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Rebuilding the Crankshafts - lets talk about the details

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malmac
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Rebuilding the Crankshafts - lets talk about the details

Post by malmac »

Hi Guys

I think it is time we pulled together the group wisdom on rebuilding cranks.

I would like to do this and I know Martyb a member here is in the process of sorting out the details.

Also Rodolfo apparently has sorted out the black art.

I think James Wonder is sniffing at the task and I have made some very basic overtures, like building a crankshaft trueing stand.

So I am sure there are those who have done it and could share their wisdom, maybe templates for press tools. On the other hand there are others like myself who would like to tool up and get on with learning exactly how to do it and avoid the pitfalls.

I look forward to seeing the posts roll in.

My trueing stand.
MAL_0793.jpg


Cheers

Mal
MAL_1052.jpg
mal - R69s
Toowoomba- Australia

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jwonder
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by jwonder »

My truing stand is not as fancy as yours, but it works. I use small skateboard bearings instead of a rubbing surface and it works great.

I am making a tool to allow me to press the crank apart but before I do any of that I am going to find someone who can fix the crank front and rear.

It makes no sense to work on the tools until I have a person who can do the finer machine work such as welding and machining of the final surfaces. I have lathes and milling machines but I do not feel comfortable with final machining of the bearing surfaces. I feel they should be hard/final ground.

The tools should be straight forward to produce.
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

sherman980
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by sherman980 »

Haven't ever personally rebuilt a BMW crankshaft (have had a number of them rebuilt by others over the years with great success), but back in the early/mid 70's I regularly rebuilt the crankshafts in the Yamaha TZ250 roadracers that I campaigned as an AMA Pro rider at the time. (They needed to be rebuilt every 600-1000 or so track miles!) As I recall, the truing stand I used had knife edge rollers on it, and the gentleman who taught me how to use it was quite insistent that stands that used flat bearing surfaces for the crankshaft to rest on could introduce errors in the measurements of crank shaft runout. Having never compared using the two different types, I can't say I know how much potential error is introduced but food for thought if you are building a truing fixture. Please keep in mind that it's been about 50 years since I last did one, so my memory may be a little "blurry" on the issue. :)
Thanks.
Chuck S

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Rodolfo850
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by Rodolfo850 »

I believe the first steps needed to achieve a successful rebuild of a crankshaft is:

* Understand how it needs to be opened, basic measurements, how to be closed again, alignment, spares and procedures.
* the most important part is to have GOOD tools to guarantee you take the correct measure and do the job without damaging a part that for sure its hard to replace and too expensive.

Since here in Guatemala the only guy who used to fix BMW cranks passed away 10 years ago, and its a small market so must guys send its cranks to USA to fix, but a box with a 20 pound parts its too expensive, and taxes increases a lot the job, so I start to sniff in this art, until I understand and make some drawings, and then later the tools.

To understand its good to read the OLD Bmw shop manuals, like R50. R60 and R25 , because it has a better procedure and good drawings of the tools to part it, and assembly later. Salis web site has some pictures also.

Last three Years I have successfully done R27, R50 and R60/2 crankshafts of my own bikes.Image

https://photos.app.goo.gl/dynauLjh4Xnr5itZ6



Regards

Rodolfo

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Martyb
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by Martyb »

Restoring these cranks seems to be like chasing the holy grail (for those of us that don't do this as a full time job) and particularly here in Australia where it seems to be a dying art and many will not touch a twin.

After restoring a few of these bikes over the past few years (never ending learning curve) and with each restoration doing more myself or building up a solid network of skilled individuals I am on the quest to successfully restore cranks. I'm very fortunate to have someone who is a skilled machinist as a day job and a restorer of older Japanese 4 cyl bikes so has extensive experience with cranks with many conrods. I have attached a couple of photos of the crank truing stand that he has fabricated and we've started using to take all measurements and to also assist in the final truing of the crank/webs.

At this stage I've got a contact for hard chroming the bearing journals, I have access to a very solid truing stand and we've pulled a 50/2 crank apart. The next step is getting in the required parts and putting it all back together.

So far so good....!
Attachments
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sherman980
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by sherman980 »

So been thinking about crankshaft truing stand details since my last post. Went and looked at a couple of original BMW workshop manuals and they all show stands using V-blocks, not knife edge wheels as I used to use for the two-stroke crankshafts I mentioned above. I can see the benefits of the knife edge wheels, but if BMW says to use a V-block, who am I to suggest something else.... Anyone else use anything other than a V-block stand for this purpose?
Thanks.
Chuck S

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Rodolfo850
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by Rodolfo850 »

my truing stand uses small ball bearing, so is a small contact surface, but doesn't make any drag caused by friction.

Other benefit it has, is I could adapt the height , since r27 and r60 doesn't have same diameters on both ends.

Take a look at the pictures on my last post. I works great

Regards

Rodolfo

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Re: Rebuilding the Crankshafts - lets talk about the details

Post by sherman980 »

Rodolfo,
Appreciate the response. I've seen fixtures that use small roller bearings and they seem like a potential improvement as long as the bearings are thin and high precision ones. Unfortunately, I didn't look at your pics. Maybe I'm a bit paranoid, but I got bit a few years ago clicking on a link on a similar website that I thought was ok, and just don't anymore unless I know in advance that it's safe, no offense intended. If you can post a pic to this site instead of the link, I'd be interested to see it.
Thanks.
Chuck S

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Re: Rebuilding the Crankshafts - lets talk about the details

Post by Rodolfo850 »

Here you can see some of the pictures on the attached link...
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malmac
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Re: Rebuilding the Crankshafts - lets talk about the details

Post by malmac »

Rodolfo850 wrote:
Wed Jan 26, 2022 7:07 pm
Here you can see some of the pictures on the attached link...
I also was unable to access your link, so appreciate your pics.

Still have some press tools to make before I press apart my first crank.

Regards

Mal
mal - R69s
Toowoomba- Australia

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