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Old 04-13-2013, 02:05 PM
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Head gasket thickness question..

Im currently getting a little seepage of coolant on my engine. Chevy small block with aluminum heads, cometic .070 head gasket. The seepage is in the rear of both sides, near the last headstud. The engine has zero symptoms of a blown head gasket, it simply seeps some coolant down the side of the block.

I was contemplating a simple retorque of the headstuds to see if that did the trick vs replacing the entire head gasket and sealing it with copper spray prior to install.

My question is, my engine currently has 11.3:1 compression and I was wondering if going to a .080 thickness head gasket would lower the compression a bit so I can run 92 octane instead of 100 octane. Any thoughts or estimates?

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Old 04-13-2013, 02:15 PM
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Typically, we like to see about 10.2:1 as an "upper limit" for 92 or 93 octane gas and aluminum heads. This varies a bit depending on chamber shape, piston head, etc.

If you provide the chamber volume, bore and stroke, we can compute the "new" compression ratio.

Jim
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Old 04-13-2013, 03:15 PM
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Calculating c.r. on a typical SBC using .070 gaskets, and again with .080 gaskets on that same motor, only lowered the static c.r. 0.14.
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Old 04-21-2013, 06:27 PM
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I have some other questions, this is my first headgasket replacement by myself. I put ARP thread sealant on the threads of the headstuds that thread into the block. I have the surface completely clean on both the heads and block and the engine was fairly fresh its just that I saw some coolant leaking between the block and heads near 1 or 2 head studs on each side. Most likely the studs needed to be re-Torqued or they werent thread sealed properly. The gasket was intact and the surface of the block and heads were pretty.

I was wondering about where you put the ARP lube prior to Torquing the nuts for the head studs down? Is it on the bottom of the washer only or on both the washer and the threads? Also When disassembling the heads, it appeared that there may have been black silicone sealant on the threads, but i cant be sure..I cant see any reason for sealant on the bolt side of the head studs..

thanks in advance.
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Old 04-21-2013, 07:27 PM
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If your meaning one of the short head studs near the headers, I had one leak when I first ran my rebuild. Same engine, same gaskets ( but .030" ) same compression (11.48). I just took it out, drained the coolant low enough to not be up the tread hole, cleaned the hole thoroughly with lacquer thinners and torqued it down again with fresh sealant, worked a charm.
I would be loathed to compromise your already poor quench by opening it up another .01" for such little drop in compression.
If theres only two outside studs weeking coolant, id just re-seal them...that's me. A lot less work for a better result. If your needing 100 oct gas now, your still gonna need it with an even poorer quench. Your cr will be barely affected.



Duke
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Old 04-21-2013, 08:46 PM
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Quote:
Originally Posted by Dajerseyrat View Post
I have some other questions, this is my first headgasket replacement by myself. I put ARP thread sealant on the threads of the headstuds that thread into the block. I have the surface completely clean on both the heads and block and the engine was fairly fresh its just that I saw some coolant leaking between the block and heads near 1 or 2 head studs on each side. Most likely the studs needed to be re-Torqued or they werent thread sealed properly. The gasket was intact and the surface of the block and heads were pretty.

I was wondering about where you put the ARP lube prior to Torquing the nuts for the head studs down? Is it on the bottom of the washer only or on both the washer and the threads? Also When disassembling the heads, it appeared that there may have been black silicone sealant on the threads, but i cant be sure..I cant see any reason for sealant on the bolt side of the head studs..
thanks in advance.
You have to put the lube on the threads and between the washer and nut. You also need to progressively torque the studs, usually divided by 3. If your heads final torque is 75/ftlbs, set your torque wrench to 25/ftlbs and torque all studs. Then set it to 50/ftlbs and repeat, then the final torque 75/ftlbs. At least that's the way I do it and no problems yet.
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Old 04-22-2013, 01:09 AM
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Quote:
Originally Posted by thinwhiteduke View Post
I would be loathed to compromise your already poor quench by opening it up another .01" for such little drop in compression.
If your needing 100 oct gas now, your still gonna need it with an even poorer quench. Your cr will be barely affected.
Bada Bing, Bada Boom, there's the honest truth.
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Old 04-22-2013, 07:36 AM
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Quote:
Originally Posted by thinwhiteduke View Post
If your meaning one of the short head studs near the headers, I had one leak when I first ran my rebuild. Same engine, same gaskets ( but .030" ) same compression (11.48). I just took it out, drained the coolant low enough to not be up the tread hole, cleaned the hole thoroughly with lacquer thinners and torqued it down again with fresh sealant, worked a charm.
I would be loathed to compromise your already poor quench by opening it up another .01" for such little drop in compression.
If theres only two outside studs weeking coolant, id just re-seal them...that's me. A lot less work for a better result. If your needing 100 oct gas now, your still gonna need it with an even poorer quench. Your cr will be barely affected.



