Block decking: how necessary?

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MopaR&D

MopaR&D

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Just got my 360 block back from the machine shop where it was hot tanked, cylinders honed (already .060" over and in good shape), and crank polished. However the guy who did the work didn't mention anything about the deck surfaces. I'm worried about this because I want to run flat-top zero-deck KB-107s with Magnum heads and I need to get the quench height right so the 10.5:1 compression will work with pump gas. How bad did the blocks tend to be from the factory in this spec, and would these pistons and heads still work OK with an un-decked block?
 
bad,
not only from one bank to the other, but along the same row
of cylinder you'll get variances. so its a good idea to always deck the block to factory specs at least.
 
The blocks are terrible from the factory.Give hive the rods&piston assembly to check the high and low of each end and you will see its off by some .020-.030,mrmopartech
 
Any engine i ever had built was decked its just peace of mind knowing the surface are flat and true,so them expensive head gaskets sit nice and flat..
 
A KB-107 piston with everything at the nominal dimensions will be about 0.012" below the surface of the deck. The Deck height is almost always bigger than the nominal 9.600 so you are going to be more than 0.012" below the deck.

I had to remove about 0.020" off the deck to get them square and the KB-107 pistons to zero deck.
 
It's only impotrant if you want to blueprint the block or if you want to acheive a desired deck height, or if the block deck is out of spec and warped. Those engines ran for DECADES with the decks right where they are. They beat lots of Chevys and Fords in the day too. Is it absolutely MANDITORY? No. It is however absolutely advisable. I am going to have mine decked just to blueprint the block. I don't wanna acheive zero deck height. I'd LIKE to run on pump gas. LOL
 
My factory stock big block was horrible so I had it decked and it was getting blueprinted anyways.
 
For what its worth.....the answer has already been given.

When custom engineering a specific compression ratio on a given engine IT IS necessary to Deck The Block to true up the height across the deck...for assurance that each cylinder will be as close to the same as molecularly possible.

It is not necessary (or worth it:angry7:) to deck a block for a basic rebuild.

Specifically on your setup...I have no experience with KB107's.
I do know that a experienced machinist sure can whittle away the willings' wallet.
 
Shoot looks like I need to take all my stuff back. I need to get the rotating assembly balanced too. I'll ask my machinist when I get the chance but how much does decking typically cost for a Mopar block that needs it? And can it be done after the cylinders are honed?
 
My shop guy called himself a Mopar man, has a BB Cuda built, he told me the Mopars were ok, most don't need it. My pistons, kb243, were to be .018 positive deck height, I checked with feeler gauge afterward and all cylinders were pretty darn close to .018, felt to me right on.
 
Factory machining sucks. I want anything I do to be an improvement, never mind when I need to get the dimensions perfect too. So everything get's square decked, which is what you need to have done. This should not be confused with decking as they are not the same and many millers can't actually square deck unless they are equipped with the right fixtures. If you can't square deck, there's not much sense in decking because the old millers can't get it that good anyway.
 
StrokerScamp said:
I am going to have mine decked just to blueprint the block. I don't wanna acheive zero deck height. I'd LIKE to run on pump gas. LOL
Click to expand...

FWIW, a zero decked KB-107 piston in a 360 with typical open chamber heads (72 cc's) will give you 9.7:1 compression. With typical magnum heads (64 cc's) the compression will be 10.6:1. Either case is a no sweat pump gas motor as long as you are using a cam with a late enough closing point of the intake valve to keep the pumping pressure within reason.

I have the KB-107/magnum head combination and 10.6:1 compression and I am using a Comp XE268H cam that results in cranking pressure of 195 +/- 5 psi and the engine runs fine on 89 octane. The closed chamber and tight quench make for a very detonation resistant engine.

FYI, the photo below is of my block being square & zero decked. Notice how the block is fixtured off the plane established by the centerlines of the crank bore and cam bore. This particular machine and fixturing is also used to bore the block so the bores will be perpendicular to the same plane. The head is changed to a boring bar. The green machine in the back ground is where the final honing with deck plates is done to the bores.


BlockDecking.jpg
 
dgc333 said:
A KB-107 piston with everything at the nominal dimensions will be about 0.012" below the surface of the deck. The Deck height is almost always bigger than the nominal 9.600 so you are going to be more than 0.012" below the deck.

