The Dodge Challenger Message Board

What's your opinion about?
Big siamesed blocks are bored to 4.500, but what is the reason to receive this name?
What is the real difference from typical big block?
Any cooling problems to continued street use or in the traffic jam?
Need to install a new radiator to keep the engine on the right temp range?

As I think you suspect the cylinders are joined together down the middle (like siamese twins) so the water jacket does not completely surround each cylinder.

I am not sure that this matters on a street engine as long as the cooling system is good. I think the old Pontiac Firebird 400 inch engine was like this. Although that particular engine was prone to blowing head gaskets if i remember right but I think that was because the cylinder bores were very close to each other.

dave-r wrote:As I think you suspect the cylinders are joined together down the middle (like siamese twins) so the water jacket does not completely surround each cylinder.

Yes, exactly that.
I don't find information about the difference between 4.320 block and 4.500 siamesed block.
Why they call it siamesed?

I don't know what you call then in Spain but "siamese twins" is an old expression that used to describe children born joined together. This used to be more common in the far east and in Siam in particular. I think Siam is what is now Cambodia or something?

Yes Dave, I know
But the reason I want to know is if this name is due the right and left cylinders position or the reduced space between the same side cylinders until no space enough to water flow.

Other thing I'm not sure is the block called "mega block", is only for overbore or something more?

They are using the cross bolt main blocks, is this system used on each hemi?

plum-crazy wrote:Yes Dave, I know
But the reason I want to know is if this name is due the right and left cylinders position or the reduced space between the same side cylinders until no space enough to water flow.

I think the name gives it away. All four cylinder on each side are cast together. No space between for water. This allows for bigger bores without the walls getting too thin.

Same with Mega block. Cast with big bores they have much thicker cylinders and smaller water passages.

plum-crazy wrote: I want to know is if this name is due the right and left cylinders position

If the left and right banks of cylinders were joined together it wouldn't be a V8 would it?

plum-crazy wrote:They are using the cross bolt main blocks, is this system used on each hemi?

I believe so. I would have to check if the stock street hemi used the cross bolt mains. I can't remember.