When I put the rocker box gasket back on, I used copper coat gasket spray on an unused metal gasket I had. I think it was a Japanese aftermarket one. Not a honda. It looked like it had a coating of shellac. Not sure if thae two coatings were incompatible or anything, but I cleaned the other two surfaces and was careful about my torquing and order of tightening, and it still leaks soon after the swap, on a 3" area of the gasket at the left front area. I'm glad it's not the front area where the spark plug cavity is, as that has usually been the case, and an unnecessarily messy pain in the ass when doing anything spark plug related.
The gasket between the head and rocker box has always been a big pain in the asson this bike in general, always leaking. The same thing happened to my single carb XL head. I had ok luck with a coat of silicone on either side of the stock metal one. Even the silicone gave way to an oil leak in the same place I'm having one now, left front, so I know it's not a warped matting surface. Or I should say, if it is, these two are freakishly both warped by unlikely odds. But I know everyone has trouble with that gasket. Some fiber gaskets hve been alright, maybe I should try that again. I heard that there is a guy out there who makes custom-shaped gaskets out of a sheet of solid silicone for any application, and they are durable and reuseable.
The only problem with using different gasket materials is that it is hard to know the right thickness. Too thin, and the cam bearings will be too tight and could be binding up and adding resistance to the cam turning, stressing your timing chain and tensioner, and the center cam journal can rub, also adding resistance. Too thick, and the cam bearings can move around. Both of these can also throw off your valve clerances since those are... wait, no, they wouldn't since you would adjust those after installing the rocker box and then they are still whatever you adjust them to.
So what someone with two absolutely flat plates of metal and taps, dies, and possibly a machine shop at their disposal should do is this: trace a stock gasket out on one of the plates, drill and tap holes in the appropriate places, and drill holes in a corresponding plate to go on top. The plates would be used to mock up the head and rocker box going together. Take your stock gasket and put it in there and torque everything up. For the 10mm and 12 (or is it 14)mm-headed bolts, shorter versions from the hardware store can be used in place, which would be fitted to accommodate the plates. They are there just to simulate the squish that occurs when the gasket is tightened. Now the plate and gasket sandwich can be measured and the thickness of the plates subtracted for the gasket-under-pressure measurement. Now you can cut gaskets from other materials and see if they are the right thickness based on Honda's intended spacing, which had been established for the cam bearings and journals to be happy. You could also use the plate to pre-manufacture some silicone coated gaskets based on the stock ones. I used brake cleaner to clean the old gasket and the surfaces before using silicone and it still eventually leaked. I wonder if it is the cleaner I was using.
Sounds like a big deal just to stop an oil leak, but if this helps a community of XL people, it would be well worth the while of a couple of individuals. What say you, XL loving machininsts or friends of machinists?
1987 Yamaha XT600 2KF (German)
STOLEN: RED XL600 in Portland
I do heavy-duty textile repair, upholstery, and design/manufacture bags.