It took a little longer than usual for me to start working on the engine of my Yamaha MF1

Yamaha MF1 engine assembly.

1)

Before building the engine of my Yamaha MF1 the crankcases are thoroughly cleaned with very fine steelwool. Thereafter they are washed in luke warm water with some mild detergent and dried.

2)

As a first step the bearing can be pressed into the crankcase halves. I always use a small press to do this. Using a hammer or so is asking for trouble, especially with small engines likes this one. The respective oilseals (with a good coating of grease) can be pushed in with your thumbs easily.

3)

Please note that when you build vertical split engines, you always have to try to fit-in each shaft between the cases beforehand, otherwise you might be faced with a tight-running shaft, The kick-starter shaft is inserted with its spring  in the l.h. crankcase. But it has to be pre-tensioned about half-a-stroke in this model.

4)

I have done this by fitting a brake-lever on the shaft, tensioning the shaft and then fitting an 8 mm bolt on the outside. I added a nylon bush as otherwise the whole thing could slip back easily during assembly.

5)

Now the crankshaft can be inserted into the l.h. crankcase. The problem was (therefore it took so long) that the crankshaft has left handed thread 10 mm x 1.25. It took some time before I found a supplier for the tapping tool I needed to make my puller. This bush has l.h. thread on the crankshaft side.

6)

The regular dynamo-puller can be screwed onto the bush now.

7)

The Yamaha crankshaft puller-housing slips over the dynamo-puller rod and the crankshaft slides easily into place without any stress.

8)

I had to fit different shims on each side as the crank-webs were touching the crankcases.

9)

The gearbox shafts are ready for fitting now.

10)

But the gearbox cluster fits easily in the ball-bearing and the bronze bush.

11)

The gear-shifter can now be installed. It has quite a few parts as ylou can see.

12)

Firstly the rear activating arm is bolted into place with 2 6 mm bolts; it has only one simple “blade” to move the gears (the MF1 has only 3 speeds).  At the same time the steel ball and spring for the locking cam have to be inserted (upper left).

13)

The gear-change shaft with the rest of the shifter mechanism is now fitted with 3 more 6 mm screws and washers.

14)

The kick-starter gear with the ratchet and kick-pawl are now put on the kick-starter shaft and locked with a seeger-clip. As the kick-starter shaft was pre-tensioned (3) and 4)) the kick-pawl is now in exactly the correct position when the 2 crankcase halves are joined together.

15)

I always use Loctite # 574 for putting engines together. It is the best there is. When you take it apart later on, you only need some thinners to clean everything again. Please note the small metal piece above the central ball bearing. That is where the kick-pawl has to fit precisely!

16)

The r.h. crankcase is now carefully put onto the l.h. assembly. I never press the crankcase onto the other half. I use the Yamaha puller again. So the l.h. threaded bush I made is screwed onto the crankshaft together with the dynamo puller bolt.

17)

The Yamaha crankshaft-puller housing is used again and by tightening the nut the crankcase half slides smoothly over the crankshaft.

18)

By carefully guiding the crankcase half over the shafts and at the same time tightening the nut further the crankcases will finally smoothly fit together. With any other method the crankshaft will twist/bend  slightly, this way you guarantee yourself a nice, free spinning crankshaft.

19)

When the crankcases are bolted together the oil-splash plate is attached as well.

20)

The gearbox driven wheel is now simply put on its shaft and fitted with a nut and lockwasher.

21)

The brass washer behind the primary gear is placed onto the crankshaft.

22)

The primary gear slides nicely in olace and meshes correctly with the secondary gear. (I had to add a small shim under the secondary gear).

23)

The clutch boss is fitted and the inner face meets that of the splines on the crankshaft.

24)

The clutch assembly is now carefully placed on the clutch boss and fastened with a nut and lockwasher as well.

25)

The old con-rod was bent and the piston worn and burnt. As the original parts (when you can find them) are still inferior to modern ones, I decided to use “today’s” Yamaha parts. As many 50 cc Yamaha engines have this 40 x 40 mm bore/stroke, this was not so difficult. A MA/SA/S/CT 50 connecting rod was used (2E9-11651-00). This rod is 2 mm shorter than the original and a little thicker. So the sides had to be chamfered slightly to fit between the crank-webs. The cylinder got a first o/s bore and I use a PW50 piston (18U). This piston is slightly “taller” than the original one, so that might help a little!

26)

The “old” MF1 dimension is 104 mm from the crankpin to the top of the piston crown. The new conrod with the PW piston measured practically the same, I only have to add one more cylinder base gasket! Also very important is that the original MF1 conrod has a plain bronze bush as piston pin bearing, the modern conrod utilizes a normal needle bearing of course.

27)

The PW piston fits nicely on the con-rod and the extra cylinder base gasket is added. Piston rings have been “tested” in the cylinder barrel for slot clearance of course.

28)

The cylinder slides properly over the piston and the clearance is okay.

29)

The engine now comes together more or less.

30)

The clutch push plate with the steel ball in its centre  can now be added.

31)

The clutch lever is fitted into the l.h. crankcase cover. Yamaha did not do things easily in those years! The lever has a washer and a felt ring on the outside, an o-ring in the bore, next comes a shim, then a large clip to secure it. The cam activates a pin and on the pin slides a collar, which rests again the clutch push plate from the previous picture.

32)

The r.h. clutch cover is now fastened with its 6 screws, it looks better again!

33)

On the left the rotor can now be fitted with a new woodruff key. Always grease the shaft well! The cam is a separate part and is fitted at the same time of course. You can see that the MF1 has worked not very hard in its life, this component shows how “fresh” it is.

34)

The stator is now attached, brushes fitted and the ignition adjusted. I do hope it is okay with the old ignition points. Point gap set on the usual 0,3 mm, ignition timing is 1,3 mm b.t.d.c.

35)

Everything has been cleaned as far as possible. The condenser is new, the brushes are made from larger ones I had. With the battery leads on the starter generator the engine is running! Hopefully the rest functions also…….