Locking small outboard in center position

[kpgraci]

I have an older Mercury 7.5 2-stroke I use for a pusher on my sailboat, It's about a 1985 I believe (next time I'm at the boat I'll get the SN and find out for sure).

I want to lock the motor in the center since I use the sailboat tiller to steer. Is there some way to do this by tightening the pivot somehow or do I need to rig up some sort of external lock of my own design?

thanks,
ken