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!
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.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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.
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.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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.
I sourced a /5 crank. Should I proceed? Need some way to contact you.
edit: found another one: https://denver.craigslist.org/mpo/d/denver-bmw-1995-and-5-parts/7140816408.html
Hi Arni, thank you very much for both of your replys. The information is very helpful. A friend here in Baltimore has a /5 crank that I may take off of him. Also looking at an R90S shortblock, and waiting to hear back on another complete engine.
Thanks for passing the crank you found along.I think my contact info is on my avatar link, still getting use to this forums set up. My email is sam.m.hintz@gmail.com
This was very helpful, thank you. The more we dig into the bike the more it looks like the guy who owned it for 8 years before me didn't maintain it well. It's a 1973 with supposedly only 30k miles on it. I knew there was something wrong with the engine based on the price but was optimistic when I asked to drain the oil and I didn't see glitter in it. Again was thinking with my heart more than my head at the time, but don't regret it at all. Excited to get back to it's former glory.
You have a 4 speed transmission. When the change to 5 speeds happened (in there somewhere) the flywheel changed as did the length of the tranny input shaft. So a different block with a different flywheel might not fit your tranny. see snowbum for the details.
I would be interested to know what is wrong with your crank and sez who anyway.
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