Duke

Your headstud issue was the same issue I was having but I chose to remove the heads and replace the gasket whit the same basket while I was in there. I was just wondering about the thickness and how it would effect compression, that's all..
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Old 04-22-2013, 07:38 AM
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Originally Posted by S10 Racer View Post
You have to put the lube on the threads and between the washer and nut. You also need to progressively torque the studs, usually divided by 3. If your heads final torque is 75/ftlbs, set your torque wrench to 25/ftlbs and torque all studs. Then set it to 50/ftlbs and repeat, then the final torque 75/ftlbs. At least that's the way I do it and no problems yet.
So no sealer on the nuts then, just build lube..gotcha.. Im up to speed on the 3 step tourqe process. Thanks!
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Old 04-22-2013, 07:40 AM
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.070 head gasket.....for real?? hmmmmmm
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Old 04-22-2013, 07:45 AM
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.070 head gasket.....for real?? hmmmmmm

Cometic C5245-070
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Old 04-22-2013, 08:34 AM
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wow....thats thick....
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Old 04-22-2013, 11:18 AM
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Originally Posted by Dajerseyrat View Post
Im currently getting a little seepage of coolant on my engine. Chevy small block with aluminum heads, cometic .070 head gasket. The seepage is in the rear of both sides, near the last headstud. The engine has zero symptoms of a blown head gasket, it simply seeps some coolant down the side of the block.

I was contemplating a simple retorque of the headstuds to see if that did the trick vs replacing the entire head gasket and sealing it with copper spray prior to install.

My question is, my engine currently has 11.3:1 compression and I was wondering if going to a .080 thickness head gasket would lower the compression a bit so I can run 92 octane instead of 100 octane. Any thoughts or estimates?
What is the engine's displacement and why such a thick gasket? Adding more distance between the piston at TDC and the head is the wrong way to go for a lot of solid reasons.

You can retorque the head bolts, this is a common and good practice. While many head gaskets are sold as not needing retorquing the fact remains that you've got a sandwich of aluminum and cast iron which have very different rates of thermal expansion and contraction. While gaskets try to absorb this movement by allowing an internal shear motion in the gasket, the thermal motion is still there with every temperature cycle so this can disrupt the gasket seal.

Compression and cam timing walk together, you can't really arrive at a suitable compresson ratio untill you know in crankshaft degrees when the intake valve seats and how long the connecting rod is as these things determine where the piston is located in the bore at the point on intake closure. That determines the trapped pressure within the cylinder, this is a reference related to the concept of Dynamic Compression Ratio.

The distance between the piston's crown and the squish/quench step of the head controls a feature called mechanical octane. This when done properly will make the fuel react as if it has 4-6 more octanes than it is rated. For a street engine the optiminum point is about .040 inch, for a competition engine that may be pushed to .030 inch or less. But as this gets tighter the risk of colliding the piston into the head becomes greater. Certainly a .070 inch thick head gasket has minimized the effect and has lost the addition of the mechanical octane feature. This becomes a lot worse if this block retains the factory piston crown to deck clearance of .025 inch as that added to the gasket has the piston nearly .10 inch away from the head.

Detonation avoidence is enhanced by getting the burn over-with by a point where the maximium cylinder presure occurs a few degrees after TDC with a minimum of ignition advance so the pressures aren't peaking before TDC. The former makes power, the latter makes detonation. To get there setting the crown clearnce tight to the head's squish/quench step causes a couple things to happen. First; the mixture on the far side of the chamber is squished with great force into the valve pocket. This both mixes the fuel and air together and places it with high density before the spark plug resulting in an easy light off, followed by a fast but not explosive burn. The second function of quench is where the close surfaces on the far chamber side provide a lot of surface to volume which sinks the heat out of the mixture on that side preventing it from reaching the self-ignition temperature thus exploding before the flame front gets there. The wider the gap between piston and head the less this effect which results in the engine's greater need for higher octane fuel and the more it will suffer from miss and late fires.

There are many ways to play with excess compresion even where squish/quench are optimized. Lower the engine's operating temperature, lower the intake temperature, richening the mixture, reducing the ignition timing lead, advancing the cam, water/alcohol injection, running stiffer gearing are a few options short of opening the motor too deeply.

But becoming more specific would requiring knowing a lot more about the motor, vehicle and how it gets used.

Torquing the head bolts requires the threads be sealed if this is an SBC because the bolt holes of the block are open to the coolant jackets. A good solution for the threads is plumbers Teflon paste pipe joint compound. This has about the same lubricity as engine oil so using that torque spec works very well. The underside of the bolt heads need to be lubricated with engine oil, that would apply to the hardened washer needed between the bolt head and the cylinder when aluminum cylinder heads are used on the build. Studs are usually epoxied in place to seal the block's threads and prevent stud rotation when the nut is run down. Since studs are a proprietary aftermarket item, so use the thread lubricant the manufacturer specifies on the upper stud threads and under the nut, again with an aluminum head a hardened washer must be placed between the nut and the head it also needs to be lubricated with the same stuff. Failure to use a hardened washer between the bolt head or the nut if using studes will cause the rotating fastener to gall the soft aluminum not only damaging the head but also giving a false torque reading which will read proper torque way before the fastener reaches the proper amount of stretch for the indicated torque reading. In other words, the head is left loose and will leak coolant if not compression.

Bogie

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Old 04-22-2013, 12:20 PM
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I am still a bit confussd....I plan on using assembly lube for my touring so that would coat the underside of the washed and nut. Is there sealant you are supposed to put on the threads of the nut? I already used ARP thread sealer on the studs into the block, I simply need to know if the other end of the stud gets sealant also. I am using head studs not bolts..
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Old 04-22-2013, 01:17 PM
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Originally Posted by Dajerseyrat View Post
I am still a bit confussd....I plan on using assembly lube for my touring so that would coat the underside of the washed and nut. Is there sealant you are supposed to put on the threads of the nut? I already used ARP thread sealer on the studs into the block, I simply need to know if the other end of the stud gets sealant also. I am using head studs not bolts..
The stud threads that come through projecting above the head on which the nut is screwed uses thread lubricant, no sealant.

Bogie
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