1980 cb750k front brakes

[Dan soar]

Hi team,
I have just pulled apart my front callipers, Pistons were seized.
I need new Pistons and seals as the pistons are badly pitted.
Problem is this, they measure 38mm x 38mm.
Very odd you might say as this dimension of piston is as rare as rocking horse shit.
Can anybody tell me what's going on?
And if this is correct where the hell I might be able to buy a new set, they are a very strange size.
Most are 38mm x 35mm.
Help please.

[TOOLS1]

The 1980 K only had one front caliper. Are you sure your bike is a "K", or that front has not been switched?
TOOLS

[Dan soar]

Yeah it's a K model, Australian variety.
It has dual front disks and a drum rear end.
The funny thing about it is the size of them darn Pistons.
Definitely 38 x 38mm.

[18Bravo]

Strange indeed.  There's one in Germany I found that's 38 x 34.
According to my research (a funny phrase I learned watching Magic School Bus with my daughter) for your model it should 42.8 OD x 36.5.   I also noticed rear pistons come in 35mm as well. Could it have been replaced with the wrong piece?  I've seen dumber things happen.

[Dan soar]

The rear brakes are drums, the front brakes are dual disks.
The bike has been badly neglected by previous owner, and left to die in the weather.
Virtually everything that could sieze has done so.
The brakes were fully siezed and I had to hydraulically jack them out.
I would say that they are the original components due to the overall lack of maintenance this machine has been subject to.
I will do more investigating.

[TOOLS1]

I would not know where to begin to look for Australian specific parts. However I do know that Shiny has made caliper pistons on his lathe. Also I have repaired pitted pistons with JB Weld. I have thought about trying bronze on one, but the JB Weld has worked just fine.
TOOLS

[18Bravo]

"However I do know that Shiny has made caliper pistons on his lathe"

That was gonna be my next suggestion. I was looking at them thinking even with my limited experience on a lathe I could probably knock one out. (given a 400 mm piece of stock to start with)

And to clarify about the rear piston, I know yours is a drum, but the thought crossed my mind the the last owner found a piece somewhere that looked right and just used it. I have a photo of a "job" Pep Boys did on my car about 25 years ago. Can you believe they switched out an exhaust and an intake valve? I didn't even know such a thing was possible. Like I said, dumber things...

[Dan soar]

Yeah I've seen some butchered apparatus in my time, you wonder what people must be thinking.
My mate is a tool maker, I will get him to turn me up some out of stainless.
I was hoping more that somebody had the low down on the strange size, that's all.