Front master cylinder sight glass replacement ?

My ZN11 frankenbike project needed the correct front brake master cylinder, along with a lot of other parts. Bought a couple off eBay, but split or cloudy widows seemed to prevail on these old parts. So I pried out the worst one and having access to a lathe and a 1" dia bar of acrylic, I made one. 2 weeks immersed in brake fluid with no adverse effects and it's UV resistant. I'm usually late for the party, but has anyone else tried route ?

I've never seen anyone replicate a plastic barrel sight glass. The KZ sight glasses I've checked had a glass button encased in a metal ring & then fitted with a seal that encases the metal ring w sight glass >> that assy is pressed into the mc orifice.

Here's a pdf w good pics from another forum for replacing the sight glass on some Yamaha models.
Similar to yours >> mc at bottom >> replaced plastic barrel sight glass with glass button.
Nissin master cylinders were used on many Japanese motorcycles.
xj4ever.com/choosing%20a%20sight%20glass.pdf

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I’m very interested to find out how the acrylic holds up over the long term. The chemical compatibility charts I’ve seen list the two as incompatible. I wound up making a solid aluminum plug for mine but I’d much rather have something clear.

EB search = master cylinder sight glass.
other internet searches: glass button. mineral glass. 18 mm or 25mm sight glass. motorcycle sight glass.

18mm w o-ring

Drag Specialties barrel type w gasket for Harleys.


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Before going down the rabbit hole to deep you might want to closely inspect the masters bore for pitting. A lot of these old masters are corrosion pitted and not worthy of rebuilding.

The 18mm looks close, the Harley one not so much. My ZN master has a single vertical slot behind the glass and no metal ring around the window. It just had a shallow groove for the O ring to seat in and beveled edge on the back side to help get it started into the O ring. By this Sat, the first blank I cut will have been soaking in brake fluid for 3 weeks. I'll clean it off Sunday and see how it's doing. The master cylinder I'm using had a good bore, but a cracked window.

I believe mine was 22mm. There was a replacement “glass” offered on eBay but upon purchase I discovered it was just a thin (less than .100”) piece of acrylic. Not at all usable. Came with a set of stainless reservoir cover screws which were slightly more useful. YMMV.

After 3 weeks of soaking in brake fluid, my test blank is starting to develop very tiny cracks arranged in a radial pattern from the center outward. I have to use the magnifiers to see them, but cracking was listed as a likely result of exposure to brake fluid. Plan A was just to easy to succeed.....on to plan B. Super glue is brake fluid resistant. So I coated the end of another blank with superglue and let it dry. It cured with a little ripple effect, but still quite clear. I gave it another coating and let it dry, a little less ripple and still clear. I'm going to turn another window and not part it off the bar. I'll dip the fluid side in superglue, including the O ring seat and see what that looks like. I may have to allow for the thickness of the dried glue coating and cut the O ring groove a little more on the next piece.