Thread:

cylinder bore

Originally Posted by steele211

When a block is bored too far is there a location on the cylinder that is the most prone to failure? The reason I ask is if a bowtie block and a regular mk IV block have 4.8 bore spacing why is it ok to bore the bowtie to 4.56 or bigger and I keep hearing .060 to .100 over on a mk IV?

steele, although Aleexi's post is technically correct, In don't think it address what you're asking exactly. It depends on how far you're boring a particular block.
Like Alexi posted, if you bore a block too far, but it is intact, you may have bored it to the point that the bores will not remain round under operation. The more power you make, the thicker the bore needs to be to remain stable. A heavily NOS or blown MK IV with a .060 bore can fail. The walls go barrel shape with every firing, and evendually fail, plus you're probably losing power up to the point that it fails. They usually fail on the major thrust side of the cylinder. Either the out side, or valley side of the cylinder depending if its a odd or even numbered cylinder.

With regards to the difference between production MK IVs and Bowties, or aftermarket, it because of the way the block is cast. If you bored a production MK IV to 4.56 or 4.60, you would have to worry about failure during operation. It wouldn't have a cylinder left to fail. It would be GONE, nothing but water jacket. The cylinder walls of a MK IV just aren't that thick.

The Bowties, and others have what is called siamesed cylinders. The walls are cast so thick that there is no room left between cylinders for water to circulate. All 4 cylinders are cast as a single block from one end to the other, with no space between cylinders. To give you an idea how thick the castings are, a production MK IV bored .060 has a thinner cylinder wall than a late Bowtie bored 4.625

Originally Posted by steele211

So if I understand you correctly gn7 what you are saying the main difference is is that the mkIV has a water jacket that surrounds each cylinder and on a bowtie block that space is solid? Makes sense to me thank you

Well, sort of.

Look at this pic of a Dodge Nascar block. It has an open deck so you can see the water jacket. You'll notice that there is no way for water get between the cylinders, only around the outsides of the them. If this was a MK IV there would be a gap between each cylinder, and the 4 wouldn't be all connected.

Originally Posted by ohflat

hey gn what about 100 over mark 4 bbc

Personally, I would sonic check any BBC after .070

Some 4.250 bore 427 blocks will go .125 easily.

Yet I split the walls of a .060 bore this year at Long Beach. Granted its blown and has some pretty serious cylinder pressure, but I never cracked one before so I think this one was a little thinner.

Originally Posted by jockorace

Many moons ago, I had a .125 over 427 (makes it 451 c.i.) blower motor that I bought from a SoCal guy. I was always paranoid with it, but ran it a couple of seasons @ 13 psi of boost. Ran great. It obviously had some decent thick cylinder walls with no core shift. Jocko

Yeah, Chevy wasn't stingy with the iron back then. I have heard of .150 (4.40) from a 427 tall deck marine block. In fact JE stocked 4.40 pistons for them until recently.