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TOPIC: Front master cylinder sight glass replacement ?

Front master cylinder sight glass replacement ? 13 Mar 2019 17:37 #800299

  • jljim
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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 ?
1981 KZ650 CSR
1985 ZN 1100 LTD
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Front master cylinder sight glass replacement ? 13 Mar 2019 18:02 #800301

  • martin_csr
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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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Front master cylinder sight glass replacement ? 14 Mar 2019 06:04 #800326

  • ayeckley
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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.
1976 KZ900A4 (1105 Project)
1976 KZ900A4 (Stock Project)
1978 KZ1000A2 (Completed Project)
1983 ZX750 A1 aka GPz 750 (Completed Project)
1983 ZX750 A1 (Basket Case)
1973 CL350K4 (Completed Project)

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Front master cylinder sight glass replacement ? 14 Mar 2019 06:16 #800327

  • martin_csr
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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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Front master cylinder sight glass replacement ? 14 Mar 2019 07:15 #800329

  • Nessism
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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.

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Front master cylinder sight glass replacement ? 14 Mar 2019 12:50 #800340

  • jljim
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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.
1981 KZ650 CSR
1985 ZN 1100 LTD

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Front master cylinder sight glass replacement ? 14 Mar 2019 18:08 #800364

  • ayeckley
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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.
1976 KZ900A4 (1105 Project)
1976 KZ900A4 (Stock Project)
1978 KZ1000A2 (Completed Project)
1983 ZX750 A1 aka GPz 750 (Completed Project)
1983 ZX750 A1 (Basket Case)
1973 CL350K4 (Completed Project)

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Front master cylinder sight glass replacement ? 21 Mar 2019 05:28 #800772

  • jljim
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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.
1981 KZ650 CSR
1985 ZN 1100 LTD
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