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305 sbc questions

General engine tech -- Drag Racing to Circle Track

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rfoll
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Re: 305 sbc questions

Post by rfoll » Sat Nov 02, 2019 9:41 am

I usually use the 10105117 gasket on my standard bore 350 engines, but I'm somewhat concerned about quench. A.028" gasket would produce about .060" quench. Maybe that much isn't a problem, but I do like to run regular gas.
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rfoll
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Re: 305 sbc questions

Post by rfoll » Sat Nov 30, 2019 12:51 pm

What is the most quench I can run and still have it work to good effect? Does the small bore of the 305 require a smaller quench, or can I get away with more? My current plan is to use a 64 cc head that will produce a good swirl. It should help, but I need to know what to target for. The factory pistons are about .030 in the hole, but I really don't know what the compression distance is. My replacement piston is 1.560 distance. If I assume the same deck and use a .028" gasket, I would be .058" quench. SCE makes a 3.96" bore gasket .020" thick, but will the extra .008 really affect the quench? If all of this worry sounds somewhat obsessive, I will have to say I want to get it right the first time. This is a daily driver with torque and fuel economy the primary goals. Barring some catastrophe, I expect this engine to never come back out of the car for the rest of my life. One of the oddities of getting old is that many of the decisions are lifetime.
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turbo camino
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Re: 305 sbc questions

Post by turbo camino » Sat Nov 30, 2019 5:24 pm

I have always followed the rule, avoid anything between .060" & .100", .058" is close enough to the 'avoid' zone that it'd make me a little nervous. Or at least a little unhappy.

The minimum safe distance to keep things from crashing changes with the bore size but only because of piston rock, the effective range that's generally sought of .020-.040" isn't dependent on bore size.
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Re: 305 sbc questions

Post by rfoll » Sat Nov 30, 2019 7:29 pm

There are .015" shim gaskets, but the bore on those are all 4.10" or more. That would leave a crevice at the deck of about .182" radial by 015" thick. Maybe this isn't a problem, I don't have to worry about emissions testing
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cv67
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Re: 305 sbc questions

Post by cv67 » Sat Nov 30, 2019 7:41 pm

Never felt comfortable going thinner than .028
The thin shims I Used almost always leaked...once or twice I sprayed copper coat (term?) and held for awhile.


Not a builder tho, so ya know what they say about free advice. Id shave those heads down its under 100 bux. Worth the peace of mind.

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Re: 305 sbc questions

Post by turbo camino » Sat Nov 30, 2019 7:54 pm

Shaving the head and using a thick gasket would claw back some compression but would kill the squish/quench. More squish and higher compression is actually more pump gas friendly than lowering compression by increasing the squish distance.
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Re: 305 sbc questions

Post by rfoll » Sun Dec 01, 2019 9:26 am

I will deck the block if necessary. I don't really know where I am until I drop a piston in the hole. I think the 64 cc head and flat top piston.028 gasket combo will allow me to cut the deck the block .010" and give me a static CR of 9:1.
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Re: 305 sbc questions

Post by F-BIRD'88 » Sun Dec 01, 2019 9:34 am

The .015" shim style head gaskets will only give you a problem IF the engine is subjected to WOT detonation.

Normal combustion forces are not a problem.

If the gasket does fail it is because it was subjected to
abnormal combustion (detonation-preignition.)

It is important that the head be flat.
The block is a bit less critical for in that reguard.
But the head needs to be flat.
Install clean and dry on clean dry surfaces.
Retorque the head bolts.

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Re: 305 sbc questions

Post by rfoll » Sun Dec 01, 2019 10:48 am

This is the gasket I have been looking at, https://www.summitracing.com/parts/mrg- ... /chevrolet I Still haven't seen any responses about the crevice created from a gasket bore significantly larger the cylinder bore, and effect of the thickness if any. My initial thought is the thinner crevice matters less than the diameter of it. The 4.10" bore .015" steel shim gasket could save me over $100 if it will work on the 3.736 bore.
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Re: 305 sbc questions

Post by turbo camino » Sun Dec 01, 2019 11:13 am

At only .015" the overhang won't affect anything, if it were .030-.040 thick gasket it'd be a different story. However I don't think the .015 gasket is a good solution if it still leaves the squish at .055-.058. If the .015 could get it down to .040, then yes.

But, if you're going to touch the block at all I don't see any reason not to take off enough to get the squish to ~.040 using a conventional (cheap) composition gasket, either the .028 GM part or plain blue .041 Felpros. I'd pick a gasket and cut the block to get the squish you want.
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Re: 305 sbc questions

Post by rfoll » Sun Dec 01, 2019 11:34 am

.015"gasket should give me .045 quench and about 9:1 CR.
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Re: 305 sbc questions

Post by rfoll » Sun Dec 01, 2019 11:37 am

cv67 wrote:
Sat Nov 30, 2019 7:41 pm
Never felt comfortable going thinner than .028
The thin shims I Used almost always leaked...once or twice I sprayed copper coat (term?) and held for awhile.


Not a builder tho, so ya know what they say about free advice. Id shave those heads down its under 100 bux. Worth the peace of mind.
Everything is very low mile, 100 K on the block, and 78K on the heads I will use.
So much to do, so little time...

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