Well, I ended up with petcock kits that had the gaskets for the three hole back and I have the two hole type....yep, I'm waiting on parts again.
So....I gota chance to try out Yamadude's o-ring fix and.....it works a treat!!!
The tach on my 1975B looks like this......
I have no clue if it's correct but it's what I have. You can see that the case tapers down so I needed two different sizes of O-rings if I'm going to cover the same area as the stock rubber buffer (for lack of a better term)
I ordered these from Mcmaster-Carr
The larger one is for the lower. I hate that I forgot to take a photo of the inside of the gauge holder, but there's a ridge inside it and the o-rings go on either side of it..I'll try to get a pic of it later.
Both are Buna-N. The lower is 1/4" thick x 3 3/8" ID x 3 7/8" OD round profile, dash number 416
The upper is 1/8" thick x 3 5/8" ID x 3 7/8" OD and is the double seal/x-ring profile, dash number 239
To use the lower I had to notch it around the two tabs on the gauge cover.
To use the upper I had to use two of the rings. Here's a shot of the speedo ( pay NO attention to the damn grey primer that's STILL all over this poor bike) you can see the the gauge sits a little above the holder. (the speedo is sitting undisturbed in the stock buffer)
With the two (stacked) x-rings in place the tach looks just about right and is really solid.
The lower one is a bit of a pain to get in.....and I'm not sure you really need it....but in this case more is probably better and heck I have them now any way.
Cost for the upper was a little under a $1 each.....for a pack of 5. The lower were around $2.25 each for a pack of 5.
Stock would be better but this is a better fix than what I was going to do and I thank Yamadude for the idea.
Gordon in NC
1975 XS650B....non running but soon...very soon.
and a few BSA unit singles