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Old 10-16-2012, 12:17 PM   #1
Jo-Nathan OP
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/5 crankshaft interchangeability

So I fried a rod bearing in my 73 R75/5 back in July and it scored up the crank a little. It's my understanding that /5's of all displacements used the same stroke. Does that mean I can us a crank from any /5 (R60 cranks are easier to find and cheaper)?
Thanks!
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Old 10-16-2012, 12:31 PM   #2
brocktoon
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Yes. RealOEM.com shows that the part number for the crankshaft is interchangeable for all /5s and up through the /6s.

I had to change the crankshaft out on my bike, and ended up going through 3-4 used cranks before I found one that fit, had good journals, everything within spec, fit into my crankcase and turned smoothly without binding. The color codes BMW used for the journals (red and blue) are a good start to get an idea for fit, but a micrometer will be invaluable in determining that nothing is out of round -- particularly on a crank of unknown origin.

brocktoon screwed with this post 10-16-2012 at 12:37 PM
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Old 10-16-2012, 01:18 PM   #3
983Bob
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The old part# for the 500/600 crank was 1121 1255 290 and the 750 was 11211259 856 including early /6 [to 76] which used the longer and stronger 10 mm bolts. The 900 part# was 1121 1262 450. The bearings are all dimensionally the same, but the balance factor will be a bit different. Since the factory now only has one part# for them all, I doubt that it will make much difference. Note that the /6 block was strengthened at the front which included a smaller opening, so an early crank [/5] will not fit through that opening, but for a /5 block, all the above cranks will fit.
Hope this helps
Bob
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Old 10-16-2012, 01:57 PM   #4
mattcfish
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Quote:
Originally Posted by 983Bob View Post
The old part# for the 500/600 crank was 1121 1255 290 and the 750 was 11211259 856 including early /6 [to 76] which used the longer and stronger 10 mm bolts. The 900 part# was 1121 1262 450. The bearings are all dimensionally the same, but the balance factor will be a bit different. Since the factory now only has one part# for them all, I doubt that it will make much difference. Note that the /6 block was strengthened at the front which included a smaller opening, so an early crank [/5] will not fit through that opening, but for a /5 block, all the above cranks will fit.
Hope this helps
Bob
+1. Use a good used R100 crank and a post 75 flywheel for the larger bolts. I bought a couple off eBay for cheap that were perfect replacements.
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Old 10-16-2012, 06:32 PM   #5
Kt-88
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If you don't fix this I'm going to make you sell it to me. You're better off selling it actually.
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Old 10-16-2012, 08:20 PM   #6
Jo-Nathan OP
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Quote:
Originally Posted by brocktoon View Post
Yes. RealOEM.com shows that the part number for the crankshaft is interchangeable for all /5s and up through the /6s.

I had to change the crankshaft out on my bike, and ended up going through 3-4 used cranks before I found one that fit, had good journals, everything within spec, fit into my crankcase and turned smoothly without binding. The color codes BMW used for the journals (red and blue) are a good start to get an idea for fit, but a micrometer will be invaluable in determining that nothing is out of round -- particularly on a crank of unknown origin.
What do the color codes mean? Where are they?

Quote:
Originally Posted by Kt-88 View Post
If you don't fix this I'm going to make you sell it to me. You're better off selling it actually.
RIghttt.....
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Old 10-17-2012, 12:07 AM   #7
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The color codes are used to match crankshaft to crankcase bearing sizes. The front and rear main bearings in the crankcase should have a red or blue marking around their diameter, though they may have faded with time. You'll want to measure the I.D. of the crankcase bearing shells and compare them against the published BMW specs. If they're scored, out of round, or worn beyond spec you'll want to replace them.

Similarly the crankshaft you buy should have color markings at the flywheel and alternator end. Again, measure their O.D. and compare them against BMW's specs for that color.

Subtract the crankcase bearing shell's I.D. from the O.D. of its corresponding crankshaft journal. You're looking difference of 0.0028 to 0.0051 inches at room temperature. That gives enough radial clearance for oil. If you're within that range, and the crankshaft turns freely, you've got a winner.

Here's some more info from Airhead guru Oak Okleshen on the subject:

Quote:
there are four choices of main bearing shells in just
the standard main bearing sizes. (red or blue ) If the original color
coding on the engine block and crank is still discernable you could get lucky and
pick the correct shell size. Also possible front and rear may be
different. There are special oversize bearings available of different colors but I
have yet to see one in any engine worked on.

More misery....Just can't stuff in any crank . Firstly, measure it to make
sure it is not a bummer. It must be carefully measured and using the shop
manual and factory SI info, at least you know what to expect. More misery.
Any measurements for bearing clearances must be done at 68F room temperature.
But to remove and refit new mains you must oven heat the crankcase and
remove original bearings. Then when cool make all your initial measurements. You
will need BMW shop manual specs. If all looks OK then reheat and install
the new bearings. Repeat the measurements in ready to install crank condition
as the shrunk in bearing shells will likely reduce size slightly. Make sure
all is still in spec. All clearance measurements must be done clean and dry.
You will need an army of precision measurement and fitting tools. When you
get all done, and may be worried if all parts are sized to spec, there is
one bottom line that tells it all.

What you want to achieve is a 0.0028 to 0.0051 inches (cold 68F and dry )
difference in bore DIAMETERS, installed bearing bore versus crank journal
diameter.. That will allow the proper radial clearance all the way around the
crank journal surfaces for a film of lubricating oil.

I have attempted to set em up for about 0.0035 ( 0.044 mm) inches and so
far has never been a problem. With the measurement and fitting efforts, total
time consumed is lengthy. Not a 5 minute job. With flywheel back on (void of
cams, chain, rods etc.) and end play of the crank properly shimmed it
should freely turn by hand smoothly and slicker n snot. If it binds in the
slightest, something is wrong. When ready to roll, no hot rodding or high speed
races. Give the fresh surfaces a chance to mate together for 600 + miles and
hope for the best.
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Old 10-17-2012, 01:49 PM   #8
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Might be time to buy a known good r100 engine and stuff it in there.
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Old 10-17-2012, 03:04 PM   #9
pommie john
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When I replaced the crank in my airhead racer I thought it was sensible to replace the bearings too so I took it to my local independent workshop who I've found to be very good.
They told me they use red bearings in every one! They claimed they had never seen a crank that needed any other size!
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Old 10-17-2012, 03:15 PM   #10
mattcfish
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The shells will have the part number stamped on the outer serface, so you dfon't need to know what color they are. Check the Max BMW parts fische to identify. Most cranks have the red shells
Most used cranks I've bought off eBay had the rods still attached. If the crank journal surface looks smooth and unscored, then it is most likely good. These cranks are over built and hardened. If they don't turn freely, they've probably been mishandled after removal, or the engine had a major seisure. Again, if the rods come with it and are themselves in good shape, you've probably got a winner.
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Old 10-19-2012, 09:26 PM   #11
bmweuro
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I have seen where the post 74 cranks will not clear the inside of the case on the earlier models. R60's have the same stroke, use the same bearing but have different counter weights.
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