Mk IV, Gen V, And Gen VI Ratmotors

[Joe C.]

What are the differences between these three designs. When did the Gen V and Gen VI come to be used.

Thanks in advance.

Joe

[Joe L.]

Re: Mk IV, Gen V, And Gen VI Ratmotors

Joe-----

There are lots of little differences here-and-there, but here are the basics:

Mark IV (366, 396, 402, 427, 454 cid)

Used from 1965 through 1991
Adjustable rockers
10 bolt, stamped steel timing cover
All PRODUCTION blocks have mechanical fuel pump provisions
2 piece rear main seal
Has clutch cross shaft threaded boss
Main oil gallery located above left side oil pan rail
Heads will not interchange with Gen V/VI except with special head gaskets and plates

Gen V (or, Mark V) (454 or 502 cid)

Used from 1992 to about 1996
Non-adjustable, "net-lash" rockers
10 bolt stamped steel timing cover
NO fuel pump boss except for one SERVICE-only casting
1 piece rear seal
Different cooling passages than Mark IV; heads will not interchange with Mark IV except with the use of special head gaskets
No clutch cross shaft boss

Gen VI (or, Mark VI) (454 or 502 cid)

Used from 1996-2001

Same as Mark V except:

6 bolt cast aluminum timing cover
set up for roller lifters (i.e. raised lifter bores and "spider" mounting bosses
provisions for cam thrust plate (for roller cam)
provisions for crank-triggered ignition
single roller timing chain only

Gen VII or (Mark VII) (8.1 Liter or 496 cid)

2002 to present

Except for stroke, I think it's pretty much the same as Mark VI 454

[Joe C.]

Re: Mk IV, Gen V, And Gen VI Ratmotors

Thanks Joe. It looks to me like the Gen V and VI are more adaptable to modern computer controls. I wonder if the basic Mk IV casting is stouter than the Gen V, or Gen VI. What design is your ZL1 block based on......is it of the original MK IV design? As far as horsepower capability, do you know if there are any differences between the IV, V, VI, VII??

Joe

[Clem Z.]

i have built 510 cu in supercharged mark Vs

that dynoed over 750 HP and 10 years later they are still running in the boats. the production 502 blocks will take 700 HP and joes mark 4 is good for 650 HP according to GM

[Joe L.]

Re: Mk IV, Gen V, And Gen VI Ratmotors

Joe----

I think that the Mark IV and Mark V/VI are about the same as far as strength is concerned. The Mark V/VI may be a bit stronger, but not significantly stronger. The Mark V/VI do have priority main oiling which places the main oil gallery above the camshaft instead of in the oil pan rail like Mark IV. That's an improvement. The Gen V/VI "borrowed" this from the 1969 ZL-1. The ZL-1 was the only Mark IV engine that ever used this configuration which is similar to that always used for all small blocks.

I think that either the Mark IV or the Mark V/VI would be adaptable to computer control. However, if one wanted to use crank-triggered ignition, then the Mark VI would be the way to go. Plus, Mark IV blocks are no longer manufactured by GM(except ZL-1); neither are Mark V. So, if one wants a NEW block, then the only way you can go is Mark VI (or Mark VII). If I were building a big block engine from an iron block, I'd use the GM PART #12561353 bare block as a starting point. This block, CASTING #12550313, is a Mark VI with mechanical fuel pump provisions. The only really noticable configuration difference between this block and the Mark IV (as originally used in all Corvettes) is the cast timing cover and the machining of the front of the block. Incidentally, this block was made possible as a result of the Tonawanda engine plant's acquisition of some of GM's production machinery used to build the LT-5 engine after that engine went out of production at Mercury Marine in Stillwater, OK. That machinery is what makes the addition of the fuel pump provisions to the Mark V and VI blocks possible.

The current ZL-1 block is kind of an "amalgamation" of the original Mark IV aluminum block, but incorporating features of the Mark VI and also features of the SERVICE-only Bow-Tie-series blocks. Of course, it has the priority main oiling like Mark VI (since, as I mentioned, the ZL-1 is where this configuration for big blocks came from in the first place). The ZL-1 has thicker deck surfaces like Bow-Tie blocks and siamesed cylinders like Bow-Tie blocks (ZL-1s always had siamese cylinders, though, so, in a sense, the Bow-Tie blocks got this feature from the ZL-1).

Current ZL-1s use the special cam bearing configuration used by all CNC Bow-Tie big blocks and NOT the configuration otherwise used for PRODUCTION Mark IV, V, and VI and NOT the configuration used for the original ZL-1. In this configuration, all cam bearings are exactly the same OD.

The current ZL-1 borrows the hydraulic roller mods built into the Mark VI blocks, including taller lifter bores, machined lifter bore tops, provisions for installing the lifter retainer "spider", and the cam shaft thrust plate provisions.

Current ZL-1s use anodized, machined aluminum block plugs with straight thread sealed with o-rings as opposed to the cast aluminum, square head plugs used for the original ZL-1 and the press-in steel plugs used for most other block plug applications on Mark IV, V, and VI.

The current ZL-1 uses main bearing caps machined from billet steel with splayed outer bolts. No other PRODUCTION big block, including the original ZL-1, ever used this type of main bearing caps. ALL others use cast nodular iron caps, including the original ZL-1s.

The current ZL-1 retains the 2 piece rear main seal configuration of Mark IV blocks and does not use the 1 piece seal like Mark V/VI/VII.

[Clem Z.]

a big change in the 8.1

the 8.1 has different head port arangement so all the intake ports are equal length

[Joe C.]

Thanks Joe And Clem

If I were building up a big block for max horsepower, then I would probably opt for the Gen VI or VII casting.
The Sallee Chevrolet 123708050 aluminum "ZL1" block looks good because of it's light weight (110 lbs).
I am very happy to know that the General, and others, are making improvements to the venerable MK IV ratmotor.

Joe