OK I'm going to link the Faria site to show you what I'm talking about...There's some basic choices as you'll see each design of aftermarket tach has a selection for the type of engine the boat runs...
Here's the three :A} diesel,B}outboard and C}inboard{all components are in an engine bay only the shaft/prop comes through the hull}and I/O{engine is in an engine bay and outdrive is on the outside of the hull probably the most common today}..
Diesel is only a 4 grand tach and I'm not sure if the "mag signal" is compatable with the 084..Speaking of diesel you'll notice some diesel tachs have a mechanical input I'd like to try one with the 084s tach drive to see if it reads correctly sometime...
Outboard is 2 cycle and a 7 grand tach
The inboard I/O with the 6 grand tach is for 4cycle like the 084 that's the input you need to find...If you look closely at Aerigh's pic of the tach backside you'll see the "4cycle only" on his also...

Each style of Faria tach will have an input for each type of engine just find the style you like and make sure you get the part number indicated for the correct inputs... The faria site has a wealth of info including how to hook up each gauge correctly look it over carefully it's all there... Here's the link to the tach part number page.. https://fariabeede.com/2-pages/prod_...edGauge=2_tach