Craftsman engine swap with double pulley

[AChrome]

My son's Craftsman mower DLS 3500 hydrostat had a 20 HP B&S intek that throwed a rod through the block. I replaced it with a B&S 17.5 engine that I had. The deck belt pulleys are the same size. The transmission pulley on new motor is smaller than old pulley so the slack in belt won't turn transmission. The old pulley shaft is same diameter but has a shoulder part way down inside that won't allow it to go all the way on so I can't just swap pulleys. Any ideas? Do you measure the pulley size by outside diameter or inside? If I get a shorter drive belt so it turns the transmission, will it affect the hydrostat drive in anyway? Thanks in advance.

 

[SeniorCitizen]

RPM can be calculated with : RPM x drive pulley Diameter divided by the driven pulley diameter .

Measure the dia. of the 2 drive pulleys pulleys and calculate the RPM difference of the driven .

This isn't the answer to your question but i suspect someone will ask for the answer of the driven after the smaller drive change .