gearbox making the nose cone look a little more oem, tweeked the water definition plate to blend into the cone and tweeked the anti ventilation plate a little just straightened it up and blended it in lost a little bit of material off each side

[hydrospeed77]

No the 6f1 was homoglated with 13t 26t

Yes, but only at a later date. It never came with 2:1 gears from the factory. The RYA homologation that includes the 2:1 gears clearly states those gears are from the 40F "679" model, and in fact shows the "675" part number as the factory gears (same part number as the 70CES 6J2 factory gears).