Subject of zero deck arose again.

One commonly published figure for Pontiac 400 of our vintage says pistons down .023 in the hole. I have a bone stock original unrebuilt '67 400 with pistons down .018 in the hole.

What has changed in the past 40 years that a factory built 400 can no longer run well, given all in mechanically good condition with reasonably low mileage (just over 77,000)?

Fuel.

And if I am not rebuilding my '69 400 (no reason to other than because it has never been) then why should I have to zero deck it to make it run well? The only change from stock is the timing set. It ran the same before it, but the ignition timing had more variation with the worn gears and chain.

And why do some engines have more issues than others, even those that were built using the same parts?

different machine operators! also a zero deck height will up your compression. why not ask tom about this one?

I don't have heat issues. My car has only heated up once, never overheated. The heat was all from the TH400 trans, which when left in drive at 0 - 5 mph for a couple of hours in 90+ degree weather, does not have much opportunity to cool.

There is something else at play here. I will have to tear it down to find out what.

Does anyone have compression height figures for the common replacement piston forgings and for factory pistons?

Different machine operators on the assembly line? I don't think that the cars were plagued with detonation when new.

Yes, the cast pistons made for aftermarket were (and are) junk. Those measurements don't surprise me. What I'd like to have seen is what the below-deck measurement was after swapping the pistons.

My 69 400 is rebuilt to original specs. It's mostly original under the hood with some added ignition electronics. It runs perfect. Doesn't ping or overheat. Has just the right amount of power. The only drawback is that I have to mix 93 and 110. Notice how I didn't mention zero decking? It isn't necesary. It may very well be an improvement. But why change a good thing? If you build it as it was when new it'll run good. Make sure you feed it right and it will treat you right.

Thank you, Jim, I have nothing against zero decking but I also don't believe it is necessary in all cases.

Can you please tell me exactly which pistons were used in your build? I believe you posted it once but I do not recall.

The stock-build '68 has the L2262F forgings. I don't have the compression height on them, but until I am home to measure the stock pistons I don't have anything to compare with anyhow.

I would not object to blending my fuel if I got 100% of the power that the engine should deliver. Running straight 110 to get average performance is not acceptable. I could do better/drive harder with the '76 block/6X-4 I have in another '68, running on 89 octane.

I recently had an opportunity to drive a well built '68 that has me longing for more power. I'd use a stronger verb, but this is a family site after all.

I found NOS pistons that were available from the dealership until the late 70's. I didn't check the deck height. Next time I have the heads off I will measure it. A compression check isn't accurate enough to calculate it that way.

The after market pistons in my '68 400 are .028 in the hole.

Thanks, Greg. Which pistons do you have?

vikki,

zero decking is used primarily to up the compression and bring a true surface to the heads. you have to allow for the quench in the head so you need larger cc heads when zeroing the block. i think your after more hp which a stock build and mild cam should give you ample amounts of! i on the other hand am planning for the after effect of getting the power... longing for more as you are!

Here is a good explantion of what 0-deck is for.


"Setting the deck height to zero above the piston appears to cause superior fuel/air mixing on the compression stroke. This better mixture then seems to burn faster after ignition, and the faster burn means the ignition pulse can be delayed slightly for optimum power. The delayed, or retarded timing, means we are further from detonation, and can then run increased CR on the same fuel. It appears to be a win-win deal. We were able to reset timing from an optimum point of 34 when running about .015 clearance, to only 30 when the deck was cut to zero. That appears to be very significant when shooting for best possible power on a given quality of gas, and I note in the just received Feb, 2000, issue of Popular Hot Rodding, the latest GM LT 1 engine is using a positive deck height of .007 to .010"(piston above the deck). Of course, we don't know the total clearance, but it certainly indicates that GM also feels the close deck is productive. Jim Hand "

Here is another

"It would make no sense to cut a good block down excessively just to zero deck it to low cost pistons. We quit using the cast pistons several years ago as they just lower the CR too much, between the 8 valve releifs and big chamfer around the outside of them. We mocked up an engine several years ago that had them installed and cc'd everything above the piston at TDC. Came up with something around 22cc, nearly as I can remember. It was around the same time that we had been working with several troubles 400 engines with the early small chamber heads. We found, through some pretty extensive testing that having the pistons .030" or more below the deck was contributing to running hot/overheating/detonation. During the course of the testing, we tried quite a few "tricks" to keep these engines cool and avoid detonation. We played around with fans/shrouds, pulleys, water pump impellers, radiators and cam timing. For the most part, everything helped...but...once we zero decked the block(s) the gains were monumental! Despite increasing the CR a bit, we found that these engines ran about 20 degrees cooler, did NOT heat up at interstate speeds during long cruises and made more power.
Since then we have been building our pump gas iron head engines right at 10 to 1 CR, and closer to 10.5 to 1 with aluminum heads. To date haven't had the first trouble with any of them on a steady diet of pump fuel. My own 455 was raced with 89 Octane fuel on a few occassions when it still used the iron 6X heads and ran exactly the same as it did with 93 octane! We even pulled the engine down last year and inspected the rod bearings after two full seasons of street and strip action, they were in perfect condition......Cliff "

