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Topic-icon 1973 R75/5: New to airheads, new to the forum

  • BMoreW/5
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1 month 2 hours ago - 4 weeks 2 days ago #6731 by BMoreW/5
Hello,

Just joined the forum, very excited to have this great resource of information at my disposal. I will preface my questions by noting that while I do have a good amount of mechanical knowledge and I do have experiencing wrenching on my 1981 Honda GL500 turned cafe/brat bike, I do not have much experience with the BMW airheads but have always wanted to own one.

I recently purchased a 1973 R75/5 for a price that I knew indicated there was going to be work required, I will admit I was thinking with my heart a little more than my head but I was optimistic and jumped at the opportunity. After purchase and digging into the engine a bit more it was determined that the problem was much deeper in the engine than I had hoped.

I took the bike to a mechanic who specializes in these bikes has 30+ years of experience and comes highly recommend by others. It appears that the crank is completely wiped out. I have been given a couple suggestions as how I could move forward at this point.

1. Find a replacement R75/5 engine and swap it.
2. Repair / re-machine the crank, full engine rebuild basically.
3. Swap the current crank with compatible one from another bike

My budget to fix this bike is somewhat flexible.. but I am really trying to avoid putting 5k into a 2k bike. I have not had much luck finding a /5 engine so far. I know that they are a bit hard to find. I was wondering if I did go with an engine swap what other R engines could I potentially put into my original frame without compromising too much of the safety.

My second questions is, if I did go the route of repairing the crank that's in there now, most likely I would have to have the whole engine rebuilt all the way up to the heads? It doesn't make sense to me to have one new part pushing around a bunch of old parts... on the flip side if we swapped the crank from another bike I don't think I would need to do a bunch of work on the rest of the engine right away.

I appreciate any and all advice you have to share. Thank you in advance!
Last edit: 4 weeks 2 days ago by Wobbly. Reason: Mods altered title

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4 weeks 2 days ago - 4 weeks 2 days ago #6737 by Wobbly
Welcome Aboard !

You didn't say how many miles, but these engine typically go 250,000 easily. If your mileage is less than that, then the crank is in trouble due to negligence... which doesn't bode well for the balance of the machine. That would make me want to find another complete engine.

I'm not versed on engine interchange between /5, /6 and /7, so I can't get into details. Best place to start is with your friends. Try the pull down menu at the top of the page labeled Airbazaar.

Keep us up to date on your progress.

Don't hide 'em, Ride 'em !!
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Last edit: 4 weeks 2 days ago by Wobbly.

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4 weeks 2 days ago #6742 by BMoreW/5
Replied by BMoreW/5 on topic 1973 R75/5: New to airheads, new to the forum
Thanks for your reply, I will give updates as the project progresses. It's looking more and more like another engine is the most cost effective route at this point.

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4 weeks 2 days ago - 4 weeks 2 days ago #6743 by Wobbly
A complete engine should be available. They made thousands of these things and they were extremely popular in the day. And you'll end up with lots of spare parts for the new engine.

The first thing to do is add your location to your profile. When you click on "Wobbly" over on the far left, it tells you all about me and where I live. When I click on "BMoreW/5" there's nothing. It's really hard to help someone that doesn't live anywhere. :unsure:

Second thing, contact your AirMarshal. He knows where all the treasure is in your local area.

Don't hide 'em, Ride 'em !!
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Last edit: 4 weeks 2 days ago by Wobbly.
The following user(s) said Thank You: BMoreW/5

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4 weeks 1 day ago #6755 by arni
You can go with many engines. 90/6 engines are routinely put in /2 frames. Just needs a driveshaft adapter. Any /6 and /7 motor will bolt up. But I would stick with 750cc or less. You just don't have much brakes. I did a 900 cc conversion to my /5 and it went a whole lot faster than it would stop.

The /5 has a double row timing chain, a mechanical speedo drive (or tach drive? it's been years), the cam and block changed for the newer ignitions, etc. mixing and matching internals could be tricky. At any rate I would inspect the valves and seats through the spark plug hole with a Streamlight Stylus flashlight, Inspect the rocker running surfaces, pull the top end and inspect each of the very expensive lifters and inspect the splines in the rear wheel (if those are worn it's a costly repair and indicates either poor PM or very high mileage.) If any of these are bad it indicates huge mileage or poor lubrication and I would abandon the engine.

I know of no one that can correctly grind the crank. It is a special deal and the local grinder can't do it and won't notice what needs to be done. But call people like Bud Provin or Anton Legardier and ask.

An option is to buy another bike that runs, has some service history and can be evaluated. Between the two of them you will get a bike to ride and a pile of very valuable and often hard to find spares. This game can really pay off, especially if the donor has rust and bad paint and is cheap. But a cheap airhead will sell quickly, you will need to stay right on top of things.

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3 weeks 6 days ago - 3 weeks 6 days ago #6763 by arni
I sourced a /5 crank. Should I proceed? Need some way to contact you.

edit: found another one: denver.craigslist.org/mpo/d/denver-bmw-1...arts/7140816408.html
Last edit: 3 weeks 6 days ago by arni. Reason: added mo'

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