Exhaust Collar Mystery

[George_1]

I am rebuilding the top end on my new to me 1975CB750. The exhaust collar on cylinder 4 has some sort of hard rubber on the inside of it. Its just over 1/4” thick and runs through the entire collar/flange. There is no other visable differences in the valves or cylinder head.

My question is why?

Note: the bike has a 4-1 delkivic exhaust, dyna ignition, and jetted carbs. otherwise mechanically stock to my knowledge

 

[George_1]

thanks in advance!

 

[madmtnmotors]

Remove the offending material, it does not belong there and I have no idea what would have possessed someone to jam something like that in there unless the exhaust spigot has holes rusted through it. If that is the case then there are better ways to address it than what is shown here. Pretty sure new spigots can be had from Honda (if you're a purist with money to burn) and even a few aftermarket sources. You'll need to pick the old copper crush washer(s) out from where the spigot meets the head if you decide to remove the spigot and put a new copper crush washer in place.