3-1/2 mechanical tach drive

[Canuck750]

I have a 1975 Moto Morini 3-1/2 that came to me with a mechanical tachometer (tach face labeled Moto Morini) and a 90 degree drive mechanism that passes through the electronic pickup cover plate, it all looks stock but I know that the 3-1/2 of this period would have come with an electronic tachometer.

The 90 degree drive mechanism take a standard flexible cable fitting on the exterior to take drive up to the tachometer. On the inside of the 90 degree drive there is a square sided shaft that somehow mates to the end of the cam shaft. The camshaft on my bike appears to be standard type, there is a round hole in the end of the cam shaft where the nut goes over the cam shaft to retain the pickup rotating disc.

My Moto Morini Riders Club parts manual has no referance to a mechanical tachometer, the 90 degree tachometer drive or the pickup cover that is used for the mechancial drive.

QUESTION: is there a mating piece that threads onto the end of the camshaft to mate with the 90 degree drive square stub? or do bikes with a mechanical tack have a different cam shaft with a square slotted hole in the exposed end?

[mbmm350s]

Hi Jim

The mech tacho wasn't fitted generally to 350s until 1979, and the parts book you have is for bikes built before 1979.

The part you need is shown in later parts books and supplements and should be same as on the mech tacho fitted 500s and 250s.
It is a long hex nut part 46.03.33 along with a spring washer, and replaces the nyloc nut. It is described as "nut with internal square"
happily no change in camshaft.
I don't have a spare one but I am sure someone will be able to help as no stock at Mdina Italia.

Hope that helps
Mark

[buzby]

On page 18 you can see the layout of the drive,

https://www.hofmann-andi.de/morini/Rica ... 250_2C.pdf

Dave.

[mbmm350s]

From parts book

Mech-tacho-nut.png (72.56 KiB) Viewed 5483 times

Mark

[Canuck750]

Thanks!! exactly what I needed,

part # 46.03.33 "nut" - Mdina Italy has no stock on these, anyone have one for sale?

[Daddy Dom]

For a new one you could try Alex Luzzi in Italy or Herdan in the states. Somebody might well have a used one, though.
DD

[Canuck750]

Herdan doesn't have any, I may end up making one on my lathe,

Progress so far on my 1975 Strada, waiting on new headers from Italy, carb arts, etc....

[Canuck750]

I was able to make a new special nut from round stock rod, lathe and miiling machine time

to make the square slot I found a great Youttube video, cut a slot along the length the width of the tach drive; 3mm and cut the depth to the center plus 1/2 the width of the drive, in this case a 14mm nut size so 7mm to center plus 1.5mm for 1/2 the width of the drive for an 8.5mm deep cut. Then use a 3mm drill bit as a spacer, srop in a 3mm wide plug and weld it in, then resurface in the mill





7mm x 1.0 internal thread



I made a new brass cupped washer as well, it was missing too

[3potjohn]

Brilliant work, better keep the info safe as you may get a request for more since they seem to be unavailable at Mdina
John

[mbmm350s]

Yes a brilliant piece of work and quite an inventive way to do it
and thank you for taking the time to describe it and up load the pictures
A question:
how did you decide about the length of the nut as you don't have a prototype to copy?
I guess so long as there is sufficient clearance all is good.
I could measure one for you if you like.

For the benefit of others reading this thread in the future.
The cupped washer is very important to properly fix the rotor,
without it the nut would bear on the soft aluminium core and
the rotor will be damaged.
Mark

[Canuck750]

A question:
how did you decide about the length of the nut as you don't have a prototype to copy?
I guess so long as there is sufficient clearance all is good.
I could measure one for you if you like.

A lot of measurements using the depth gauge of a caliper and then trial fitting by leaving the 90 degree tach drive off the cover and recording the clearance to the cover face then subtracting the protruding reach of the tach drive bolted into the cover, a few fine shaves in the lathe and I think I got it. The brass cup washer I made has a thicker depth so the nut I made would probably be too short with a stock cup washer.

Thanks
Jim