1972 R50 to R75 conversion

[Raceydog]

Ok. The time has come to start working on the 1972 R50/5. One thing I have planned to do is convert it to a R75. Anyone know what parts I need to do it? I am guessing I will need pistons and heads. Also, is it possible/adviseible to put in a 5 speed? What years will work? Any input will really help.
Jimmy

[flash412]

Ok. The time has come to start working on the 1972 R50/5. One thing I have planned to do is convert it to a R75. Anyone know what parts I need to do it? I am guessing I will need pistons and heads. Also, is it possible/adviseible to put in a 5 speed? What years will work? Any input will really help.
JimmyPistons and CYLINDERS. You CAN use EVERYTHING else. Though you might want to get R75 heads, carbs, and carb tubes. A 5-speed ain't recommended. If you keep the R50 carbs, you will need to rejet.

BTW, if you put 750 jugs on there, the bike will flat get up and run away from normal R75/5s (out of a corner, but not on top end), because you have the lower rear end of the R50.

[Raceydog]

Great. Will these cylinders work? They are on ebay. The item number is 220182372651. He has both sides. Also, any idea what this seat might go to? There is a two inch gap between the seat and tank when the seat is down. It also hits the taillight on the back bottom of the seat.

[20774]

Also, any idea what this seat might go to? There is a two inch gap between the seat and tank when the seat is down. It also hits the taillight on the back bottom of the seat.

Looks like the seat on my /7. IIRC, the /7 tank is longer, so the seat is shorter. On the earlier models, the tank was shorter, so the seat was longer to match up to the tank.

[lostboy]

While you can use R50 or R60 heads on an R75 cylinder, it won't pull above about 5000 RPM. The R50/R60 heads will not flow enough air and will give you a sky-high compression ratio.

Any '71-'75 R75 cylinder will fit your crankcase. '76-'77 would require boring the case or turning the cylinder base outside diameter.

The seat on the bike shown is a /7.

[flash412]

I dunno about that seat being /7. That same-looking seat was used on /5s. Now if somebody recovered a short wheelbase seat with the pattern used for a LWB seat and then put the SWB seat on a LWB /5, it would look like that photo.

Yeah, this certainly IS a convoluted theory, typical of interweb allowing-as-how with NOT ENOUGH INFORMATION.

I took the trouble of tracking your eBay item number. That bike is 1975. Putting a short wheelbase seat on that bike will appear just like the picture.

Maybe you should ask the seller.

[20774]

I dunno about that seat being /7. That same-looking seat was used on /5s. Now if somebody recovered a short wheelbase seat with the pattern used for a LWB seat and then put the SWB seat on a LWB /5, it would look like that photo.

It's a little hard to tell, but I can almost make out the lever for the door that covers the forward under-seat compartment on the /7-era seats.

[Raceydog]

It does have the little door with the lever in the front and a plastic document compartment underneath. So I'm guessing everyone agrees that it's a /7 seat.?

[Raceydog]

The bike in the photo a couple of posts back is my 1972 R50.

[flash412]

It's a little hard to tell, but I can almost make out the lever for the door that covers the forward under-seat compartment on the /7-era seats.D'oh! I didn't catch that.

[dlearl476]

My only queston would be why? R75s are a dime a dozen, R50's relatively rare, especially in the US. Unless you're using the bike as an everyday driver and NEED the extra capacity, I'd simply restore it as an R50. I've owned an R60 (my first road bike) and an R75 and to be honest, I prefer the smaller carbs of the 50/60s because there's more room for my big feets.
My next /5, if I ever get another, will be an R50.

[Raceydog]

I got the bike because it is an R50, but I want the extra power so I can take it on longer trips that would require driving on the Interstates. I also just want more power to keep up with the people I'm riding with. I'm also going to turn it into a "cafe racer". Of course I'm not going to do anything to it that can't be undone with a wrench and screwdriver.

[mmiller]

camshafts are different as is the final drive ratio. R50/R60 carbs won't fit directly on R75 heads. HTH Miller

[lostboy]

Yes, the R50/R60 cams are different than the R75-R100 cam, but the R50 cam works surprisingly well with an R75 top end. The carbs are different,too, but if you're buyng cylinders and heads, a set of carbs shouldn't stop you. The R50 is rare, so save the old parts should you ever decide to restore it.

[robertklee]

Pistons and CYLINDERS. You CAN use EVERYTHING else. Though you might want to get R75 heads, carbs, and carb tubes. A 5-speed ain't recommended. If you keep the R50 carbs, you will need to rejet.

BTW, if you put 750 jugs on there, the bike will flat get up and run away from normal R75/5s (out of a corner, but not on top end), because you have the lower rear end of the R50.

What's the difference in R50/5 heads and R75/5 heads?

[lostboy]

The R50 head is a completely different casting with a much smaller chamber and a very wide squish band. The chamber is so much smaller that the valves are longer than the R75, in able to reach the rockers (and not require different valve springs).
The ports are much smaller, as is the carb mounting stub.

[robertklee]

So...anyway, I've got a R60/5. Same goes for it?

[lostboy]

Yes.