Going to bore 4.0L... Need answer ASAP.

[BIIHawg]

I've got a 4.0L from a 91 Explorer that I am having bored out. I have found different opinions on the web about the safe zone for boring this block. So here's the question:

What is the safest bore one can do without risking the cylinder walls failing?

I am looking at .040 over (to eliminate a rust pit in the cylinder wall) which I have heard is fine

Please inform me with your wisdom.

Thanks in advance.


Just one more question. If I weren't in this bind:

Is there a benefit to boring an engine? i.e. noticable power increase.

 

[rangerbert]

I had a 94 4.0 bored out the machine shop that did mine told me no more than 20 over. But I don't know if you can go more than that or not just go the least amount that you have to. Then you'll be fine.

 

[Bird]

.040 is the absolute max you should go on a 4.0 .............. .020 is preferred. I've found that the earlier blocks 'seem' to be a little thicker that the later blocks - wouldn't even consider boring a 98+ block past .020

Bird

 

[bottledgt]

why even bore it at all if you dont need to. 4.0s suck anyway

 

[Dangeranger3]

why even bore it at all if you dont need to. 4.0s suck anyway

I am looking at .040 over (to eliminate a rust pit in the cylinder wall) which I have heard is fine

 

[MAKG]

It would be cheaper to get a used 4.0L from a junkyard. It's not like these are rare or expensive engines....

If you borescope it, you can spot blow-by quite easily. Same deal for leakdown testing.