The great o-ring, seal, gasket, nut, bolt, sizes used on the XS650 thread.

The o-ring that goes between the plastic reservoir and the metal body on 1977 and later front brake master cylinders is 39.7 x 3.5.
I covered this pretty thoroughly in this thread:
Jump to post #13 -- there is a lot of other good info in post #13 of the above thread regarding that o-ring.
Just a little follow up on the orings I used, "Buna-N" as I believe it was mentioned back in the thread DogBunny linked to above is NOT recommended for brake fluid.

Now the bike I used the Buna-N oring on at that time has not had any seepage of any kind. However back at that time I had, for kicks, tossed a spare one into a jar of DOT 3 brake fluid. It did not seem to have a problem sitting there for months. I was working on the master cylinder for the 1978 so I popped that oring into the groove and assembled the thing on the bike. I did notice that it seemed a slight bit softer and not quite as tight going together. Well a few days latter I noticed a slight dampness under the plastic reservoir. figured I had just spilled a little and to took time to seep out. Well after a few time cleaning off the fluid I decided to drop back and try again. This time using an oring that had not been soaked in DOT 3. So far a couple months later no sign of seepage.

I 'm not too worried about the two I have in place at this time but if I was doing this on another bike I would be leaning heavily towards getting the Honda oring price is not that bad compared to some rubber parts for these bikes.
I was asked to post the master cylinder reservoir O-ring here so:
3.5mm material x 39.7 ID
Sulphur cured EPDM material
Shore A, 70 durometer

My source was Marco Rubber mainly because they are fairly local to me and had them available to order online. I got 50 pieces which was probably the minimum and therefore a lifetime supply.
Some O-ring chemical compatability charts:



  • O-ring Material Guide.pdf
    115.6 KB · Views: 357
  • O-ring_ChemicalCompatibility.pdf
    342.6 KB · Views: 297
You just gave me a memory jog Steve...
Here's the Parker Hanifin O-ring handbook.
Got an O-ring question? This guide has the answer.... guaranteed.


  • ORD 5700 Parker_O-Ring_Handbook.pdf
    9 MB · Views: 478
It would be good to have the size of the o-ring that goes between the tank and a vacuum petcock. It ends up oval in a groove but it starts out a regular round ring.
I just ordered some 47mm OD x 3.1 mm o-rings for this, I've been using #29 from the red box of metric o-rings assortment but I've run out. That #29 is about 48mm OD x 3.5mm and that's just a little thick. I'll edit this post when they arrive and I check the fit.
Put one in and it's a good fit. usual lightly snug the screws finish pushing the o-ring into the groove, tighten screws done. A bit of lube on the o-ring helps.
Last edited:
I just ordered some 47mm OD x 3.1 mm o-rings for the late model vacuum petcock to tank seal, I've been using #29 from the red box of metric o-rings assortment but I've run out. That #29 is about 48mm OD x 3.5mm and that's just a little thick. I'll edit this post when they arrive and I check the fit.
O-rings arrived from China look good in the petcock groove with enough bulge to make good seal, but I haven't installed the petcock to a tank yet.
From @RustiePyles recent post
A list of bolts by dimensions for stator and primary cover (engine side covers) on an XS650


Copper washers
M6-4, get more, use them along bottom of RH cover and the oil filter cover (in spite of what 5T sez) :sneaky:
Last edited:
Yes, those 6mm long screws are perfect for the BS34's because they milled the shaft flat on both sides of the area that the butterfly plate fits through. But for the BS38s, the shaft doesn't have flats machined into it, it's fully round for it's entire length. You'll want 8mm long screws for it .....

Here's a BS34 shaft with a 6mm screw on one side, 8mm screw on the other. Clearly the 8mm screw is too long .....


It could be useful to know the thread and size of the hollow allthread that holds the signal stalks on. Along with where to buy 2" pieces of it.
The thread is 8mm x 1.25mm. Suppliers like McMaster Carr sell metric "vented" screws in a variety of types. The hole may not be as big as you would like but enlarging should not be too difficult. 40mm is as long as I have found them so far. Good luck.
Here's a 50 pack to fit the head bolts at Amazon for 10 bucks. Less than Mikes want's for 4.
I used this item in an experiment which required vinegar, I was trying to make a copper, zinc, vinegar battery, but these washers are not solid copper, they are copper coated, after just a few minutes of being submerged in the vinegar, the copper started coming off. Buyer beware, these are not solid copper, only copper coated.
There's a flood of crap on the market and that's suspiciously cheap for full-on copper washers.