Good article, but there are more than a few differences. I don't have closed chamber heads. Total above-piston volume is in the range of 5 cc at TDC excluding valve reliefs, which add another 6 cc, totalling only half of his reported 22 cc. And there is no problem at all with temperature, even after miles of freeway running or cruising around town the engine is around 190 degrees with 100% stock cooling components.

I am not saying that zero decking is bad, but why not just use pistons of the proper compression height?

You could build it to stock specs. Then all you would have to do is find some 100 octane leaded fuel and you would be good to go.

Or you can take the advice of people who have been through all this and have a motor that has 50 more HP,runs on 89 octane unleaded and no over heating.

PS, open or closed chambers still 72 cc's (unless heads are62's then about 75 cc's)

You agree that a stock 400 will run great as it was built from the factory if you use fuel like was available back when they were built.
It seems that these experts(no sarcasm, these are real experts)that you are quoting are talking about some blocks that have been machined improperly right from the factory. They speak of 'troubled 400's' and blocks that when they are built have the pistons 'more' than .030 in the hole. They aren't talking about 'ALL' the 400's, just troubled ones.
Here is my thinking; I look at the two choices that you outline. Choice one is to keep my very stock car's engine just as it was in 69. Find higher octane fuel, which is available, and drive it like it's 1969. And it does run perfectly like that. No detonation or overheating. I would like to give an example of the performance which is hard to do without taking you for a ride. The best I can do to give you an idea of the performance in just a sentence would be to say that it will brake the tires loose when it hits each gear and it's an automatic. To me, it has just the right amount of horsepower for fun on a road course and for cruising. I've had a few 1st gens with 400-500 HP and they are a lot of fun but it's all overkill. They make a lot more noise and can do much more 'fun and obnoxious things'. Even after having quite a few 'altered to be fast' cars, I'm very happy with what a stock 400 car can do.
Choice two would be to have around .020 milled off my original block that hasn't even been overbored. This would raise the compression so you will definitely have to relieve the heads to get the compression back down somewhat. Then I also would have to alter the stock cam profile. Their theory is that this will allow me to retard the timing to burn the crappy fuel.

8point,
I know you have told us before but please tell us again; What compression are you running? If it's less than stock then what did you have to do to the heads to get there? Since you have a lively specimen, what gas do you use and how does the car run? If you are burning low octane fuel and have high compression, how do you know that you have no detonation? Since you have headers it may not be heard.

Jim, Yes I have stock or better compression. (.030 over 400 and 0-deck, but am using TRW forged pistons that claim 9.8to 1, so probley over 10 to1.

I run 93 octane but have tried 89 and didn't see any difference.
My Doug's headers are well sealed and are not to noize.
Also I have overdrive trans so in 4th gear and converter locked up at 50 MPH I'am turning about 1,700 RPM and I can step on it and it loads the motor down alot but still don't get any detonation that I can hear.
I built my 400 to this exact spec.

KRE dyno'd the 60916 cam w/HS 1.65 rockers in a 10 to 1 400 engine with unported heads and made 419hp/453ft lbs. Rhoad's lifters .
You will find strong power right off idle, the "real" power starts around 2500-2800rpm, choose converters and gearing accordingly.

No one can determine if the cam alone would help/hurt with detonation issues, too many variables. We have a good many 400 engines in service in the 9.8-10.2 to 1 CR range with this cam and no troubles with running hot/overheating/detonation. All of them are zero deck with filled crossovers and polished piston tops/combustion chambers.......Cliff

The other main isue is dynamic compression. My car tested at 165 LPs
--------------------------------------------------------------------------------

Noted Pontiac Tuner Dave Miranada who used to be involved with Superjet Specialities in Queens, New York.

He deduced that the detonation threshold of a Pontiac V8 was right at 185 psi cranking compression.

Dave also came to the following conclusions:

The optimum (maximum) cranking cylinder compression pressure for a performance/street application using 93 octane fuel = 160 -170 psi
Chosing a cam with a late intake closing will help with this.The Crower 60916 has this built in.

Vikki's orignal question was she wanted more power and not have to by $5.00 fuel.

