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How important is head gasket thickness on a street engine?

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Rembrant

How important is head gasket thickness on a street engine?

Gents,

I need to pick up a set of head gaskets for my 302. I'd like to get it buttoned up this week.

Does it matter if the gasket is 0.039", or 0.041", or 0.047" thick? Totally stock street engine, but bored 0.040" over. Heads have been resurfaced and block decks have been resurfaced. I don't know how much was removed between the two...I'm assuming somewhere between 0.005" and 0.010" based on previous experience.

The Fel-Pro stock replacement gaskets show that they are 0.047" thick. From the little bit of reading I have done, it looks like the originals were 0.040"-0.041", but I'm only going by internet hearsay.

I'm just curious how much it matters and how much I should care lol. If there's anything in-stock locally, it's likely to be stock-ish original replacements, and nothing more. I'll order something specific online if I need to, it's not a big deal, but just wondering what you guys recommend for preferred head gaskets and preferred thickness.

1994 F150 4x2 Flareside. 5.0 w/MAF, 4R70W, stock.
1984 F150 4X2 Flareside. Mild 302 w/ 5spd. Sold.
1980 F150 4X4 Flareside. 300i6 w/ 5spd. Sold in 2021.
1980 F100 4X2 Flareside. 351w/2bbl w/NP435. Sold in 1995

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

I think it only matters when you are trying to get the compression ratio spot-on, or if you are trying to get "quench", which takes the thinnest headgaskets.

In your case I'd go with the stock replacement ones from Felpro.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

mat in tn

Re: How important is head gasket thickness on a street engine?

well now that is almost a loaded question. ask that question in a room of ten engine builders and you get twelve different answers. i always start with purpose. engines are always purpose built. even if the purpose is to be general use where good enough across a wide range is the goal. i generally will say to stay with stock size on stock rebuilds. if the block was decked and the heads milled. knowing how much is important when making these decisions. often the piston tops out a few though below the deck and that reduces compression but quench also. i like quench spec to be 35-40 thou. if the pistons are 5 below deck i would like a 35 thou gasket, where if the piston is at deck or a thou above i would be wanting 40 thou gasket

85lebaront2

Re: How important is head gasket thickness on a street engine?

Administrator

In reply to this post by Rembrant

We went through this on Darth's new engine to get the compression I wanted without being too high. I have a compression ratio worksheet I downloaded. I will send it to Gary and you, that way if Gary wants to add it to the site he can. There are several variables in addition to the normal bore diameter, combustion chamber volume etc.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

Rembrant

Re: How important is head gasket thickness on a street engine?

Thanks Bill,

I used that chart and another one online and my compression comes in right around 9.0:1 for this engine, and I'm fine with that. I had to make a couple of assumptions, like comb chamber CC size and piston valve relief size as I'm not going to go through the trouble of measuring them. Worst case it might be 8.9:1, and on the high side it might be 9.1:1, so either way it's fine for what I want to do with it.

Thanks gents.

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

In reply to this post by 85lebaront2

Bill - Take a look and see if you approve: Documentation/Engine/Compression Ratio Calculator.

I added the # of cylinders input and changed the calculation to use that. And I added a cubic inch result in addition to the cubic centimeter result.

Thoughts? Changes?

Gary, AKA "Gary fellow": Profile

85lebaront2

Re: How important is head gasket thickness on a street engine?

Administrator

Looks good to me. This is what I used on Darth's new engine, in fact the numbers are from that build. Gasket thickness is compressed thickness and obviously the larger the bore, the bigger effect the gasket thickness has on compression ratio.

Hopefully this will help some people with engine builds.

Bill AKA "LOBO" Profile

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

Thanks, Bill.

Gary, AKA "Gary fellow": Profile

Lima Delta

Re: How important is head gasket thickness on a street engine?

In reply to this post by Rembrant

I was just debating this very thing with myself.
I have a set of the Fel-pros that came with a gasket set and which compress to .047. Problem is, after doing the math back and forth a bunch, to get into that ideal quench zone I need to go with something more like this: https://www.cometic.com/products/C5514-027
I think these are something north of $100 each with a 6 week pre-order. I had to ask myself if it was really worth it for me to spend an extra $200+ to get a .020" closer fit for my cylinder heads on a street engine that will only have about 8.5:1 compression with the thicker gasket.
I don't think so.

Lucas
"The truck" - 1985 regular cab F250 4x4 - 351W HO, C6
"Beige Beast" (project) - 1981 regular cab F250 4x4 - 300 straight six, T18

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

How far down in the bore are your pistons? If they aren't pretty much at deck height you won't get to the ideal "quench" with any gasket. That's because you need about .040" between the piston and the head to have good quench.

Summit says you should be at "0.035 - 0.045 in." So with even the thinnest gasket you mentioned of .039" you can't get there w/o being zero-decked.

