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Autor Thema: Shaker crank  (Gelesen 364 mal)

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BSA Bob

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Shaker crank
« am: 28 März 2021, 12:04:35 »

I've removed the sump pan & seen my R26 crankshaft.
It's a "Shaker" crank. The crank has the small webs (#3 on the comparison chart) with a steel rod (checked with a magnet).
Because the engine's seized I could only get one picture which I'll try to post, but the small webs can be clearly seen.
So at least I know why it ran so roughly.
If I decide to go ahead with this the crank will of course need to be repaired or replaced. Does anyone have opinions on whether the large or small crank is the better? I'd have thought that the crank with the large webs would give more momentum & pulling power? But slower acceleration?
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Shaker crank
« am: 28 März 2021, 12:04:35 »

rolf

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Re: Shaker crank
« Antwort #1 am: 28 März 2021, 12:15:29 »

you don´t feel any difference
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4Taktix

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Re: Shaker crank
« Antwort #2 am: 28 März 2021, 12:17:41 »

Hi Bob,
theoretically you were right. But the 4,5 kg ( ~ 9 lbs ) Flywheel overrules that.
I urgently recommend to go with a stock-crank with the large webs.
It makes more sense to achieve pulling power at low revs with e.g. a smaller carb, but that will make it chewy at high revs.
Depending on your individual preferences.
For more "instant response" I'd rather lighten the flywheel, than to go with the small-web-crank.

Cheers,
Sascha
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BSA Bob

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Re: Shaker crank
« Antwort #3 am: 30 März 2021, 02:14:03 »

Thanks for the replies.
I've read a thread about the crankshaft started in 2019 by JR, on Page 8 of the mechanical section. I understand from this that the alloy rods often gave trouble, & that they can't be obtained anymore? That means my crank can't be used.
So as you say, Sascha, I should really get a full web crank. This would probably have to be a replica.
As you & Rolf say, the flywheel is more important than the crank as far as acceleration's concerned.
I'd use a standard carburetter & try for a pretty standard engine.   
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Re: Shaker crank
« Antwort #4 am: 30 März 2021, 12:40:24 »

Hey Bob,

that's all right! Get a new crankshaft with big webs and Steelrod, and that's it. The old one is not taken by any of the dealers in return, as it is useless. Nobody is offering a refurbishing of the Aluminium rods, as there is a friction bearing with this kind of crankshaft. It's nothing that can be simply replaced like with the needle bearings of the Steelrod.

BR

Steffen
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Re: Shaker crank
« Antwort #5 am: 30 März 2021, 12:41:17 »

Yes, that's true. Chances to get an alloy rod in useable condition are close to zero.
They can't be overhauled to stock condition, most engine-shacks refuse to.
Also they are notorious to be delicate concerning heat-resilience.
Our well known dealers usually offer cranks "in exchange" (don't know the correct term for that in english)
meaning you sent your worn out old crank and get an overhauled or new crank for a lower price.
But none of them accepts the "small-web-crank" for exchange.
I once asked Uli's for that and he told me to better use it for door-stopper, no other use for such.

I got no idea, how chances are down under concerning new or overhauled cranks for the 250 cc BMWs.
But for overhauled cranks you should be aware of the method they did the bearing seats in particular.
Recently I had an "overhauled crank", which already was in use after restoration and was fine so far, as the former owner told me.
But after pulling off the bearings I chucked it on my lathe to regrind the flywheel-taper and supported it with a steady-rest with rollers on the bearing seat.
And guess what - after a few revolutions the "spray-welded-plating" came off the seat in 3 pieces like nut-shells.
The crank was junk then.

Cheers,
Sascha
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Re: Shaker crank
« Antwort #6 am: 01 April 2021, 02:27:19 »

Well folks, thanks again for these replies. I'm afraid the project will stop for a while now. I'm  deciding which of 2 bikes (the other a 69s also in poor condition) to try to fix. I've had both in the back of the shed for many years & will attempt to do one as a retirement project. The other would go to a good home. But before I do anything I need to raise some money.
And Sascha, sorry to hear the story of your crank. It can be so hard to get proper work done on these old components.
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