J.V.IRONHANDS

I have to replace a Gen V. block, as these are no longer made ,my GM dealer says it's replaced with the Gen VI. Only difference is Gen VI has a fuel pump boss in block where Gen V block didn't. Also some minor differences on timing chain cover but everything else is the same. GM book also states Gen 4 blocks can't be used with Gen V heads, they will leak so you have to stay with Gen V or VI block. Also is Gen IV, V, VI mean the same as Mark IV, V, VI Blocks? Need opinions , Thanks. John

Dennis Moore

Gen V and Gen VI 454 short blocks are the same except for the roller camshaft, lifters, pushrod, timing chain cover and fuel pump boss. The roller camshaft requires a hardened distributor gear. The official names of the engines are Mark IV, Generation V and Generation VI but even GM has a hard time keeping with these names sometimes calling the MarkIV a GenerationIV and calling the Generation VI a Mark VI.
Sincerely
Dennis Moore

formulaF302

You are correct in that Gen IV blocks cannot be used with Gen V or VI heads....they will leak. The head bolts are different too. As to the fuel pump boss I believe that the Gen VI does not have a cast in boss for the mechanical fuel pump....lets see if someone else on the site can verify that please however. I know for a fact that some special head gasket modifications have to be made also if you use aftermarket Gen IV style heads on Gen V or VI blocks, but it can be done and the stock Gen VI bolts will be too long in such an application. If anyone has a part number for head gaskets already made for using Gen IV or Merlin heads on a Gen VI block I would appreciate the information...everytime I pull up a GM part number for that it has been discontinued, superceded, etc.

Offshore Addiction

the gen VI is a better block and rotating assy,GM had severe issues with soft cranks in the GEN V motors and of the numerous problems of retro fitting the mark IV motors,as far as I know and from my experience Ive never had a single problem putting any year head on any year blocks,even the merlins,darts, and brodix all retro fit any year,the gen VI does have a better crank and hasn't run into any severe faults at this time that I've ever run into..

formulaF302

its been about 2 years now since I put my Gen VI motors together, so I'm a little bit foggy on the exact details...but after cracking the stock heads on one of my Gen VI motors I went to Merlin heads and do specifically remember that we had to punch some extra coolant holes in the head gaskets to use the Gen IV style heads on the Gen VI blocks. There is a difference in the water passages between Gen IV and Gen V & VI. I also specifically remember that my engine guy showed me why Gen VI heads will not work on a Gen IV block because of a different steam hole or water passage that does not get totally covered by the block casting, thereby allowing water to get into the lifter gallery area. I believe it was near the front area of the port side head....sorry bout being so foggy on it...musta been all the Bud Lite consumed during that project......does anyone else on the board know the specifics on this?