Long story,short version.I have an airhead that I bought last year and is a mish mash of parts.It has a 1975 R75 twin shock frame with a 1974 R75 motor.The transmision is a 4 speed with a kick start.I would really like to put an R90 or R100 motor into it and possibly a 5 speed tranny.Before I try to locate and buy a motor and transmision I need to know what problems I might run into.I tend to think that a bigger motor will drop into the R75 frame alright but I'd like to be sure and will a five speed transmision fit onto a R75 drive shaft? All and any advice will be greatfully received, John
I have, for 19 years now, been riding an R60/5 with an R100s engine and 5 spd trans. I put it all together myself, and though it's been a long time ago, I don't remember there being any challenges. In the swap, I got a higher output charging system from the r100, but the only difference in wiring is that there was an extra wire between the stator and the diode board. The transmission also is a non issue. There's really nothing to it. I guess that the only glitch in my whole rig is that I also used the r100s final drive, a 33/11, instead of the 37/11 from the R60. Because of this, I have a significant speedo error. R75's came with a 32/10, which is pretty close to what the bigger bikes came with, so it may not be worth swapping that out, but if you did, it wouldn't produce much of an error. If you use your r75 carbs, make sure you change the jetting to that of the larger engine you use.Good luck!
There is no problem doing this. If you get matched parts. The frame for the '75 R75/6 is the same frame for a R90/6. Or an R100 motor bolts right up also. If you get a 5speed from the R90 vintage it will have the long input shaft but if you get a later trans they have the short input shaft. So the deal with the input shaft is that they use a different clutch/flywheel setup. If you get an older motor and a newer trans then you need the clutch to match the trans. Newer clutch. But if your motor is old enough to have 10mm flywheel bolts then you can't use the newer clutch because the carrier doesn't bolt to a 10mm crank. You can always change the input shaft of the trans if you can find the one you need. And it is expensive, if that's all your opening the trans for. So really you want to buy a working motor and trans pair, already a team. Because other wise you need to know what you are doing. If you have all the time in the world you can ask and hopefully you get the right answer, soon. The motor you have right now has 10mm flywheel bolts. How good does the bike run as it's set up right now? Is the trans trouble free? No real advantage to taking out a well built 4speed just to get a trouble prone 5speed. It's easier and sometimes not much more expensive to just buy another bike. If I really wanted to do this I would do it 1 step at a time. I'd do the trans or the motor first. Then next year I'd do the other.
Like the guy from Silver Spring said, leave it. What I'm adding is this: if the engine is fine just put a bigger top end on it. The frame was not at the peak of it's evolutionary climb until the 77 model year, but most of that don't track race don't care. R75 bikes run just fine. If you happen to stumble on to a topend from a 1974 or 75 R90, go for it! it's a direct bolt on with no mods.
A friend has a 73 or so 750 with a buggered motor and I have a fresh 100 to transplant.Just waiting for the motovation.
Thanks to all for the much needed advice!The four speed tranny works ok and I think I will keep it on the bike plus I get to keep the kick start which is a plus.That leaves me with the option of an R90 top end or a complete motor swap.Which years motors will match up with the existing gearbox without changing shafts or clutches. Thanks,John
Any thing from 8/1969 until 8/1980 will drop right in with little or no mods. alternators interchange 8/69 until 8/75 starters from 8/69 until 8/76 points will be the same 8/69 until 8/78, then points cannister 79 & 80. all exhausts interchange 8/69 until 8/80 (except for R65 and 40mm RS in 77) If you go looking for a 5 speed, the very best in you year range are the ribbed case models from 79 and 80. Avoid the 1974 5 speed.
"Avoid the 1974 5 speed"-----I have always been told the same thing....What is the weak spot of the 1974 5 speed? I have a 1974 5 speed behind my R100S Engine & sidecar. I've abused in horribly and it just keeps working fine. I salvaged out a couple 1974 models so decided to "use up" what I had on hand but can't seem to make it fail.:eek1
Well sometimes they do seem to hold up remarkably well. The '74 5 speed I mean. I had one in my first R90 back in the early 80's. That bike was younger than the one I own now and it looked worst, go figure. Some people want a '74 5 speed because it comes with the kick starter. 1st problem, The kick start of a '74 will break. One of the parts was not hardened right, I think. I forget what the part is called, there are several pieces that go into making a kick starter. Mine was the common problem I think, it sort of half worked but even when not being used it hung down a bit. I got used to that. Come to find out later that even when they do work they can't always start the bike. The part that was not hardened got a supplement from BMW so the problem could be fixed. I don't think this part has been available for years tho. There was a guy on Ebay that had 2 of them. He wanted $74 each. I didn't have the money. I wanted to buy one just to put on a shelf and pray to. #2 and #3, The shifting forks and/or grooves break. The dogs break. maybe #4, I think there are some other parts that are unique to '74 trannies. Not bothered to get into this because I don't own a '74 trannie. I do own a complete set up for a '74 kick starter but not the hardened gear. The Getrag 5 speed transmission was first used in the '74 year. Some of the worst problems were fixed the next year but this transmission kept evolving almost yearly. It even got another problem engineered into it when they eliminated the output shaft snap ring. (I think after '81 or '83). Maybe wirewrkr will do a complete evolution of the 5 speed for us. I'm very impressed by the list for which trans to use, two, three posts above.
I liked the option of the kick start when I mated it to the R100S engine....but didn't really think about the compression forces involved when trying to kick it manually. It used to succeed when the engine was warm....then I added a heel shifter....if I was to try to kick start it now, the starter shaft would hit the heel shifter and put it into 2nd. gear. Kick starter is now more useless than it was before.
I think you should get an award for that modification. Most knowledgeable Beemer Nuts will tell you to not bother with a kick starter on the Airhead but some still have one. I really think it's just the looks. It does look like it belongs there. The parts are worth money. I'm saving my KS set up till I think the market maxes out. Then I'll put it up. Maybe ten years from now. There was this guy, forget his name, he wanted a kick starter for some replica bike he was building. This was some big deal with him. I wrote back and forth several days with him. Found him one that would work, and it was rebuilt, and then he got pissed at me because I was not impressed with this project bike that nobody would ever ride! (Just thought that was funny so I threw it in) The real reason to not spend any effort or money on a kick starter is that the electric starter works so good. You don't need the kick. The electric system, even tho it does have problems, is very dependable. So you say what if I got a dead battery, left something on? Just push the bike or get a jump. Both probably work better than the KS anyway.
My bike is an '84 frame according to the title, but an '83 according to the numbers, a 74-79 front end, complete with mismatched ATE calipers, and the 83 or 84 Brembo master cylinder, Snow Flake rims, and an R100/7 engine on it. It's in a million pieces right now, but it's going back the way I found it. A true STS (South Texas Special). It had a carb button on the tach that says "Terry's Tire Sales Harlingen TX, but I have found no such a place when I searched for it. I may try and get a part from every R Bike year, if I can verify it, and put on it, LoL. I doubt I'll do that though. Too bad the kick starters never were very good.
