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LS 243 milling

General engine tech -- Drag Racing to Circle Track

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turbo camino
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LS 243 milling

Post by turbo camino »

How much can 243s be cut down before they get too thin? Is it possible to get down to ~58cc (.050"ish)? I'm trying to puzzle out the best way to get a Gen4 5.3L from 10:1 up to around 10.8 or so.
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Re: LS 243 milling

Post by 1980RS »

turbo camino wrote: Mon Aug 31, 2020 3:01 am How much can 243s be cut down before they get too thin? Is it possible to get down to ~58cc (.050"ish)? I'm trying to puzzle out the best way to get a Gen4 5.3L from 10:1 up to around 10.8 or so.
I would run them the way they are and use the .040 Cometic head gasket. Going form 10:1 to 10:8 to one is not going to make that much difference in performance, some porting will work out better. You can cut .006 for every one cc so 61cc with the Cometic should get you where you want to be.
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Re: LS 243 milling

Post by turbo camino »

If I could get there using only the gaskets it would be worth it, but the heads would still need to be cut ~.030. Stock replacement MLS gasket at .052" thick is $50/pair vs $160/pair for the .040" Cometics. Not this again...
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Re: LS 243 milling

Post by CGT »

While not a 243 head...I have cut over .080 at an angle... off of a LS3 head....knocked about 10 cc's out of it. 14:1 compression, no issues. All the production LS heads seem to have the same basic deck thickness. I wouldn't be scared to do the same to a 243 casting myself....if it was a NA deal anyway.
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Re: LS 243 milling

Post by turbo camino »

Yes, N/A, heads will be shadetree-ported before install (I have a spare set so they can be ready before teardown), cam swap (not bought yet, probably something like 215/223 112*). What would that typically make, 430-440HP?

I'd definitely feel better about .040" squish compared to .052", but at what cost? The LS2 has the same squish distance with the same head casting and tolerates 10.8:1, AND the stock tune has about 10* more spark timing in it than the 5.3L tune. So I'm not sure there would be any benefit other than peace of mind to tightening it up in a combo like this.
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Re: LS 243 milling

Post by tenxal »

turbo camino wrote: Mon Aug 31, 2020 11:04 amIf I could get there using only the gaskets it would be worth it, but the heads would still need to be cut ~.030. Stock replacement MLS gasket at .052" thick is $50/pair vs $160/pair for the .040" Cometics. Not this again...
If you want to use an el-cheapo head gasket, you'll need to cut the heads. Since the heads need to be cut anyway, mill away and shim per your whim. ;) The head thickness is more than adequate.

Though the 'economy' of not using a quality gasket that can be reused multiple times escapes me. :?
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Re: LS 243 milling

Post by CGT »

tenxal wrote: Tue Sep 01, 2020 8:02 am If you want to use an el-cheapo head gasket, you'll need to cut the heads.
Well the GM gasket is a very nice gasket, MLS, and reusable also if someone wanted to do so. GM#12589226 and can be had for around 30.00 if you do some shopping around. So I can see wanting to use it, I've been there in the same situation with trying to warrant paying a decent amount more for the cometics just to optimize quench...and the GM gasket is nicer than the cometic in my opinion, which makes it an even harder decision :( It just does suck that its .051ish thick....

Turbo, Ive run quite a few LS's with a .036 cometic and factory pistons that are .006-.007 out of the hole on all aluminum engines. And I 'd probably do it on an iron- aluminum combination also to be honest if it still had tight fitiing factory pistons. Not sure if that would get you to your desired static compression. I would run a .030 cometic with pistons .006 out of the hole on an all aluminum engine, if it was for myself and I was the one doing the warming up of the engine :lol:

If you have access , look at a L83 5.3 head gasket. GM#12622325. Its for a direct injected motor, and their are some differences. One being that it doesn't have holes for the upper row of 8mm head bolts which were omitted on the later engines. The other that the hole in the gasket for the dowel's are a bit small since they were moved in the later design. But everything else is good, and those two things aren't big issues...to me anyway. But...the gasket while also around .050 thick the individual layers is made in a way that I think could be used as individual layeres , layers removed etc.... Its something I aim to to try someday on something of my own. Not saying it's cost effective when considering the time it would involve, or that its the solution to your current deal. But I thought you may find it interesting.
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Re: LS 243 milling

Post by 1980RS »

turbo camino wrote: Mon Aug 31, 2020 10:07 pm Yes, N/A, heads will be shadetree-ported before install (I have a spare set so they can be ready before teardown), cam swap (not bought yet, probably something like 215/223 112*). What would that typically make, 430-440HP?

I'd definitely feel better about .040" squish compared to .052", but at what cost? The LS2 has the same squish distance with the same head casting and tolerates 10.8:1, AND the stock tune has about 10* more spark timing in it than the 5.3L tune. So I'm not sure there would be any benefit other than peace of mind to tightening it up in a combo like this.
Just a note to you, My friend has a set of 243's on his 5.7 Camaro. When he put them on and they tuned it the car made more power with less timing and the 243 is a really efficient head. I think it made best power with 24°.
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Re: LS 243 milling

Post by turbo camino »

The stock 5.3L spark timing was a complete joke, like 11* in the areas where a 6.0L file has 20-22*. I still get no knock after copying in the TBSS maps.

I have been making assumptions in doing the compression calcs. 65cc chamber, .052" gasket, zero deck - is that right? This is an aluminum block LH6, I've never had one apart before so I don't know how they typically measure out, especially the deck clearance. But using those assumptions the calc comes up to 9.93:1, the service manual claims it's 9.95:1.
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Re: LS 243 milling

Post by CGT »

turbo camino wrote: Tue Sep 01, 2020 12:16 pm especially the deck clearance
I've never seen one that didn't have the pistons above the deck...usually .006-.008. I get 9.99:1, but I'd have to measure the bore diameter of the factory gasket to know that for sure...Im guessing its around 3.9, because its also retro compatible with 5.7 litre engines.
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Re: LS 243 milling

Post by turbo camino »

Well it shouldn't matter I guess... if you add 1.5cc of gasket overhang and subtract 1.5cc from the piston height it's a wash. That's probably about what the volumes of each would come out to.

Knowing the piston is usually above deck makes me less unhappy about that .052 thick gasket, at least.
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Re: LS 243 milling

Post by brentry »

I have a 5.3 block that are .003 in the hole.
Only one I have seen in the hole. Not sure if it a 5.3 thing.
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Re: LS 243 milling

Post by turbo camino »

Anyone have a source for OE-quality stock replacement valves? 3 intakes are bent in one of the core heads. All the aftermarket valves I have seen are complete shit quality compared to the stockers. Even good used valves? Anybody? FML.
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Re: LS 243 milling

Post by turbo camino »

Anyone used GM gaskets, 12498544? $50/pair, .051" thick, but listed as 3.91" which happens to be the same size as the 243 chambers. The smallest "5.3L" gaskets are 3.95" bore.
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Re: LS 243 milling

Post by CGT »

turbo camino wrote: Mon Sep 07, 2020 8:33 pm Anyone used GM gaskets, 12498544? $50/pair, .051" thick, but listed as 3.91" which happens to be the same size as the 243 chambers. The smallest "5.3L" gaskets are 3.95" bore.
I'd jump on that. 12498544 has a MSRP of 51.93 ea. Dealer's cost is 38.95 per. So its either a mistake, or someone is wanting rid of some old stock real bad. Regardless, that's a bargain.
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