Crankshaft question

Opvoeren en modificeren van de FS1

Crankshaft question

Berichtdoor bmw02touring » 17 okt 2013, 15:30

Hi all, I´m new here. 40 years old from Sweden and just got an Fs1 dx 1978, that I´m going to restore and tune this winter.

Now to the question that I can´t seem to find the answer to anywhere. Back in the Days when I was 15 and last drove mopeds, it was commonly known that the old Fs1 (pre 1980) was much faster then the newer ones, given the use of the same cylinders, carbs, exhaustssystems.

The reason to this was probably due to the 2 holes drilled in the crank (2 on each side).


Can anyone tell me why these holes are in the crank? It should reduce crankcasecompression, but the holes in the crank are the only thing that I´ve found that are different in the older Fs1s compared to the new ones.
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Berichtdoor mpro09 » 17 okt 2013, 18:42

to be honest, i dont think pre 1980 fs1´s are faster than the newer ones
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Berichtdoor Tombo » 17 okt 2013, 19:11

I agree. And in my opnion the difference between a crankshaft with holes and the ones without isnt really big. My fs1 did about 85/90 km/h with a crankshaft with holes
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Berichtdoor bmw02touring » 17 okt 2013, 19:50

Thanks for the replies.
Well in Sweden back in the mid eighties they were faster for sure.
One thing I don´t know if its true either is that the pre 1980 Fs1s had only 2 transferports, whilst the 1981 and forward had 3 ports.

The question still remains, What is the purpose of the holes in the crank? What did the engineers have in mind when they drilled them?
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Berichtdoor Tombo » 17 okt 2013, 19:56

Maybe for balancing purposes or something.

If you are talking about newer FS1’s they probably had a rotating valve inlet system. If you still have the slow rotating valve in it, then you can say that an older one with (probably with a reed valve inlet) is faster indeed.
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Berichtdoor bmw02touring » 17 okt 2013, 20:02

Tombo schreef:Maybe for balancing purposes or something.

If you are talking about newer FS1’s they probably had a rotating valve inlet system. If you still have the slow rotating valve in it, then you can say that an older one with (probably with a reed valve inlet) is faster indeed.


As far as I know the Swedish FS1 allways had the reed system, no rotating valves.
I dont thing its as simple as a balancefactor.

Well, I´ll have to try my old crank with holes in my Engine. If it doesn´t run well I´ll change to a newer one.
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Berichtdoor joosie1234 » 18 okt 2013, 15:59

bmw02touring schreef:
Tombo schreef:Maybe for balancing purposes or something.

If you are talking about newer FS1’s they probably had a rotating valve inlet system. If you still have the slow rotating valve in it, then you can say that an older one with (probably with a reed valve inlet) is faster indeed.


As far as I know the Swedish FS1 allways had the reed system, no rotating valves.
I dont thing its as simple as a balancefactor.

Well, I´ll have to try my old crank with holes in my Engine. If it doesn´t run well I´ll change to a newer one.


I actually do think it's that simple. To have a higher crankcasecompression they are sometimes filled-up with more lightweight plastics. The difference could be in the rotating valve intake, carburator, or the used cylinder.
Alles wat ik wil is oud en ruig.
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Berichtdoor NIX » 18 okt 2013, 18:36

All crankshafts are balanced and have about the same volume. The old ones are standard drilled, the newer ones are cast a bit differently. The difference in speed is caused by something else, I would say it's management of perception and some deformation during storage.

I hear people seriously try to convince me that standard 50cc FS1's with only a fast pipe, 16mm, FS1 cilinder and fast disc would do 100 easily and some even reached 130. Most of the time I don't even reply. Some times I ask if 130 was done in 4th or 3rd gear.

Fizzies are quick, parts are cheap, but they are not magic.

The last types like 2RU or 2RT were a bit less quality. Frames rust more and the crankcases are of softer aluminum. But they run all as fast as you tune them.
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Berichtdoor bmw02touring » 18 okt 2013, 19:00

The question remains. If the holes arn't à balancefactor, what are they for!
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Berichtdoor NIX » 18 okt 2013, 21:32

They are a balance factor. But it's just standard balancing, not individual. All old crancks were balanced with the same size holes.
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Berichtdoor bmw02touring » 18 okt 2013, 21:41

Ok. Then I know.
Is there any other difference between the old and the new Engine?
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Berichtdoor NIX » 18 okt 2013, 21:52

Many, but most components can be used, sometimes as a set.

The gear on the crankshaft has 6 splines old type and only 1 with a woodruff key on the newer types,
The old type clutch house is steel with a bronze ring around the bushing, new is aluminium, no ring, bottom plate old type has short studs with thick long bolts, new type has long studs with normal bolts,
Kickgear and in between gear can have different teeth,
Sprocket on old type is bolted, new type is with circlip,
Very old types miss the front bolt on the ignition cover,
Different types flywheels and ignitions,
Very old type shift forks have guide pins locked with split pins,

And some other details, but as said: most parts can be exchanged, sometimes in pairs or as sets.
Kijk eerst even in de FAQ.
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Berichtdoor bmw02touring » 18 okt 2013, 22:15

Thanks Nix!
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