# Crankshaft model BMW R 75



## MuellerNick (May 1, 2013)

Hi!

Long time no read. I have been distracted by other things.
And as a side-side-side-side-project, I made a bunch of crankshafts for a BWM R 75. This is a sidecar motorcycle from WW II.
I had the idea while working on something else. For me, it was quite obvious to build exactly this crankshaft. First, the motor(cycle) is quite famous. Second, in my little villige, there is a guy making spare parts for them (I do make some grinding jobs for him). He has all the original plans.
So I asked for the drawings and also got a crankshaft.

This is the result:



5 of them, together with the original on a drawing (part of) the crank. If you are clever, you can detect the scale. Right! 1:5

Pictures were taken with a flash, the finish looks much nicer in real.












Jig for the second milling op. The brass pin indexes the second crank pin relative to the first.



With the work in the jig. The jig for the first op looks similar. Just shallower, because in the first op, the center web is machined too.
The outer web is milled with an endmill. The crank pin is milled with a circular saw, like the inner web.

This crankshaft is only intended for look and see. For playing at your desktop, or attached to the keys, or ...

It is made out of stainless (1.4305). Machining almost takes 1 hour. Can't rush the sawblade ops, or the blade will start to climb out of cut.
The original is pressed together, for the conrods. As I don't want to add them, a single piece is good enough for the purpose.


Nick


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## Hopper (May 1, 2013)

MuellerNick said:


> Hi!
> This crankshaft is only intended for look and see. For playing at your desktop, or attached to the keys, or ...
> 
> Nick



... to build the rest of the bike around. I've started full-size vintage bike restorations from less. 
On yer bike now...

Very nice work BTW. I know a few BMW nutters who would love one.


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## canadianhorsepower (May 1, 2013)

very nice work


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## MuellerNick (May 6, 2013)

Last but not least: [ame="http://www.youtube.com/watch?v=YCaAXpDC7Ag"]A video[/ame]

Enjoy!
Nick


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## jwcnc1911 (May 6, 2013)

You've got all those prints... am I the only one having this thought right now???


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## MuellerNick (May 7, 2013)

I don't have them. But a good friend in my little village.

Nick


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## Mitchg07261995 (May 7, 2013)

are you planning to sell a couple crank shafts?


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## MuellerNick (May 7, 2013)

> are you planning to sell a couple crank shafts?



Only if someone wants them. hahaha
I made a few, because it really looks nice if you arrange them in an array. I did not intend to build an engine around it. One is in the bag of my trousers now. But still, I can spare some of them or make a new run.

I like crankshafts, they are like the heart of an engine. A friend thinks the same. So he will go to Porsche (at Weissach) next week and ask for drawing of a 911 (and other engines) crankshaft. He has some connections there, so things look good.


Nick


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## jwcnc1911 (May 7, 2013)

do you have access to drawings for the complete bmw engine?  I'm thinking that would be a pretty sweet model.


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## Mitchg07261995 (May 8, 2013)

MuellerNick said:


> Only if someone wants them. hahaha
> I made a few, because it really looks nice if you arrange them in an array. I did not intend to build an engine around it. One is in the bag of my trousers now. But still, I can spare some of them or make a new run.
> 
> I like crankshafts, they are like the heart of an engine. A friend thinks the same. So he will go to Porsche (at Weissach) next week and ask for drawing of a 911 (and other engines) crankshaft. He has some connections there, so things look good.
> ...



I would be interested in one Nick, not to build a whole complete engine but to have on the book shelf  How much for just one?


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## Mosey (May 8, 2013)

Better still, have him bring home a 911.
Mosey


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## MuellerNick (May 8, 2013)

> Better still, have him bring home a 911.



He has one already. And I can't afford one, I have to stay with my $5 bicycle.



> I'm thinking that would be a pretty sweet model.


That would be a nice model, admittedly. I had a deeper look into that. The fins of the cylinder barrel and the cylinder head are quite thin, even on the original. So a reasonable scale would be 1:2. But still quite a challenge to cast.

BUT! That would cost a lot of money and take a lot of time to develop. At a scale of 1:2, the displacement would be 750 / 2^3 = 93.73 ccm. So that should be a good runner.


Nick


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## mu38&Bg# (May 8, 2013)

Some of the guys who have been here longer will remember this build.

http://www.homemodelenginemachinist.com/f31/bmw-r7-motorcycle-engine-13661/

Greg


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## MuellerNick (May 8, 2013)

> Some of the guys who have been here longer will remember this build.



Thanks for reminding me of this excellent work!

There are two points that would stop me building one in that scale:
* Idle RPM is too high
* I don't like milled billet when there should be a casting

Don't get me wrong, this work is extremely good. I personally would like these feature on my list. Even if I'd have to make the flywheel out of solid tungsten or densimet. 


Nick


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## mu38&Bg# (May 8, 2013)

Absolutely, castings are wonderful if you can do them. Idle speed is one of those things that doesn't scale well.

On my mind for some time now is a BMW M88, but that will likely never happen. I'd end up cheating with a bead blasted CNC block and head. Nick, I don't know how you manage to get so much done and still have time to make videos and post as much as you do.

Greg


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## MuellerNick (May 9, 2013)

Made a crankshaft data sheet.
This is the first draft:
View attachment r75-1.pdf


Nick


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## Septic (May 25, 2013)

I think the urge to make them "just because you could" is the very Essence of what drives many machinists... I'm a huge BMW boxer fan myself (currently running an 80,000mile R1100GS) and I wish I'd thought of it first...


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