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Small block chevy Head Gasket concern.

4K views 21 replies 7 participants last post by  Onefunponcho 
#1 ·
I'm putting together a .030 over 350 chevy. The short block was a put together some time ago by someone else. (I received it by bartering parts) the pistons are cast dished with small valve reliefs and seem to displace around 11-12cc in the dish by my measurements. I measured the deck height and the piston is quite deep about .035 under the block deck.

I'm putting a pair of summit vortecs on it that have 67cc chambers.

The camshaft is the Lunati Voodoo #VDSC-262 its 220 intake @ .050 and 227 exh @ .050 lobe separation is 112 the lift is 468/489

My concern is this, because the pistons are so deep I'm not sure i'll have adequate compression for the cam if I run a standard head gasket. I'd like to use a standard gasket because the block was never decked. I think they may be more forgiving but I'm not sure that's a fact.

A steel shim gasket (by my math) puts me right in the sweet spot for compression but i'm concerned about how well they will seal as i have not personally decked the head or block. Though they look good with a strait edge.

Do you all have any input toward what would be the best head gasket to run in this application.

Thanks in advance for your opinions.
 
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#2 ·
Many so-called "performance"pistons these days have shorter than stock compression height. Just doesn't make sense to me. We used to call these "rebuilder" pistons for use by shops that would go beyond zero on the deck.

If you can't get the block decked and the surface is not new, I'd suggest a .028" composition gasket like GM 14096405. Then again, a thinner shim might work, and might not.

Are those Summit branded heads? If so, you should find out how much they can be decked. Maybe you can get the chambers down to 64cc or less. That will help compression, but you'd still be stuck at .063" quench height, at least .013-.023" more than optimal. But hey, I've seen factory quench at .075" or more.
 
#4 · (Edited)
Here's the issue: Squish/Quench

With todays cat-piss pump gas, you need a tight 0.035" to 0.045" squish/quench to combat detonation. With the piston down 0.035" in the hole, the best squish/quench you can achieve will be 0.050" with a Fel-Pro 1094 head gasket. Now, that may or may not work to quell detonation, depending on your static compression ratio, ignition timing and fuel/air ratio.

But when you move on up to aluminum heads, the shim gasket probably won't work. Aluminum heads need a thicker composition gasket that will yield a little because of the dissimilar growth factors between the aluminum heads and the cast iron of the block. If the gasket cannot absorb the movement, then the aluminum will fret itself. If the heads fret at the fire ring, you may breach the head gasket and have to begin all over again by taking a surface cut on your new aluminum heads and either exchanging the pistons for some taller units or cutting the block decks.

So, you are left with the problem of making it right now by either changing pistons or cutting the block decks......or cripping by for now with iron heads and a shim gasket and doing all the work later to run aluminum heads. Your choice......:eek:

.
 
#5 ·
I agree with the Techinspector....fresh short block, so right now is the ideal time to slip the pistons back out, remove the crank and cam and the incidental parts.....and take the block in to the machine shop and get it decked to make it right, rather than just tossing it together and crossing your fingers.
Then bring it home and slip it right back together with all the same parts, rings and bearings included....the only bearings to get replaced are the cam bearings, because the machine grabs the block by the front and rear cam bore, so they have to come out.

A good flat deck, tight quench, and the compression ratio increase you need all in one easy step.

If you can't afford to do it right the first time, how can you afford to do it the second time when it bites your parts?? It will cost a bunch more to redo it down the road compared to right now before you waste gaskets, sealers, and fluids...not to mention the work time and aggravation.
 
#7 ·
You sure its 0.035"?

You measure at the pin area where there is the least amount of rocking and really needs to be done with a dial indicator zeroed to the deck, pretty hard to get an accurate reading with a dial caliper.

The reason I ask is most stock 350's are right around 0.025" down and with a 0.018" shim gasket puts you right into the sweet spot.

0.010" less quench will only put you down 10 ft/pds or so depending on the build if its mild...not like your knocking on the door of high compression where every little bit of quench is needed.

Not always a great reason to tear it apart again especially if its broken in already, I wouldn't do it...not worth the trouble.
 
#8 · (Edited)
Most cheaper "rebuilders pistons" have a shorter pin hgt & will sit lower down the bore.You want your quench in the .035" to .045".However, depending on the actual piston used, it's debatable wether a dished piston offers much, if any benefits from quench.If the band around d the circumference of piston is wide as in newer modern pistons, there may be some benefit.If the band is narrow & has a chamfer to the outside edge, quench won't have much of an effect.To realize & gain the full benefits of a proper quench, you need an adequate flat section on the piston, as in a flat top, or, D cup & a quench pad on the chamber side.With a 67 cc head, a flat would put you just about rite on compression ifusing a piston with a proper 1.56" pin hgt.
 
#10 ·
FWIW, Summit sells a set of decent flat tops with a 1.56" pin hgt for less than $100.They are prolly as good as what you currently have.Now would be the time to swap.Just be sure they use the same ring pack as your current pistons.
These would pretty fix all of your current issues & offer up alot better performance.
 
#12 ·
Thanks again guys for all the input, I believe I'm going to run the 1094 gaskets and roll the dice since the piston shape doesn't sound like it's ideal for quench anyhow. I know if I pull apart the short block I will replace everything with much stronger components. Turning my budget bomber into a meticulous (expensive) build very quickly.

I will post an update in the future and let you all know how it worked.

I'll even let you know if it didn't. So some can enjoy an "I told you so" moment.
 
#13 · (Edited)
I understand your point,but,for $86,this could make a really significant impact on the performance of your build.These pistons will set your SCR at approx. 9.9:1.With the chosen cam you listed that has an IVC(intake valve closing) of 59*,your actual running compression(DCR) will be approx. 8.2:1.With your current pistons,SCR will be approx. 9.1:1 & the DCR will be down approx. to 7.6:1.
If your current pistons use a 5/64,5/64,3/16 ring pack,the $86 would be the only added cost & it would make way more than an $86 impact on your motor's performance & efficiency.
 
#15 ·
Its worse after a recheck

After work I remeasured the deck height being more careful to measure in line with the wrist pins as mentioned by 4jawchuck and its worse than I thought. Its a whopping .045 in the cyl!! Yikes, I don't think they make gaskets thin enough for that.



The math is all pointing towards these pistons not running well at all with this cam:smash: Perhaps I can give the old ones away to be used for ashtrays.

Ill be ordering the flat tops from Summit today to go with the shim gaskets they seem like one hell of a bargain, Thanks guys for bringing these to my attention .
 
#17 ·
The summit racing pistons arrived yesterday and Ive had them pressed today. I mocked up cylinder 1 and things are looking much more promising now. I've measured the deck and its at .027 deep now. With the .015 gasket it should yield 9.89 CR and a .042 quench. Good call on those pistons.



Now if I can just get a good seal with those 1094 head gaskets it should go like stink :thumbup:
 
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#20 ·
if the head is as clean as the block you will have no problems getting the felpro steel shim to seat up. They are coated and seal up well. Its not going to be an issue for you as long as the heads is tq down right and everything is very clean during the install.

1094 is rubber like coating that will help it seal. Its not just a steel shim its not hard to get them to seal up at all.
Duly noted. Would be ideal for situations where piston is down the hole .025"-.035". Guess I've been influenced by old wives tales, because I never liked the idea of steel shim gaskets on even new surfaces. Could have benefited from using .015" instead of .028" on at least one occasion.
 
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