I had to remove about 0.020" off the deck to get them square and the KB-107 pistons to zero deck.
Click to expand...

Not only that, but each deck is a different height so you end up with a different compression ratio deck to deck. Unacceptable.
 
Correct, square decking puts both decks at 90° to the bore centerline, and exactly parallel with the crank centerline. Plus, a modern miller uses a single cutter spinning at a much higher rate. Because there is only one cutter, the plane of the cut is almost perfectly flat. Where a miller that uses a disc with many cutters has to be tipped so the cutters dont hit on the opposite side while it rotates. This can create valleys at the center of each deck surface that can be anywhere from .001" to .005", plus, the old millers rely on the machinist leveliing the block. If he's a little off in any direction, the cut will be messed up. If you need a tight quench, you need modern equipment to machine the parts.
 
You always, always deck the block and heads. Does not matter if it is for a racer or a grocery getter. Always deck them. I have been building motors since the mid sixties and have deck all of them, no matter what. Never go cheap on the motors. Not the place to save money.
 
66silverfish said:
You always, always deck the block and heads. Does not matter if it is for a racer or a grocery getter. Always deck them. I have been building motors since the mid sixties and have deck all of them, no matter what. Never go cheap on the motors. Not the place to save money.
Click to expand...

I agree, especially for a engine that you want the max performance/ max reliability out of. Us mopar guys running stock blocks already have an issue because the 4 bolt per cylinder design is prone to blow head baskets when running big compression, nitrous, or boost. Without having the most-likely warped deck machined to make sure that everything seals best that it can, you could have issues down the road with combustion sealage. Careful not to get tons milled off because then the intake manifold will also need to be machined to match.
 
I see what you guys are saying but it's an issue of block decking costing almost half a paycheck LOL (I asked him today, $110). And I'm already going to pay more than that to have the rotating assembly balanced. I suppose I don't have much of a choice if I want to use these pistons and heads correctly but I'm at least going to try to measure the decks myself and go from there.

dgc333, is there a way I can calculate the cranking or corrected compression for my engine? I'm using this Voodoo 256/262 cam. I tried to compare specs with your XE268 but Lunati and Comp seem to rate their lobe opening/closing degrees differently. Looking at their lineup my cam is roughly two steps below yours in duration and lift. I'm also at 7000 ft. above sea level so that will affect my final compression.
 
This calculator will give you the dynamic compression http://www.wallaceracing.com/dynamic-cr.php

You should be able to compare the dynamic of mine with the XE268 and the Voodoo you are using and if they are reasonably close you should be good to go. If everything else is the same I would expect the Voodoo to have a higher dynamic compression; according to the Voodoo specs the intake closes 34.5 degrees ABDC and the XE268 closes 60 degrees ABDC.

If you are going to go with a 10.5:1 static compression ratio why not go with a bigger cam to take advantage of it?
 
If you are looking for perfection, have it decked....but then also have all the rods equalized in length....crank stroke 'indexed'.....heads cc'd and corrected....all valve faces cut identical as well as all stem heights cut to exact perfect rocker geometry......lifter bores checked and corrected/bushed to actual 59*......custom pr's......and the list go's on.

Just giving the guy 4 rods and pistons to do a 4 corner check can still leave you with variances.

jmo
 
Kid - most of the calculators are for sea level. I don't usually worry about that but at 7K feet you should if the car's going to stay that high.
 
dgc I used the calculator and found my dynamic to be around 8.6:1, about the same as yours calculated. Although I wonder about the intake valve closing degrees because the Voodoo 268 cam is rated at 39 ABDC, while your Comp XE268 is rated at 60. Does it make sense that these two supposedly similar cams would be so far off in that regard?
 
The Comp "268" intake lobe with .477 advertised lift is smaller than the "268" intake lobe with .494 advertised lift of the VooDoo. When I started looking at the lobe shapes the Comps are about 1 step smaller.. So the Comp XE268 lobe is the equivilent of the Lunati Voo Doo line's 262 duration intake lobe. The Voo Doos are taller and wider so that makes sense that the closing point would be later if the overall duration was longer even thos they are both "268" lobes.
 
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