Why does Jake hit Ambrosch? What does Jim’s grandfather suggest Jake do to prevent further problems with the Shimerda family?

When Jake visits Ambrosch to retrive the horse collar he had borrowed, Ambrosch returns with a collar that is ill used and borken. Jake hit Ambrosch because he is angry that Ambrosch isn't being honest with him.

Jake jumped off his horse. I saw a wave of red come up under the rough stubble on his face. “That ain’t the piece of harness I loaned you, Ambrosch; or if it is, you’ve used it shameful. I ain’t a-going to carry such a looking thing back to Mr. Burden.”

Ambrosch dropped the collar on the ground. “All right,” he said coolly, took up his oil-can, and began to climb the mill. Jake caught him by the belt of his trousers and yanked him back. Ambrosch’s feet had scarcely touched the ground when he lunged out with a vicious kick at Jake’s stomach. Fortunately Jake was in such a position that he could dodge it. This was not the sort of thing country boys did when they played at fisticuffs, and Jake was furious. He landed Ambrosch a blow on the head—it sounded like the crack of an axe on a cow-pumpkin. Ambrosch dropped over, stunned.