Whatever happened to tall-deck 427" BBCs?

[PackardV8]

Our boneyard is filling up with these old dogs. Occasionally, someone will pay enough for us to strip out a steel crank, but are these no longer a cost-effective way to get street cubic inches?

[fastvette]

For me, the way I see it, there is no real advantage to the tall deck. You can get pretty much the same stroke in a short deck. Now if you could get a bigger bore in it that would make it worth while. I wonder why GM went to a tall deck with them.

[Walter R. Malik]

For me, the way I see it, there is no real advantage to the tall deck. You can get pretty much the same stroke in a short deck. Now if you could get a bigger bore in it that would make it worth while. I wonder why GM went to a tall deck with them.

You will never get a 4.750" stroke crankshaft in a short deck.

[Erland Cox]

They are tall decks because they have an extra ring.

Erland

[PackardV8]

They are tall decks because they have an extra ring.

Erland

Yes, they use 4-ring pistons; hell-for-stout, essentially being a diesel-spec DUALOY, with an Al-fin bonded iron top ring groove insert.

[SupStk]

What the truck tall decks offer in deck height they are restricted by the oil gallery placement for building over 4.375" strokers. With that restriction there is no advantage going with the mark IV truck block.

[PackardV8]

You will never get a 4.750" stroke crankshaft in a short deck.

What the truck tall decks offer in deck height they are restricted by the oil gallery placement for building over 4.375" strokers.

I've never built either, but know the tall deck always has the 3.76" stroke, so maybe the lower end is the same as the short deck?

[Ratu]

How much heavier is the tall deck block than the regular deck block?

[dannobee]

Yup, 4 rings, but another benefit for the medium duty trucks was that the higher deck height necessitated longer intake runners to make it all fit. Longer runners = more low end torque. While I'm not privy to the current Pro Stockers, just a few years ago the deck heights were well below that of a small block chevy (~8.7" or so compared to 9.025" sbc). The driving factor for the shorter deck height was to be able to shorten the length of the intake runners and still get the straightest path to the cylinder.

[Walter R. Malik]

What the truck tall decks offer in deck height they are restricted by the oil gallery placement for building over 4.375" strokers. With that restriction there is no advantage going with the mark IV truck block.

I have personally put a 4.75" stroke crankshaft with a smaller journal using a 6.750" rod in a tall deck block with no problems from the oil gallery along the side ... actually, done it several times.

I can not get a long enough rod in the standard short deck for more stroke than 4.375" stroke.

[PackardV8]

Hi, Walter,

Thanks for the clarification.

With the standard 4.25" bore and 4.75" stroke, I get 540". What bore did you use with these?

Reduced rod journal to what diameter?

How many tall-deck blocks do you need today? FWIW, there is still the occasional market for the OEM steel crankshafts, which basically zeros out the block.

[Walter R. Malik]

Hi, Walter,

Thanks for the clarification.

With the standard 4.25" bore and 4.75" stroke, I get 540". What bore did you use with these?

Reduced rod journal to what diameter?

How many tall-deck blocks do you need today? FWIW, there is still the occasional market for the OEM steel crankshafts, which basically zeros out the block.

The rod throws were 2" and the blocks used 4.32" bores.

I am semi-retired now so, I don't need any.

[ClassicComp]

Damn I had a customer looking for 2 of those a month ago..

[VMC]

What the truck tall decks offer in deck height they are restricted by the oil gallery placement for building over 4.375" strokers. With that restriction there is no advantage going with the mark IV truck block.

I have personally put a 4.75" stroke crankshaft with a smaller journal using a 6.750" rod in a tall deck block with no problems from the oil gallery along the side ... actually, done it several times.

I can not get a long enough rod in the standard short deck for more stroke than 4.375" stroke.

FWIW;

In 1995 I personally built a 4.500"X4.500" 572" using a MKIV Bowtie short deck block. I had custom JE pistons made with a 1.145" CH and used a set of Manley 6.385" rods. The block was decked to 9.790" which left the pistons down .010" in the hole.

I know everyone screams from the top of their lungs that the rod ratio is all wrong and that it'll wear out the cylinder walls in no time, and that the skirts come too far out the bottom of the bores, etc., but after several years of street driving a solid roller lifter failed necessitating a teardown. Save for one piston that got hurt (hunk of lifter body got wedged between the pin boss and a counterweight), the engine showed absolutely no signs of abnormal wear. The damaged piston was replaced, the hone was given a light kiss and it went right back together with a fresh set of rings.

As far as interference issues go, the block required surprisingly little clearancing at the pan rail area to clear the rods, but the inside edge of the bores by the cam tunnel definitely required relieving.

With the availability of custom pistons these days, this is actually a very easy build. If you can find a set of BBC rods with the once-popular but now somewhat uncommon 6.405" length, you can use shelf pistons with a 1.125" CH.

I *ALMOST* built another 4.5"X4.5" 572 last year, but I stumbled on a screaming deal on a set of NOS Probe pistons that were exactly suited for what I wanted to build (low compression torque motor for my dually tow rig), but they were 4.560" bore with a 1.218" CH for a 6.385" rod and 4.375" crank. Since I had to buy a crank & rods anyways, I went with the over-square approach this time.

If someone *really* felt froggy, maxing everything out they could build a 4.630" X 4.500" 606" short deck. With a set of early style Dart heads that looked like the early GM castings on the ends, a couple cans of Chevy orange paint, a pair of plain-jane chrome valve covers and a C454 under a 14" open element air cleaner, one sure could have a lot of fun with it.

[PackardV8]

I know everyone screams from the top of their lungs that the rod ratio is all wrong and that it'll wear out the cylinder walls in no time, and that the skirts come too far out the bottom of the bores, etc.,

For true. Same but different, when I was building SBFs, I was very skeptical of the 347" being durable. Now, any kid with a checkbook can build more SBF power with a 347" than any 289"/302" we/Shelby/Roush ever dreamed of.