4.? with 4in bore

[NvrGrwUp]

Thinking of trying to go to a 4in bore on a 4.0. Has anyone had much luck with this or should I just forget about it.

[Plechtan]

Try this thread: http://www.jeepstrokers.com/forum/viewt ... ?f=5&t=601

I have also bored a block to 4" but the cylinders get pretty thin in the front and the back. the (left and right)sides of the cylinders usually have about 1/4" of material, and if you remove a 1/32 (.032) that puts you to about 5/32, not the best but probably ok for a street engine. The problem is with the front and back, the outside of the cylinder is not a circle, the cylinder wall has a flat on the front and the back (the outside of the cylinder in the water jacket), so although the cylinder is 1/4" think on the sides, the front and back usually read more like 1/8". It would be ideal to maintain a .090 minimum wall thickness, but depending on your block you may end up with a .080 or less cylinder wall thickness at the thin spot. What is the thinnest you can go? well everybody seems to have an opinion on that, generally .062 seem to be a number for the absolute minium. I am sure you will be able to find somebody who went to .050 and will claim they never had a problem.

Some of the later WJ blocks have .300 or more material on the thrust side( cam side) and the cylinders are generally thicker. I guess the simple answer is that has been done.

[Cheromaniac]

Thinking of trying to go to a 4in bore on a 4.0. Has anyone had much luck with this or should I just forget about it.

It can be done but it's risky. If you want to be safe don't overbore beyond +0.060".

[5-90]

I've talked to a couple people who have done this.

1) Use a RENIX (1987-1990) block - they're thicker. AMC had a habit of overbuilding, and part of the 40# the block lost when ChryCo lightened it was in the cylinder walls.

2) Have the block ultrasonically checked for core shift. If the cores did shift, make sure there is more material on the major thrust side (the machinist should know what to look for) than on the minor thrust side, since that part needs to be stronger.

Both of these caveats apply to the 242ci/4.0L block - don't bother with anything else (even though AMC tended to overbuild, the 199/232/258 won't have enough material to support a 4.000" bore - unless you sleeve it. Even then, it may not work - even with wet sleeves.) The 242 only has to go up 1/8" to hit 4" even, while the other sixes would have to go up 1/4"