For the meeting with South Africa, Luke Price comes in at fly half in place of Angus O'Brien while Torin Myhill (ankle) and Tom Phillips (shoulder) are unavailable due to injury.
Ryan Elias starts at hooker with Javan Sebastian joining him in the front row, replacing Alex Jeffreys at prop.
There is a reshuffle in the back row with Jordan Collier at openside, Ospreys man Jon Fox stepping up from the bench to start at blindside with Oliver Hitchings completing the back row from No. 8.
Josh Helps leads the side for the third successive game.
In the backs, Ashton Hewitt will be looking to continue his impressive try scoring form with Steff Evans and Josh Adams completing the back three.
Garyn Smith continues at inside centre despite the return from injury of Tyler Morgan with Aled Sumerhill outside him.
Tuesday's game with England saw Wales produce some top-class rugby, superb individual scores from Hewitt and Smith putting them 15-10 up with just seconds remaining.
But after the full time hooter had sounded, a late converted try saw Wales end the game on the wrong side of the result.
"The England result was disappointing but what was more important was the way we played," said Wales team manager Phil John.
"We were excellent throughout the game, both in defence and in attack and I know Allan was pleased with the performances of all the boys.
"Javan Sebastian gets a chance at tighthead against South Africa with Luke Price coming in at 10.
"We're really looking forward to the game and hoping to end the tour with a victory."
On home turf, South Africa have already secured wins over both England and France and will be full of confidence ahead of a clash with Wales.
"South Africa have been impressive so far and they will certainly provide a tough test," said John.
"We're expecting a physical contest but if we defend like we did against England then we are more than capable of getting the right result."
Wales Under 18 vs. South Africa (KO 3.30pm local time):
15 Steffan Evans (Scarlets) 14 Ashton Hewitt (Newport Gwent Dragons) 13 Aled Summerhill (Cardiff Blues) 12 Garyn Smith (Cardiff Blues) 11 Josh Adams (Scarlets) 10 Luke Price (Ospreys) 9 Tom Williams (Cardiff Blues), 1 Keagan Bale(Newport Gwent Dragons)2 Ryan Elias (Scarlets) 3 Javan Sebastian (Scarlets) 4 Josh Helps (Scarlets & Captain) 5 Rory Thornton (Ospreys) 6 Jon Fox (Ospreys) 7 Jordan Collier (Ospreys) 8 Oliver Hitchings (Cardiff Blues).
Replacements from: Alex Jeffreys (Newport Gwent Dragons), Leon Crump (Cardiff Blues), Luke Garrett (Newport Gwent Dragons), Adam Beard (Ospreys), Oliver Griffiths (Newport Gwent Dragons), Dan Brooks (Ospreys), Angus O'Brien (Newport Gwent Dragons), Dan Jones (Scarlets), Joe Thomas (Ospreys), Tyler Morgan (Newport Gwent Dragons).