IIRC, they are M8 fine thread so you should be able to do something similar to what I did when I removed the valves from Eccles' caps
"Since I had to remove the valve stems to re-fill the forks I decided it would be a good time to replace them but I still didn't have any suitable screws. But I do have a lot of 5/16-18x1.75" bolts and a metric tap & die set so I cut the threaded parts off of a couple of the 5/16" bolts, dressed the ends in the vertical lathe and cut M8x1.0 threads in them to match the valve stems, then installed them with copper sealing washers "
I don't think I'd use allen grub screws though, because 1) I don't think they would seal as well as a hex head and 2) I hate having to clean out the heads of the few allens my bikes have every time I need to remove them.
That's pretty expensive for a couple of little bolts. Nobody in your area carries metric hardware?
For that matter, a set of metric taps & dies is pretty much a necessity (when you work on the engine you should always chase the threads) and it wasn't hard for me to cut the inexpensive 5/16" bolts off and cut threads in them.
BTW: I hope you checked that the threads in the holes you need to plug are the actually M8x1. I think they are but I'm not 100% sure.
I had considered, cutting threads. But, I was glad to see these plugs would actually recess into the cap.
Oh, and you want to talk expensive. I looked into buying, just, the air valve, and drill and tap my original, solid cap.
Those buggers are $25.00+US. NO friggin way.
Definitely M8x1, which is why it was so hard to find something locally.
Eveything I found in 8mm was 1.25 thread pitch.
Also, I trimmed down some 1/4" id vinyl hose, to use for a seal. Couldn't find an actual o-ring or washer, in the a size that worked.
And, before anyone asks, no, the oem o-rings will not work.
Because, the sealing surface of these plugs is about .5mm (or 1/32") larger than the same part of the original fittings.
Another possibility would have been to tap the holes out to 3/8"-16 and use hardware store bolts but if you did that you would never be able to go back if you ever decided to restore the bike to original.
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