1971 R50/5 Restoration

I am restoring a 1971 R50/5. The bike sat for more than twenty years. I managed to separate and dismount the cylinders and pistons with some difficulty and much ingenuity. The cylinder has palpable raised areas that do not come off with a Scotch Brite pad. The pistons seem fine. The rings will need to be removed and are no longer serviceable. I think the pistons, once the rings have been removed and the grooves cleaned, seem pretty good. No scratches. It seems obvious that boring and new 1 over pistons would be the best way to go. But, I have noticed that 1 over R50/5 pistons are nearly, if not impossible, to find. This bike is not, nor ever will, be my daily rider. It would probably be registered as antique, cleaned up a little ( petina on the aluminum only modified with a nylon brush and soap) and ridden rarely. ( I am short and ride a 2012 Honda Shadow VT750RS as my daily driver.) I am wondering if I should just have the cylinders honed, clean the pistons and re-ring them and live with what I get or pursue some other alternative like moving up to R60 parts as they seem more available. $ is not a huge consideration but the less expensive the better. What say You?

Welcome to the forum! Also well to your "little" Airhead!! Not that it matters much, but how many miles are on the bike? Likely the sitting around has done more damage than riding. Also, where are you located? Maybe there's some expertise near you. You might also want to consider checking into the Airheads Beemer Club. They tend to have tech days at various times at which you could get some help and meeting other like-minded owners.

If you're not after a daily driver or plan on a lot of miles, your approach to just cleaning up as best as you can might be a good one. You seem to have the skills and knowledge to tackle this. You would only be out the effort to hone and re-ring. If things don't work out or you end up using a lot of oil, then you can consider your next options.

Bumping to 600cc might be OK, but you probably should consider going to 750cc. Used prices should be reasonable. If you were to really consider a more ambitious direction, Siebenrock makes a top end conversion (keeps the same heads, though) to jump to 1000cc. Somewhat pricey but there are some improvements in the engine parts versus stock.

Let us know how it goes!

The "little" airhead ( something I have been called on occasion) only has 11000 miles on it. It belonged to a friend who has since died. He sold it to me a couple years ago for a pretty good price. The restoration is as much a remembrance to him as it is to have an interesting, unusual moto to play with now again. As I just got the heads off and pistons out today I am contemplating and inspecting my options. I still have not figured out if the condition of inside of the cylinders is just "gunky" and could soaked off, or gently polished away, or if there is some more extensive pitting. I do know that if I use the stock stuff I will need rings. I have not looked hard yet but would appreciate suggestions on sources for those.
I will try to put some pics up to show what I have to work with. The tank is nice yet. The seat is very good. There is rust some places and I will need new mufflers but those are not hard to come by. I need to spend several hours with a rag and soapy water and some detailing tools to get the dust out of hard to reach places. I also have chrome to polish.

I hope anyone who has tried a restore of this kind will speak up. Can the inside of cylinder be lightly sanded with super fine sandpaper (800 grit or higher) or some other compound and still have any hope of the bike running? Give me your advice. Thanks, littleguydavid

Cylinder service

David

I have a set of R 69S cylinders that were pretty bad at Millennium Technologies and they are getting them squared away at a very reasonable price. If your pistons are in good shape with the cylinders a little sketchy they might be able get them fixed up with out a new set of pistons. Look them up and talk to them, it can't hurt.

I own a 1971 R50/5 like yours. I think you have a fine idea just to clean up the cylinders and install new rings. Save your money for now. Get it running and go from there. You know that R50/5 is not a tall bike? I'm going to guess that you will love riding it! It surely is a different animal from what you ride now and will give you a different experience. Good luck and enjoy!

Looks like this vendor in Germany has your rings, and other engine parts.

https://www.meyer-bikes.de/index.php?page=gruppe&path=/00000081/00000075&idx=53

Check with your local dealer for parts. Could become a valuable friend although he'll probably have to order in most of the parts that you may need. I use the parts section from MAX BMW To get the correct part numbers and just hand the list to the parts person. This is usually greatly appreciated!
I agree with the re-ringing based on mileage as long as there's no pitting in the bore. Use the pistons that you have as long as they clean up. I'd want to keep the stock bore even though the R50s are somewhat slow. They will still keep up with traffic as long as you're not racing Save your money for the little things like new cables, tires and proper mufflers.
With that mileage you might want to pull the transmission and replace the original rear main seal ( It should be white), oil pump O ring and do a good clean up of the area. Lube clutch splines that may not have been done at the factory.

If you decide to head deep for the oil pump o-ring, read all about blocking the crankshaft. When the flywheel is off, there's nothing to prevent the crank to shift forward off it's locating pins. If that happens, you'll have to go even deeper to fix the problems. Snowbum talks about blocking the crankshaft...simple to do.

I took the cylinders to machinist friend who has been motos before. He looked them, touched them and mic'ed them and is pretty sure hone and re-ring is all that is needed. Anything after that might be overkill. He will know more after he ( or his shop motorcycle expert owner) does the honing. He will mic them again after evaluating the surface of the wall. Then I will have to get the standard rings or 1 over pistons and rings. I appreciate the referral to the German company and I have found a source in England but am wondering if there are any others, perhaps domestic, for those rings? thanks for any help. littleguydavid

Parts not from England

There are plenty of sources here in the states! I'm not saying NOT to get parts out of England; just to do the research to get the best deal.

to R60 or not to R60

It looks like hone and go may not be a viable option. Yet to be determined if 1over will suffice to cure the roughness inside. 2overs for R50 seem hard to come by. I am wondering about the sanity and pitfalls and benefits of boring out my R50 heads to R60. How much other stuff would need to be changed for the bike to run right? Rejetting, cams, etc. I think I read that there is actually nothing ( or very little) that needs to be changed to make the bike run properly with 60's. I welcome some feedback.

Probably wouldn't need to change the cams...the R60 series post 1969 had a mild cam timing installed and if you were trying to upgrade an R60/5, say, you would have to consider what the effect of the stock cam has on the resulting change. But not an issue taking an R50/5 and trying to fix it like you're doing.

I'm guessing you would try and put 600cc pistons in the new bore. The stroke on the 500 versus the 600 cc engines is the same. What I'm not sure of, and maybe someone else knows, are the critical dimensions of the 600cc piston the same as the 500cc? Namely, what is the distance from the middle of the wrist pin to the crown of the piston. Because of the engineering done on the R60/5, I don't know if the piston was also modified to make it work with the mild cam timing.

Certainly, you would need to work with the carb settings to make the bike run OK. Generally, the carbs are somewhat the same over time...of course the R50/5 was CV carbs while the R60/5 had slide carbs. But changes could be made to the original CV carbs to get the proper performance out of the bike.