There is no replacement part # for our brake fluid sight glasses, and the only part I can find is on eBay with about 450% markup, and the guy wouldn't even tell me what kind of adhesive I'd have to buy for his kit until I bought the glass. That was a big turn-off to me because I want to know the hidden costs.
I found some alternatives though. Here's a website where you can order glass for watches in any thickness and any diameter to within 1/10 of a mm of what you need. I'm sure there are many others: http://www.esslinger.com/watchcrysta...eralglass.aspx
. Edit: This place
has them as well but 3x at a time and $15 minimum:
At the first website, they're about $2.50 vs $15 for the eBay kit. With shipping it comes to a whopping $3.50 to your door. I bet they would also be suitable for the crankcase oil sight glass if you got the right size.
I got some measurements with my micrometer. They're taken using my old (1986) plastic sight glass and old o-ring. Bike is a 1986 VN750, stock.
Stock plastic sight glass
Inside groove for o-ring: 21.3 mm O.D. average (slightly oval)
Ridges inside/outside o-ring: 22.4 mm O.D. average
Sight glass bore: 22.50 mm, dead on in 3 spots. I'm pretty confident on this reading. (your glass must be a hair smaller)
O-ring I.D.: 21 mm just touching (This is old and somewhat flattened)
O-ring backspacing: 2.5 mm from slotted readout face
O-ring thickness: about 3 mm deep
I'd suggest glass 2.5 mm (back of o-ring) to 4 mm thick (center of o-ring depth) depending on how much contact you want with the o-ring.
RTV is supposed to hold up to brake fluid. I plan to use super black because I have it on hand.
Photos of my installation when I get my glass in the mail. I'll try to remember to update this thread.