First trip into a 4.0

[new2xj]

Pulled my head today to see how bad things were and was looking at #1 piston it had front marked on it and under that it had 50 not .050 are there fifty over pistons or is this just a cast number thanks for any help offered

 

[b5blue]

Never heard of any piston overbore in that range.

 

[tjmotter]

The short answer is yes. The long answer is that the starting point for Jeep 4.0L pistons is a 3.875 bore despite the reality that the 242 started at 3.895. As such, piston manufacturers start counting .020 below your starting point so your "real" overbore is 3.925 which equates to 0.030 over but since the piston manufacturers start .020 lower, they would stamp it with a 0.050 designation. You can buy a 3.935 which would be 0.060 according to the piston manufacturers but in reality a 0.040 in your case.

Confused yet?

HTH
Todd

 

[Talyn]

The stock bore is 3.875", not 3.895". I don't know where you got that info, but its 100% wrong.

As for the "50" on the piston, its not the over bore measurement. I have seen it on stock sized pistons. However, I have forgotten what it means.

 

[tjmotter]

Got it from Wikipedia:

"The 242 cu in (4 L) engine introduced in 1987 was an evolution of the 258.[11] It had the same 3.895 in (98.9 mm) bore as the 2.5 with a longer 3.414 in (86.7 mm) stroke giving it a displacement of 241.6 cu in (3,959 cc).[12]"

 

[Talyn]

Check the #12 footnote.

"XJ 4.0 L". Service Manual 2000 Jeep Cherokee. DiamlerChrysler. 2000. p. 115. "bore x stroke = 3.875 x 3.414 inches"