Reed cover valve size

Anyone know what size the OD of the reed cover valve is on a 1984 GPz550?

I need to get caps to plug the valves, but I none of the local auto shops around me carry vacuum caps. 5 shops, no caps!!!

I need to order them online, and my cheap Harbor Freight calipers' battery died so I can't really get an accurate measurement.

I have an 83 GPZ, should be the same cap. I used a pipe tap and threaded the inside of the nipple. I can't remember if it was a 1/4" or 3/8" tapered pipe tap though. Installed a threaded plug and it was sealed air tight. Local hardware store should have all the parts.

You could also just run a hose between both of them. Unless you want something more visually pleasing. I'm assuming this engine has the ports set up the same as my 1100, so it might not make any sense if yours are not.

Unfortunately, I don't have a workshop where I can tap some threads.

I would use a hose to run between both of them, but again, none of the local autoparts shops have the right size. Only Napa allows you to buy it by the foot and they only carry up to 3/8".

If I had to guess, I think it's like 7/8", but I don't know for sure.

I bet you could find some heater hose the correct size.

Rubber vacuum caps eventually split easily.

You can use 1/2" copper pipe caps. You have to heat them near red hot and quickly hold them in place and press them on.

We did a few that came out nice, but it's easier with two people. It might help to put the covers in the freezer first.

The copper will be discolored on the surface, but a little touch up with 500 grit cleans them like new. (Of course you can then paint them black.)

Would be easier just to make aluminium plates to blank off.