hi everyone,

this topic deals with piston to deck height clearance...long post

my motor is at the machine shop now. the machinist/builder is a trusted name in the area and has built many Pontiac motors for over 20 years.

my plan is to have the block deck zeroed in relation to the piston top. I have read from many post that "zero deck" or at minimum of 0.010 in the hole is best as it is good for combustion efficiency and quench. also, it will keep the engine cooler and will be less prone to pre-ignition.

so, my engine builder believes that it is not necessary to zero the deck height in relation to the piston top. he believes that 0.020 in the hole will work just fine on pump gas with the cam im using. so, I did more research and have found several post that 0.020 in the hole will work and that I will not have any problems.

im just a bit frustrated/confused now. not sure what to do or think. I guess I can do either as I probably wouldn't notice much difference. right? maybe add octane booster if needed?

on that note, please give me your own experiences and advice on this. if you have similar builds, please help as it will be greatly appreciated.

thanks!!

some might not agree with my build so please don't hate.

my build...

68 firebird
400 0.030 block
#13 heads cc'd at 77cc's
sealed power forged flat top pistons
forged I-beam 5140 rods
summit 2802 cam
1.5 rockers
msd ignition
stock cast iron intake
Qjet carb
TH400 tranny
13" 2500 hughs stall converter
3:55 gears

1) I am not a machinist, but....

My machinist agreed that it was best to zero deck on my build for all the reasons you listed. But you have to consider ALL the details involved. if you do zero deck, you best have the CC's in the heads, and overlap in your cam, to manage dynamic compression. You have to be able to trust your builder.

You'll likely never know the difference once the engine is installed.

thanks for the reply.

I do trust him. and I don't know this for a fact but I sense that hes kinda stubborn and builds them the way hes always built them, regardless of what the costumer/me thinks or wants.

cam LSA 114
77 cc heads

What is the engine intended for. Maximum performance or just a driver. There's a lot of speculation on the topic of zero deck.
I would be concerned more with having a flat square deck surface than anything else.
If an engine has run fine for the past 45 years the way it was there's no need to get all fancy with buzzwords off the internet.
As long as a machinist can do all the required machine work properly and accurate and the engine is properly assembled and properly tuned then you can get another 45 years of use out of the engine.

Most good engine builders are stubborn...comes from years of doing it the customers way, and then the customer is not happy, and blames the engine builder.

Do it his way, then it's all on him to deliver a reliable build you'll be happy with.

thanks for the replies.

Bigchief, built for a fun weekend street cruiser.

great points and much appreciated. my grandpa was a stubborn old fart but he knew engines and how to tune them well. im sure hed say the same thing already said in the previous posts.

Just installed this one I built a while ago. 1967 YT block with 670 heads with dished ICON pistons and aftermarket forged rods. The deck was squared and pistons are .013 down in the hole. 9:1 compression.

Good question, and you need all your engine's parameters by actual measurement not listed or assumed numbers.
I'm not familiar with the sealed Power pistons but they most likely have 8-16 CCs of valve reliefs, depending on the number of reliefs and depth of cut, which will drop the compression.
Static compression ratios mean little, cylinder pressure and dynamic compression are the important numbers.

I use zero deck height, that provides good quench, depending on the shape of the heads combustion chambers and provides for better fuel air mixing. The only negative I can think of with a zero deck height block is if you blow a head gasket and need to resurface the deck to correct an erosion, the pistons will have to be shaved as well, or replaced with lower height pistons.

Ask your machinist the reasons he thinks 20 in the hole is superior to zero deck, his/her answer may enlighten us all.

I would guess that the machinist may not feel it's a superior condition to leave them 020 in the hole(aside from maybe the reason stated) but that it isn't going to make enough of a difference to cut it. Besides, like Blue said, if compression's good at 020 then why cut it and then have to make it up somewhere else like dished Pistons or thicker head gaskets. Don't over think it. It does matter how much, exactly, you're talking about too. My latest block was just squared and I was lucky to keep all the Pistons just barely below deck. In fact one of them ended up a couple of thou above. So be sure what the measurements are before you make any decisions.

Firebob, if you had the block squared and just one piston was higher than the deck was the deck machined on a plane not parallel with the crankshaft bore? Or do you have one piston slightly taller than the rest? Or was the crankshaft machined with one journal greater stroke than the others, no that would give you two high pistons.

You could be correct about the reason for not zero decking, the machinist may have the block all squared up and doesn't think the extra benefit of zero decking are worth the cost of re-machining if it's not a performance engine build.

Well apparently one piston or rod combo was a couple thou longer for some reason or other. Not sure exactly. 7 rods came out of the old motor. One of them was new. 6 of the Pistons were older. 2 of them were newer versions with coated skirts. All were previously used and given to me so who knows. I suppose I should consider myself lucky they were as close as they were. Everything I do is on a pretty meager car budget so, basically, you get what you pay for. My opinion is not to sweat the small stuff too much. I love a challenge like trying to build a rocket ship with a rock and two sticks.

Bigchief,
engine looks nice!! what cam did you go with. cant wait to hear how it runs. keep us posted.
Bluebird428,
the sealed power pistons have 4 relief valves and are listed as 6-7cc. thanks for the reply. im having a heart to heart with the machinist today so if I find out any more words of wisdom ill be sure and share it.
Firebob,
thanks for the advice. much appreciated.

Summit 2802 cam with the USA made Johnson lifters
Waiting till we have a day under 113 degrees outside so I can fire this thing up.

Did you balance that rocket motor?

I didn't. The machine shop did...supposedly. Who really knows when you get all your stuff back and you pay the bill. You just have to hope you chose an honest shop and they did everything you paid them to do. I do know they called me and I ended up making an additional trip down there with my set of rod bearings and rings that I hadn't thought to bring with the rest. They said they needed them to do the balance so...I'm fairly confident. Haven't had her up into the 100+ range yet though. Fingers crossed.

Just wondered what the weight difference was between the two newer coated pistons and the six older versions. I guess we wouldn't know if they were balanced to a set before.
If you're using a piece of granite and a couple of sticks of west coast Douglas Fir, that rocket should be fairly strong.

Yeah, no telling. Especially since I have no idea of their history. They may have been previously balanced or they may have come out of different motors. Doesn't matter as long as they match when they go back in I suppose.