Would like to know where i can put a knock sensor on 3 rib block ( Big port came with AE86) ?

The factory spot on a smallport is high on the block, intake side, between cylinders 2 and 3.
That is the best position but you should be able to find an unused 8mm tapped hole somewhere
on the intake side if you use a "donut" type sensor. If you want to use an OEM Toyota sensor
I'm pretty sure you need a 10mm tapped hole (might be 12mm?) so you will need to check the
thread and see if there is a suitable tapped hole available anywhere on the block.



The upper part of the pic shows where the knock sensor gets in the way if you want to use RWD
water pipes on a smallport block.

Cheers... jondee86