I actually wasn't losing brake fluid!

[5twins]

Other model Yamaha Specials (850, 1100) ran smaller front discs. 650 Specials used the same large front disc as the 650 Standard. A smaller disc was never used on the front of any 650.

I agree with Gary, a properly set up single front disc is all these bikes need. I've had to panic stop several times on my 650 and that single disc performed admirably. It had the front tire howling but not locked, which is what you want.

 

[gggGary]

Seres sorry about the jab (wink) I can not verify I have good rotors at this point, I do not trust my off the bike runout set up after a few runs on several rotors this morning and I'm a bit busy to mount them up on wheels on forks right now.
(gggary the slimeball parts peddler)

 

[gggGary]

It's not all that easy to get the skinny on which rotors were large and small but this list is all "small rotor" and will not work with 650 forks.

This Yamaha 1J3-25831-50-00 DISC,BRAKE is used on these models and components:

1976 XS500C REAR DISC BRAKE - CALIPER
1977 XS2-D FRONT DISC BRAKE-CALIPER
1977 XS2-D REAR DISC BRAKE-CALIPER
1977 XS500D REAR DISC BRAKE - CALIPER
1977 XS750 FRONT DISC BRAKE - CALIPER
1977 XS750 REAR DISC BRAKE - CALIPER
1977 XS750-2D FRONT DISC BRAKE - CALIPER
1977 XS750-2D REAR DISC BRAKE - CALIPER
1977 XS750D FRONT DISC BRAKE - CALIPER
1977 XS750D REAR DISC BRAKE - CALIPER
1978 XS500E REAR DISC BRAKE-CALIPER
1978 XS650SE REAR DISC BRAKE-CALIPER
1978 XS650SE-11 REAR DISC BRAKE-CALIPER
1978 XS650SE-114241 REAR DISC BRAKE-CALIPER
1978 XS750E FRONT DISC BRAKE - CALIPER
1978 XS750E REAR DISC BRAKE - CALIPER
1978 XS750E-227501 FRONT DISC BRAKE-CALIPER
1978 XS750E-227501 REAR DISC BRAKE-CALIPER
1978 XS750E_227 FRONT DISC BRAKE - CALIPER
1978 XS750E_227 REAR DISC BRAKE - CALIPER
1978 XS750SE FRONT DISC BRAKE - CALIPER
1978 XS750SE REAR DISC BRAKE - CALIPER
1978 XS750SE-31 FRONT DISC BRAKE - CALIPER
1978 XS750SE-31 REAR DISC BRAKE - CALIPER
1978 XS750SE-311211 FRONT DISC BRAKE-CALIPER
1978 XS750SE-311211 REAR DISC BRAKE-CALIPER
1979 XS650SF REAR DISC BRAKE-CALIPER
1979 XS750F FRONT DISC BRAKE - CALIPER
1979 XS750F REAR DISC BRAKE - CALIPER
1979 XS750SF FRONT DISC BRAKE - CALIPER
1979 XS750SF REAR DISC BRAKE - CALIPER
1980 XS650SG REAR DISC BRAKE-CALIPER
1980 XS850G FRONT BRAKE - CALIPER
1980 XS850G REAR BRAKE CALIPER

 

[gggGary]

sseres this would be a good setup for you. note; he spills what rotors to use.

http://www.xs650.com/forum/showthread.php?t=1282

 

[XSLeo]

XJ750 rotors start around $25 each on Ebay. A left side caliper starts around $20. There is one that comes with the mount bracket. Item # 111116463271
On the calipers make sure it looks like your. They used several styles of caliper, only one works.
Lines and the dual banjo bolts are easy to find.
I have a stock 81 rotor you can have for $10 plus shipping. It shows some wear but still very good shape.
Leo

 

[sseres]

here's an update. After looking at the cost of adding the 2nd rotor, and not really knowing what the breaking power of my bike should be, I decided to see what it would be like with the single rotor. I decided to put the Mike's one on:

It comes with gold spacers that move the calipers over a bit because it's a lot thinner of a rotor. It's a bit of a different fit and until I get my fork back together, I won't know if it all fits right or not.

 

[XSLeo]

From the reading I have done the rotor from Mike's works well. It should work fine for you.
The spacers move the caliper over not because the rotor is thinner but the offset is shallower.
If you lay your old rotor on a flat surface with the center up then measure from the surface up to the top of the center you will find it is about 3/4 in or for the metric fans 19.5 mm. if you measure your new rotor you will find it measures less. maybe somewhere around 1/2 inch.
The difference will be the thick ness of the spacers that came with the new rotor.
Leo

 

[sseres]

so, this was interesting. When I installed the wheel with the gold spacers, the new rotor rubbed up against the inner aspect of the caliper itself.

I added a couple of smaller (but thick) washers to get it from touching.

Not sure if there's something about the forks that isn't quite right, like if there was a modification done to make that "wrong" rotor that was on there to fit. I really jammed on the brake a bunch yesterday and everything seems to be flight ready. Thoughts?

 

[5twins]

When you took your 1st pic, was the axle fully tightened? I ask because tightening the axle pulls the wheel and rotor over towards the caliper. The axle is stepped and acts as the wheel spacer on the left (speedo drive) side.

 

[sseres]

I did... it occurred to me the spacing when I tried spinning the wheel after the brake caliper had adjusted itself to the rotor that there was this scraping sound that wouldn't go away... the picture showing the rotor in both cases is with the nut fully torqued.

 

[XSLeo]

Yes, sounds like it's time for a test ride.
The caliper looks very close to the spokes. As long as they don't touch you will be good to go.
Leo

 

[sseres]

test ride: check! Love the new braking power! Thanks, forum mates!

 

[gggGary]

A bit of a heads up here. Not sure if you got new longer caliper mount bolts. The bolts are a close fit for length, add the spacers with the stock bolts and you will have very (too) few aluminum threads holding the calipers on the bike!
It might be necessary to buy a bit longer bolts and trim to length.
Confession time; I need to get this done on my own 79.

The need for "extra" spacers bothers me too. You might want to assure yourself that both fork tubes are straight....

 

[5twins]

I have another suggestion. Instead of spacing, and in this case double spacing, the caliper in, why not space the disc out? Measure your current spacer stack. Looks to be about 5mm? If your old incorrect disc is a 5mm thick one, you have the perfect material source to make a spacer ring for under your new disc.