OK, I'm doing a 350 build and my blocks at the machine shop. I'm having it cleaned, magnafluxed, bored, honed, yada yada yada. Now I'm just wondering if I should get a decking? I've heared that its something you should do, but is it like totally neccesary for a rebuild or is it like if ur head decks all scratched and stuff? I'm naive in this area, can someone enlighten me? Thanks.

 

Not unless it warped, 90% of engine builds do not require it.

 

It's done for 2 reasons.

One is to clean up the surface, to make it as friendly to head gaskets as possible. That's usually a good idea.

The other is to bring the deck height (the distance from the centerline of the crank to the deck) to some exact known spec. That's more of a blueprinting type of thing.

Stock SBC blocks come from GM at 9.025", -.000" + .015" or so. Lots of uncertainty in the compression ratio of any given engine as a result (which always makes me laugh when I see people argue about stuff like the LT1 is 10.4 or 10.5, no it's actually 10.44, yeah right... could be anything from 10.2 to 10.5 and still be within factory tolerances). Race engine shops will cut it to exactly the height you specify. Which is not the same as "take .020" off" or whatever, because they all start out at random different heights. "Zero deck" height for a race build is usually 9.000" which is the same height as a stock SBC rotating assembly. That puts the surface of the piston exactly in the same plane as the deck at TDC.

 

if you have the block at the shop anyway i think it's foolish not to, if i had no other reason to take a block to the shop i wouldn't. i do usually have to wait 3 weeks or more at my shop to get anything so time is something i consider.

 

One complication that goes along with zero decking is preassembly.
Unless your motor is assembled and you're re-using all the original parts, you'll have no way of measuring it to find out what the real "zero deck height" will be, other than to assemble it.
If you're getting new pistons and the rods are being resized the compression clearance will be different than it was before.
Preassembly is a good thing, first you get the block bored and honed, then you fit the rotating assy together without the rings and check all the clearances. Then you take the block back to the machine shop and have it decked and have them r&r the cam bearings at that time and have it boiled clean, balanced etc.
It's extra work but worth it.
Preassembly is particularly nessecary with strokers, cause there are always clearance issues there.
But that's how perfection is achieved.

 

88 what if you measured from the crank C line to the deck 9.025" i think is the spec do you think that might work without any preassembly?

 

Hi ede.
That would be one way to do it, but for myself I prefer to preassemble for a number of reasons.
Tolerance stack up is one. When you consider that there's a slight error in each of four components that go into an assembled block. The crank throws, the rod CtoC length, the piston compression height, and the block deck height itself, it seems like it's a good idea to check it if you're shooting for a true zero deck.
I've found out by doing this process on a few different assemblys, that with factory parts the error seems to be larger than with aftermarket parts.
Block deck height can vary from front to back, and with factory rods the CtoC length can vary by as much as .010", particularly if they've been reconditioned.
I've always figured that that's the reason they put the piston below the deck to begin with.
On a current project, IIRC, a set of speed pro pistons with some scat rods on a factory crank in an unmachined block was between .030 and .026 below the deck when I think it was supposed to be .025, if you're going by the advertised specs.
The original question was, "is it nessecary?"
Not at all... I'm just being extra picky.
When you're talking about pushing your pistons up to within .035" of contacting the head, I think pickiness has it's payoffs.
That's why blueprinted motors are expen$$ive, they have to build em twice (or more).

 

Ive only had 3 engines zero decked, all 3 were for performance use, the 355 that is now in the bird, the 402 that was in the truck and the 427 thats waiting to be droped in the malibu. None of these engines needed to be decked but I had an extra $60 per engine to blow.

 

It would be foolish to skip it.

Have it cut down to 9.005 and you should be safe with almost every popular piston on the market.

Its all about proper quench, getting close to zero deck height leaves only the gasket as the variable, Felpro makes the good stuff at both .039 or .041 to suit your needs pending your final deck height spec, keeping your quench height under .045 will help maximize the combustion process.

 

I wanted to bring this back up.

I'm taking my motor apart next week. One of the things I wanted to do was measure the deck height (or installed height of the pistons rather), and zero deck it, then run a .041 gasket.

Now looking at the specs for my pistons, it says:

"Application Notes Deck clearance: 0.022 in. Skirt clearance: 0.0015 in. Weight: 610g. "

So, I'm assuming, what speed pro is saying is, if installed in a 9.025" factory block, the height between the piston and deck will
be .022". So assuming my block is factory deck height (and not taller), i'd have to take off .022" to get Zero deck?.

I'll measure everything next week when i'm done racing, but I wanted to run this buy you.

-- Joe

 

Factory blocks are usually about 9.025", but they vary quite a bit.

So, NEVER give your deck spec as "take x thousandths off", since you have no clue what the height started out life as, or whether it's the same on both sides, or even whether it's the same for all cyls (do you know that the deck is parallel to the crank from front to rear?); instead, give it as, "set deck height to 9.000" (or whatever).

In this case, your pistons are designed to be .022" lower in the hole than the OE ones; which is typical for rebuilder pistons, since a slightly lower CR is far less catastrophic than the pistons coming up out of the bores of a block that had to be decked for maintenance reasons, and hitting the heads.

Therefore, if you want exactly a zero deck height, you would probably want to deck it to 8.978".

Measure EVERYTHING you have ANYTHING cut.

 

Ahh. So they're .022 LOWER than stock, not .022 from deck ? ugg.

I'd think cutting the deck down that low would cause some manifold alignment issues to start with..

-- Joe

 

Werd... I had my shop preassemble everything, check my deck height, and then zero deck it. Only way to go in those situations. Cost me extra, but I put the motor together for performance, and wanted the quech just right. It was straight and true, so I didn't need it, but felt it was worth the money to blueprint the motor to the specs I wanted...

 

From stock form, how much did you take off and how did it affact both intake manifold alignment, and serpentine bracket alignment?

-- Joe

 

I really don't know exactly how much they took off at the shop. I have that sheet in the file somewhere w/ all the specs, but I think I remember him saying it was right at .025". He preassembled everything, then measured and cut it down to zero.

Intake alignment has been fine, no issues that I have seen. Only problems I had were clearancing the middle bolt holes from the newer bolt intake to the older bolt style heads I ended up w/. The outer 8 lined up great though. Intake is sealing fine, and no problems from that.

Belt alignment is great... Perfect actually. I had absolutly no issues w/ either of those things...

 

I am putting together a 406 currently- when shopping for machine shops I went to the new one in town connected with the new speed shop. They wanted $350 to bore, torque plate hone and deck the block.

They were pushy and insisted I didn't NEED to do a mock up assembley and measure things to calculate my desired CR and quench clearance.

I asked many, many people who have built a lot of engines and they all agreed that doing a test assembley is the only correct way to find out what you need taken off.

Needless to say, I went to another shop who COMPLETELY understood and agreed with my means of determining how much to take off.

And I got the block hot tanked, manafluxed, bored and torque plate honed for $200. The block still needs .0025" final honing, he did enough so I can fit the pistons in without rings to do my trial assembly, measure the deck, and he will deck it before he does the final clearance hone.

 

Absolutely. My machinest said "Measure each cylender, then use the heighest one. calculate how much you want me to take off, and I'll do that. If you dont pick the highest, and the piston sits .005 beyond deck, it aint my fault".

I was curious as to how TRW announced their specs. To get an "idea" how much I may have to take off before taking this assembly apart.

Gonna race the car wednesday, then start taking it apart thursday.

-- Joe

 

Measure twice, cut once.