final overbore?

How far can you bore the 89 4.0 out to?
I removed my head and noticed 50 stamped on the top of the pistons.How much farther than 50 over can I go on this motor?I would like to have it bored out another time so I can make sure the cylinders are not tapered before I throw a rebuild kit into it.Thanks

It's commonly accepted that, except for some engines, a maximum overbore of .060" is right on the edge.

However, there are, as I said, exceptions.

A number of engines are listed as having a maximum overbore of .040" - many of them imports.

Some engines can be taken out a great deal if you can find sleeves for them.

For a cast iron block like our 4.0's, a cylinder wall thickness of .095" is considered a dead minimum - but the castings are thick enough, on quite a few earlier engines, to handle overbores of up to 4.000" inches - but a sonic inspeciton is required to make sure you'll have the wall thickness to get away with it.

However, why are you considering boring your engine? Is there a taper or out-of-round problem? Excessive ridge? You can probably go from .050" over to .060" over without any trouble, if you're worried about things. I'm planning on taking at least one block out to 4.000" once I get one that passes ultrasound...

5-90

yup,you guessed it,excessive ridge coupled with piston slap that didn't do #2 cylinder any good.

I thougth that there was a thread about all 4.0L pistions have a head stamp on them of 50, it don't mean oversize, it was a plant/Casting number?

You need to mic the bores, the 50 on the pistons is normally a casting number and has nothing to do with the over bore. Standard overbores are .020, .040, & .060.

That is good news, I would be surprised if you were bored more than .020 over on a rebuild.

My local machinist told me that the 50 on top of my pistons was a metric number which translates into 20 over.

jeepdeepfreak said:

My local machinist told me that the 50 on top of my pistons was a metric number which translates into 20 over.

Click to expand...

He was wrong, most have 50 stamped on them, I have no idea what it stands for.