That screw in the picture has to do with the lock for the starter.----Recoil should only work with motor in neutral.-----Do not adjust anything on this motor if you are NOT sure of it or the results !!!
The o-ring inside can be replaced in 3 ways without damage to housing.----Pull bushing out after running a 5/16-NC tap in for 3 threads.-----Using the factory tool to drive it out ( I have 4 of them )----Or believe it or not you can remove / replace o-ring without removing the bushing.----And...
I try to point out that some of thee older motor really well built , easy to to troubleshoot and repair.----Some folks simply do not understand.----What is your opinion on the design of Mercury lower unit , with special tools required ?
A few years did use thermostats.----Found at the top of exhaust cover.----A spring thingy on the thermostat cover.---Was for easy maintenance and inspection.