Be careful with gasket sealant on old honda bikes. I've got a CM400 1980. Soon after I bought it, I noticed that head and bottom cylinder gaskets were both getting wet and I got quite concerned.
I soon discovered that this was NORMAL if the bike had not been ridden for a while because the Honda gasket material is designed to absorb hot oil and swell to make a tight seal with very little specified pressure applied on engine mounting bolts.
Sure enough, less than 500 km later, the oil stopped leaking as the most experienced mechanics had predicted. I'm not sure Honda gasket material is still designed like that but all Honda motorcycles and scooter engines I ever owned were from that period (mid-70 to mid 80's) and they all use simliar oil swelling material.
I later found in the service manual that using any sealant could actually CAUSE more leaks as well as poor cleaning or improper cleaning (using tools harder than aluminum).
Other brands of motorcycles more recent models and automotive industry may use other type of gaskets designed to used or designed to be replaced by gasket sealer. Lots of mechanics also use sealers because they don't have to wait for new gaskets when they perform repairs on an engine.
I don't beleive in miracle cures that applies to any engine type. My old Honda motorcycles had accurate service procedures and head gasket installation did NOT include using any kind of sealer. If using a sealer, what would be the correct torque to apply
Don't assume that it would be the same.
Except for rare lemons like the early Honda Magna (see
http://en.wikipedia.org/wiki/Honda_Magna) and a few known problems, I would think twice before going against Honda engineers advice.
Doing things the old way is often the best way to go with old bikes ! For instance, just try switching an old bike on synthetic oil (even JASO certified) : gaskets will often start to leak, wet clutch will start slipping, and it may start buring more oil simply because the engine is running too clean (too much detergent additives).
Using mineral oil (which is much better quality that it was in the early 80's) remains the best choice.
Old Honda engines were not as economical in gas and did not run as clean as they now do, but why on earth am-I still driving a 27years old motorcycle ? You must love them as they are intended to be. Also keep in mind that today's replacement gaskets are often better quality than OEM Honda which are often discontinued for ages anyway on a '73 !
I just had my CM400 cylinders and head refurbished to new. One of my valves oil seal failed, too much oil got into that cylinder so piston rings got gummed up and I lost compression on that side. Everything was still within standard specs, but I had to get new rings and have the valves cleaned and readjusted. In retrospect, It is no big surprise to see rubber oil seal fail after nearly 30 years, but I have a lot of respect for engineers that designed such a reliable engine.
Good luck !
Martin
