The gas flow through the cylinder head is so convoluted that it's unlikely a stock 235 would hurt itself once it's out of 1st. or 2nd. gear on a SM-420.
Jerry
It's like a built in governor.
So, I won't bother with a tach. I have a cheap one but it doesn't work well. It's powered by a button battery and you wrap the sense wire around a spark plug lead.
