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Old 12-13-2010, 04:51 PM   #1
Xcuvator OP
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Turning An Airhead Crankshaft?

I need a good crank for an R90 and I have heard some of the numerous stories regarding the resulting problems encountered by people who turned their crankshaft. Hardening and incorrect side radius were the two issues that stick in my mind.

Has anyone out there ever got it right? I see the over sized bearings are still in the parts fisch, so it makes me wonder. I think Ed Korn used to turn /2 cranks, but I'm not sure

Because the two R90 Ebay cranks I have measure about the same as what I have, I checked the current new price, 1175.91$. WOW!

Anyone out there with a good track record doing it?
Thanks
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Old 12-13-2010, 05:13 PM   #2
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Perhaps in the future it might become tenable, but at present they're so widely available I can't see it being cost effective. The incorrect radius is the reason I'd heard from a dealer wonk who had grief years ago.

Good cranks are so easy to come by I've never had to deal with it myself.


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Old 12-13-2010, 05:16 PM   #3
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Quote:
Originally Posted by Xcuvator View Post
I need a good crank for an R90 and I have heard some of the numerous stories regarding the resulting problems encountered by people who turned their crankshaft. Hardening and incorrect side radius were the two issues that stick in my mind.

Has anyone out there ever got it right? I see the over sized bearings are still in the parts fisch, so it makes me wonder. I think Ed Korn used to turn /2 cranks, but I'm not sure

Because the two R90 Ebay cranks I have measure about the same as what I have, I checked the current new price, 1175.91$. WOW!

Anyone out there with a good track record doing it?
Thanks
Everything I've read, everything I've heard, and all those I've talked to that are credible say do NOT turn the crank. With that, a good used crank should be easily had for a decent price.

As far as /2 vintage cranks go, they are a completely different type of crank than a /5 and later crank, so there is not comparison 'twixt them in this regard.

YMMV--

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Old 12-13-2010, 05:56 PM   #4
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I am one of the people who has said: "good used airhead cranks are very easy to come by."
Just for because, I have had two of them in my parts stash. Upon close accurate examination, both are either out of spec on the main bearings or very close to it. I'm sure there are good used ones out there. I just need to find one.
Just a little bit of mishandling, can do alot of damage to a crankshaft.
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Old 12-13-2010, 07:15 PM   #5
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It's common prctice in Germany to nitride the cranks, and removing enough material to create a new main or rod journal would reduce its' thickness. Dunno how thick/deep the nitriding goes, but...

Any competent crank-regrinder should be able to creata a new radius for each bearing where the newly-ground journal meets the crank cheek. That's been do-able for decades. It's not magic; just one of those little details that most folks don't realize until it's too late.

Getting thicker bearings to fit the now-smaller crank journals may be a problem, however.


If it's something that CAN be done, these guys will know how: Falicon

Good luck!.
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Old 12-13-2010, 08:32 PM   #6
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The issue with regrinding cranks is not the hardening thickness, but the fact that you have to remove the bob weights to get the grinding tool in there. When they are rivetted back on, most places don't do it as well as BMW and there are many instances of the bob weights flying off, through the crankcases at high RPM.
This is a bad thing .
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Old 12-14-2010, 07:58 AM   #7
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Originally Posted by kixtand View Post
Everything I've read, everything I've heard, and all those I've talked to that are credible say do NOT turn the crank.
[devils advocate]

Those people probably don't turn anything.

I'm reminded of the one time I went to the Model A Club meeting and asked around for a good shop to weld cast iron (the block had a crack or two). They all (including the author of a Model A book I have) just told me to get a new block. Yeah, ok... Found a shop that welds cast iron all the time, and the block is good to go today.

[/devils advocate]

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there are many instances of the bob weights flying off, through the crankcases at high RPM.
Tom Cutter, I believe, told a story at an MOA national about the weights flying off at speed, through the engine block, through the frame, through the tank, and past the riders head; never to be seen again..
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Old 12-14-2010, 02:25 PM   #8
Max Headroom
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Quote:
Originally Posted by pommie john View Post
The issue with regrinding cranks is not the hardening thickness, but the fact that you have to remove the bob weights to get the grinding tool in there. When they are rivetted back on, most places don't do it as well as BMW and there are many instances of the bob weights flying off, through the crankcases at high RPM.
This is a bad thing .
John, I agree that the rivets are a potential problem, but the greater issue is finding a company to grind the cranks with the correct radii and heat treatment. The original radii are given a rolled process instead of a grinding process which makes the crank stronger. The re-grinding process is unable to duplicate this, and even a correctly re-ground radius isn't quite as strong as a virgin crank will be.

I operated a crankshaft grinding machine many years ago when in the reconditioning trade, and am painfully aware that many cranks were re-ground by other companies without due regard for the radius area. I have two broken airhead cranks in my shed which had both been re-ground and subsequently snapped through the big-end radius area.

In my humble opinion, the likelihood of this happening would be greatly reduced if the airhead Type 247 engine had a centre main bearing between the two big-end journals. Of course, this would add a considerable complexity to the design and casting challenges, and make the engine up to 50mm longer. As it is, the crank endures a lot of flexing during operation, and it is this flexing which makes the journal radii so critical.
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Old 12-14-2010, 11:20 PM   #9
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Originally Posted by Max Headroom View Post
John, I agree that the rivets are a potential problem, but the greater issue is finding a company to grind the cranks with the correct radii and heat treatment. The original radii are given a rolled process instead of a grinding process which makes the crank stronger. The re-grinding process is unable to duplicate this, and even a correctly re-ground radius isn't quite as strong as a virgin crank will be.

I operated a crankshaft grinding machine many years ago when in the reconditioning trade, and am painfully aware that many cranks were re-ground by other companies without due regard for the radius area. I have two broken airhead cranks in my shed which had both been re-ground and subsequently snapped through the big-end radius area.

In my humble opinion, the likelihood of this happening would be greatly reduced if the airhead Type 247 engine had a centre main bearing between the two big-end journals. Of course, this would add a considerable complexity to the design and casting challenges, and make the engine up to 50mm longer. As it is, the crank endures a lot of flexing during operation, and it is this flexing which makes the journal radii so critical.

Nice writeup Max, Have you ever thought checking to see if there are any staff openings at Motorcycle Classic magazine?
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