Gary, AKA "Gary fellow": Profile

Lima Delta

Re: How important is head gasket thickness on a street engine?

Hi Gary,
I can't remember the source now, but I have been using 0.060" as (let me rephrase) maximum quench - which I guess is different from the "ideal" zone. I'm no expert.

To answer your question deck clearance in my case is 0.032".
So,
.032+.047=.079 vs
.032+.027=.059 (just under the "max")

All that said, I still wonder at the importance of getting an ideal quench in lower compression engines.
When I was shopping for pistons there wasn't a single piston I looked at that, when combined with stock rods and a stock deck, achieved an "acceptable" quench without boosting the compression to something close to 10:1.
I know it's not unusual to mill the deck to get close to zero clearance for a good quench... but if this is so important for an 8.5:1 engine, why didn't Ford design the block that way in the first place?

Lucas
"The truck" - 1985 regular cab F250 4x4 - 351W HO, C6
"Beige Beast" (project) - 1981 regular cab F250 4x4 - 300 straight six, T18

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

My understanding is that "effective quench" is when the gases in the combustion chamber are squished so tightly that they move rapidly and both cause effective mixing of the fuel and air and prevent detonation. But that by .060" clearance you don't have "effective quench".

Motor Trend says "It's not uncommon for old-school, low-compression, smog-motor, stockers to have over 0.060-inch quench at TDC. We've even seen some '70s-era Chrysler 440 engines with the piston 0.150 inch or more down in the hole at TDC. Originally, this was done to lower compression and slow the burn rate in an effort to reduce detonation. Current theory is that this is counterproductive, producing more unburned gases and such a slow burn that the detonation tendency actually increases. Don't run more than 0.060-inch quench, even if you have a need to run a low compression ratio."

But to answer your question of "but if this is so important for an 8.5:1 engine, why didn't Ford design the block that way in the first place", I don't know. My guess is that it wasn't that important to them.

So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it.

Gary, AKA "Gary fellow": Profile

Lima Delta

Re: How important is head gasket thickness on a street engine?

Gary Lewis wrote

So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it.

Yeah, that's my take as well.
I might be more concerned about it if I was shooting for something like 9 or 9.5:1 because the increased compression would push combustion closer to detonation anyway (esp on 89 octane) and having a good quench would mitigate that. But to get good quench with one of these engines, looks like it's going to cost some money in modifications.
I have about 310k kms on the original 5.8 HO in my truck now. By the book it has 8.3:1 compression and probably doesn't have a good quench, yet I tow with it all the time and only recently have been getting some slight pinging if I'm on the gas too hard while climbing this one hill near my house.

Lucas
"The truck" - 1985 regular cab F250 4x4 - 351W HO, C6
"Beige Beast" (project) - 1981 regular cab F250 4x4 - 300 straight six, T18

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

Yep, you should be fine. And I agree that it would take a lot of mods to get to effective quench.

Gary, AKA "Gary fellow": Profile

Rembrant

Re: How important is head gasket thickness on a street engine?

In reply to this post by Lima Delta

Lima Delta wrote

Gary Lewis wrote

So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it.

Yeah, that's my take as well.

It's one of those things that a guy can really get dragged into a rabbit hole with online. People often refer to something as being superior, or better, or ideal whatever. I always have to ask the question, what is the unit of measure in determining that something is better?

I have checked several 302's now, and from the factory the piston to deck heights on them were in the 0.014"-0.015" range, with roughly 0.040" compressed thickness factory head gaskets. That puts them in the 0.055" range for quench.

I just assembled my little 5.0/302 today, and it is basically zero-decked now. I installed the stock spec Fel-Pro head gaskets that are supposed to be around 0.047" compressed. I suppose it is still not 'ideal', but it is better than it was originally.

I asked the question originally because somebody had asked me online elsewhere, and it got me wondering. I suppose I could have ordered the other gaskets that were 0.040" thick (and twice as much money), but who in the heck would know the difference? Not me, not the truck, nor would anybody else that ever drove it know...lol.

mat in tn

Re: How important is head gasket thickness on a street engine?

ive been reading alone and enjoying the thoughts and theories. we all have some. i have my own. having just finished another build i decided to put it in my flareside . im in the habit of "trying things". building my own parts etc. this 5.0 got bored .020 over and decked. pistons are approx.003 above the deck. heads milled also and gaskets are .044 thickness. ive now put 1500 miles on it and will do a compression check soon. zero pinging and i run 87 as a rule. power is great and mpg went up a little too

Gary Lewis

Re: How important is head gasket thickness on a street engine?

Administrator

So you have .041" of clearance, or quench, and it seems to be working. Cool!

Gary, AKA "Gary fellow": Profile

mat in tn

Re: How important is head gasket thickness on a street engine?

working great! my only complaint is the dang rev limiter engine pulls like a freight train til it hits that . one hiccup and shifts