Not sure what pistons they are Vikki. There are no markings on them other than over bore size. I think they may be "Grizzly" pistons? I have seen a set of them available on E-bay recently.

HIJACK ALERT:
8 point, are you saying that my compression tests should read 160 -170 PSI? When I had my '62s installed my readings were 175-185 PSI. I now have heads with 60cc chambers and the same pistons and cam. I'm guessing my readings will be significantly higher now...? I'll have to test this. Can actuall CR be calculated using this info vs. chamber volume?

I realized that you are giving an alternative to paying for higher octane fuel. Let's boil that down. It's not $5 a gallon. You can do it with just adding the racing fuel at a three to one ratio. 1/4 racing fuel. Super unleaded is $3 a gal. and racing 110 is less than $5 here. Not a big difference anymore. So you can run a stock 400 at less then $4 a gallon. So you can do all the work that Cliff and company speak of and save yourself $1 a gallon. IMO Since I already have vehicles that I use for most everything else but cruising that will burn the crappy $3 fuel then it's not that big a deal to pay $1 more a gallon when I'm driving just for pleasure. What's that work out too, ah...., about a nickel a grin!

lol jim.

vikki,

if your up for the grin factor now having experienced a true seat-fo-your-pants feel, i would build the 400 i brought to you. deck the block, get 87 cc heads, forged pistons and a 1/8" longer stroked crank. this should get you in a good range for power without hurting the wallet. there are a few cars here locally that run dam near 11.0 to one and they do so on low octane fuel. its the quench factor that allows them to do it. granted they are chevy's but the principles are the same!

also decking is used for more than just cooling factors. read a bit more on this befor you cut an original block. or do as i suggest and use the 400 you picked up through me.

Photo of after market pistons...

Here's the dilemma.

I could put fresh #62 heads and a blueprint cam and lifter set in. With no other changes from factory config, running 100 octane as specified, my engine should run just like new. Will it? No way to tell until I try.

Or I could spend a couple thou at the machine shop and have the block bored for the first time plus decking plus everything else suggested, along with the fresh heads and cam and HEI. Should it run better? It darn well better, for the money.

Factory stock was a pretty good package. I'd be happy with the power the car should have had the day it was delivered. This car is a cruiser, but I still need to exercise my right foot on occasion.

Numbers matching is an important criteria, and cutting a numbers matching block is not high on my list of things I want to do.

Here are two shots of a '67 Pontiac 400, original engine. Block upper surface retains a factory stamping adjacent to this cylinder. The piston-below-deck distance is very small. This is close to TDC as I could get without using a dial indicator.

If zero decking is good, then why are thick head gaskets or gaskets with a big bore (one size fits all) not an issue?

Vikki, are we talking about your '69? If so,I recall you having an awfully high rearend(consider this a compliment if you like). Perhaps you should start there...

No need to muck with the factory gears if the engine won't play nicely more than halfway through the power band. With the gears that are in it, I never have to run above 3000 rpm. It's when I get on it and push 4000 that trouble starts (running 100+ octane). Changing the rear gears without solving the issues will just keep my maximum speed down. I do like being able to run top down at 85 and still hear the stereo.

I know that even 2.76 can smoke the tires if the engine is happy.

Thick headgaskets can be an issue. Some people try to use them to drop compression and avoid detonation, but for the same reasons that zero-decking works, thick gaskets can aggrevate the problem. Between .035" and .040" clearance is optimum, but you have to be sure that is what you have with the compressed gasket (and all 8 pistons)or you can smack the head.

You stated earlier that your info shows the pistons down .023. I don't believe this is correct. Especially considering what your '67 shows. Calculate the CR using .023 deck height, 75 head cc, .040 gasket thickness, etc. Would that give you the factory advised CR? I'm guessing it will be lower. I bet the .018 figure is closer...

From what I have heard, rule of thumb for calculating is .020 deck height for a stock Pontiac. And .041 for the gasket. But that's literally 'splitting hairs'.

Jim, you hair splitter!

I measured this one at .018" but I need to set it up properly with a deck-mounted dial indicator to get the true reading. It is not more than .018, possibly less. It truly is splitting hairs trying to find the various degrees of TDC.

The original head gaskets are far thinner than .039 or .041.

Are they the steel shim gaskets? If so, that's the answer as to why the pistons are down the bore, to maintain the proper piston to head clearance (squish) of around .040". Squish is an important part of power generation and detonation control and the engines were originally designed for the proper amount of squish.

