I have an old "Shimano SG A-52-42-30" Crank, but I need to replace the 42 with a 36 Chain Ring.
I searched in my city but had no luck.
What measurements should I look for? Or should I replace the whole Crank? (i need to keep the 52 + small budget)
I live near Spain and can buy from its "Decathlon."
https://www.decathlon.es/es/search?N...to%20bicicleta
Thank you

The most important consideration is the number of mounting bolts and the bolt circle diameter (BCD). This determines what will fit and also the smallest possible chainring.

Two common BCDs are 130mm, typically used for road, and 110mm typically for mtn bikes.

I suspect that you have a 5 bolt road crank with a 130mm BCD, in which case the smallest possible ring is 38t.**

Confirm by measuring (on center) between two adjacent bolt holes and multiplying by 1.7 (round up).

** Note----- 38t 130mm chainrings tend to be pricey, 39t is the traditional standard, readily available at low cost.

You can use a ruler to measure the bolt circle diameter (BCD) as described on this page. Measure between adjacent bolt holes and then use the chart for the appropriate number of crank arms (four or five, symmetrical or asymmetrical) to make sure your crank will accept a 36-tooth ring.

Chainrings for Shimano triple-ring cranks are usually interchangeable with rings from other manufacturers, as long as the BCD matches. In other words, if you can't find a 36-tooth Shimano ring of the correct BCD, you can probably safely order, e.g., a Stronglight ring.

I see that the Decathlon pages for chainrings show BCD dimensions for the Stronglight chainrings, so you should be able to find an appropriate 36-tooth ring, assuming it's in stock and is otherwise compatible (e.g., a "narrow-wide" chainring designed for a single-chainring crank won't fit).

Chances are that's a 130 BCD if it's old, so no 36t will fit. Shimano 52t chainrings didn't come on MTB cranks, and later road 110 BCD cranks weren't triples.

So 38 or 39t are your smallest practical choices.