I ve seen many engine models inside grabcad . But i dont know how they get the dimensions of each and every part for modelling the same.
Do you draw the diagrams yourself and then model it or do you get any reference from internet or do you simply give your own dimensions??
When I am trying to clone anything, I start with an internet search for drawings of the item in question and others (different manufacturers, different sizes of the same items, etc) like it. I then model what I can directly from the drawings. I scale the drawing for the missing dimensions...it's usually fairly accurate to make some educated guesses along the way. The real trick is to get "educated" so your guesses are reasonable.
There is a website with many different model engines. Just make sure you look at the drawings thoroughly, some people leave out main dimensions and makes for frustration when you made some parts and are now missing the crankcase or something similar.
please check the tutorials