When switching to Fel-Pro or other composite gaskets from the steel shim gaskets, the deck should be surfaced by the amount of difference in the thickness of the two gaskets in order to get back to what the factory designed. So, if you have .020" thick steel shim gaskets and go to .039" thick composite gaskets, the deck should be machined down by .019" - which results in zero-decking the block. Other engines have the pistons sticking out of the bore by as much as .020" (or more) to compensate for head gaskets that are .060" thick. Why are the new gaskets thicker? Because they seal better.

The squish effect: the distance between the piston and the head closes up as the rpm goes up and the rod stretches. The squirting effect of the close tolerances speeds up the burn so much that no extra spark advance is needed above about 3000 rpm (depending), even though the time available for the burn can be as little as half (6000 rpm). I've used as low as .032" on a short H-beam rod engine without contacting the head at high rpm (7500) but the minimum clearance varies with the rpm, rod length and design. An aluminum rod engine needs an extra .020" - .025" between the piston and head because of the stretchy rod.

If you're not going to use steel shim gaskets in your engine ever again, there is no problem with decking the block for composite gaskets and using pistons with the stock compression height.

Some piston manufacturers do offer pistons for certain engines (SBC for sure!) with a higher compression distance to achieve a zero-deck without surfacing the block. Such a piston for a Pontiac 400 would have a compression distance of 1.735" (1.717" + .018"). Or use steel shim gaskets with the regular pistons like the factory did.

What about obtaining zero deck by ordering Ross custom pistons with the pin hole located to place the pistion top .018 higher? Then you don't need to machine your no's matching block to get the piston to the top of the hole.

To think that all this zero deck stuff is because of the later, thicker, gaskets. There's no doubt that cutting a block the same amount as the gaskets have grown is a 'wash'. Makes you wonder why it's not mentioned by Jim Hand or Cliff. It reminds me of a Neurosurgeon recommending back surgery.
Now I can throw a monkey wrench in all of this. I have the thicker gaskets on a stock build without any problems. If I pull my heads and use the original style thinner gaskets I can expect to gain what(other than a leak LOL).???? Would my engine run a tad cooler? It does creep up to 210 on a 100 degree day when stuck in traffic. Honestly, it's no fun driving the convertible when it's 100 degrees in traffic anyway.

'68 Bigbird, I like your idea. Thought of it myself, then wondered why we need to buy custom pistons to get pistons that are "stock" replacements.

Here's another interesting twist.

I cleaned up one of the head gaskets from this engine. Thickness at cylinder rings is.... .042".

Composition is very soft, maybe babbit type, with two layers of metal wrapped around a fibrous center core.

Gasket is shaped to fit the cylinder bore, including the notches. The ring matches the bore closely.

I guess you can't believe everything you read! ARGH

I have a set of stock Fel-Pro gaskets here...if I have head gaskets I will do a direct comparison.

Just checked unused Fel-Pro and they are .060 on the edge and .075 at metal ring. How much do they compress when torqued?

Fel-Pro: 4.306 top to bottom. Original: 4.166 top to bottom. (Original gasket lined up underneath for comparison).

That makes a pretty substantial pocket between the head and the bore with the Fel-Pros.

I've read, somewhere, that the FELPROs are .040 compressed... Mst catalogues indicate the compressed/installed thickness.

So much for that theory! I should have checked my parts book first. # 9792955 - for use when an increase in comp ratio is desired - .033 - .028" thick. That certainly seems to point to thick gaskets as standard and like Vikki has documented. Apparently the Royal Bobcat head gaskets took this one step further and may be what I was thinking were stock.

I know that the OHC engines all had steel shim head gaskets so I made the assumption that the V-8s received the same type of gaskets. Fel-Pro usually is .039" compressed.

TRW L2323F had a CD of 1.735 and a bore of 4.125 in standard with a .980 pin and a dish of 14.5 cc. That might work for someone out there. Or get a set of 455 (4.151 bore or +.030" for a 400) pistons and use custom rods that are 6.863 long.

The extra room around the bore doesn't amount to much. Using a bore size of 4.150 for the stock gasket and a thickness of .042, the volume is 9.1 cc. Using a bore of 4.210 and a thickness of .039 for the Fel-Pro gasket, the volume is 9.2 cc. The important number is the piston to head clearance.

I too was thinking of the OHC shim gaskets as "normal" as that is what I found when I tore a few down. These are not at all stiff, they could be easily formed and also deformed just by lifting them off the block. And they are certainly well-compressed!

I also see a substantial difference in one hole in the Fel-Pro set. Could/should the block passage below be opened up to match the newer gasket? (On a non-matching block, of course )

I've also seen some replacement pistons with noticeable bevels at the top corners, unlike the stock castings.

I am interested in knowing who is running which pistons (brand and part #) on a 400, if you had your block decked, which head gaskets you are using, and if your combination is fine on performance and temperature.