Llanharan put in an impressive forward display, which gave their backs a solid platform to play off. Fullback Matthew Taylor, centre Dan Atkins and flanker Justin Jones all touched down, while Codey Rees added 11 points.
Meanwhile, second-place Bargoed took advantage of their promotion rival's slip up with a 26-10 win over Tredegar. Lee Pearson and Nicky Cocklin both scored tries, but it was fly-half Steffan Jones who stole the show with 16 points from the boot.
Tredegar were hampered by poor discipline and lost three players to the sin-bin. Their only try came from second-row Jordan Williams and Darren Pearce added a conversion and penalty.
In a mid-table battle at Riverside Park, Blackwood recorded a bonus-point 39-6 win over Rumney, who had won four games in a row coming into the game. Fullback, Nathan Griffiths, scored a double while Joe Baffada, Rhys Morgan and Andrew Coupe all scored tries.
Caerphilly remain anchored to the foot of the table after their 10-5 defeat at Bedlinog. The home side's narrow victory allowed them to move eight points clear of the bottom, and they have a game in hand over Caerphilly.
Beddau gave themselves a better chance of surviving by beating Ystrad Rhondda 22-6 at home. That win allowed them to overtake Ystrad and move out of the relegation zone and build up a vital three point cushion.