Fork/caliper incompatible... what gives?

[pasteurized]

I bought most of the stuff to do a dual disc upgrade on my '77 GS750. I bought forks from an '81 750E, and calipers from an '81 GS1000G. Looking at the parts fisches on alpha-sports.com, it looks like these ought to fit without an issue. However, one I receieved the calipers they look nothing like those in the fisches for 80 or 81. They look exactly like those from a 78 or 79 GS1000 (any submodel). They even have the round pads. The problem is that the mounting bolt holes are not far enough apart on the calipers to line up with those on the forks. They are about 1/4" too narrow to fit on these forks. I contacted the seller of the calipers and he says the frame of the bike says it was manufactured in 9/80 and since it only had ~10k miles on it he doesn't expect anything was changed out. I can post pics of the calipers if it helps to id them. I have pretty much everything else to do the swap but I don't want to go enlarging holes on forks if I can find a set of calipers that just fit. I like to do these things the right way ;-)

Any idea what the calipers on a 80/81 GS1000 are supposed to look like? They are supposed to have squared pads, right??

[focus frenzy]

then that bike has/ had a front end from a pre 80 model bike! (the so called 81 1000G)
my G was made 4-80 and has the same calipers as a 80/81 (and many other) 750, not round pads

my calipers look like this

PS: I put forks from a 80 1100E on my bike (due to seal eating rock pit) and the calipers bolted right up

[8trackmind]

80/81 should be square.
outside to outside on the caliper mounting boss is about 82mm
I measured a 78 550, 78 1000, 79 850, 80 550, 81 650 calipers and they were all the same spacing and will interchange. Hope this helps.

[pasteurized]

Measurement from outside of the two bolt holes is ~81mm. I just went out and measured the distance on the forks I have (from an 81 750E), and it is 86-87mm. I removed the stock caliper from my stock '77GS750 and the "new" left caliper bolted right up.

Here are some pics of the calipers in question, can anyone id them?

http://img48.imageshack.us/img48/9757/dsc000012fj.jpg

http://img62.imageshack.us/img62/7761/dsc000027iv.jpg

http://img48.imageshack.us/img48/5173/dsc000030lc.jpg

http://img62.imageshack.us/img62/161/dsc000042ed.jpg

[earlfor]

There is a complication. I owned a single front disk 79 750E which has the same front end as a single disk 77 750E. I also owned a dual disk 81 750E. The fork tube spacing on the 81 dual disk 750E is wider than the tube spacing on the 79 750. You cannot simply install the 81 fork tubes with the mounts for dual calipers into the 79 fork trees. The dual rotors will rub on the fork lowers. You must have the fork trees from the 81 also.

Been there, done that.

Earl
[QUOTE=pasteurized]I bought most of the stuff to do a dual disc upgrade on my '77 GS750. I bought forks from an '81 750E, and calipers from an '81 GS1000G.

[pasteurized]

Thanks Earlfor. Just one more thing I need to buy for this project... :-/ I wish I had just been able to buy an entire front end instead of trying to piece everything together.

[omaharj]

this project... :-/ I wish I had just been able to buy an entire front end instead of trying to piece everything together.

That might be the most useful piece of knowledge to come from this thread, Would our experts agree that it's better to change everything rather than bits and pieces?

[8trackmind]

That might be the most useful piece of knowledge to come from this thread, Would our experts agree that it's better to change everything rather than bits and pieces?

I'm no expert but I don't agree. If you are lucky and find a front end great, but it's not that easy.
I have parts from a 81 650, 79 850 and a 80 550 hanging off the front of My bike. Everything works together just like it's supposed to.

You know what would help the most? More people sharing their findings when they do major swaps like this. It would save us all time and money.