I am told that the early C-1 engines all originally came with steel, versus composite, head gaskets. I am also told that it is better to install composite head gaskets in connection with the rebuild of an engine that does not involve re-decking as the composite gaskets will compensate better for any irregularity in the block or head surface. Can anyone here shed any further light on these issues? Also, if an early C-1 engine is rebuilt using a composite (versus steel) head gasket, is this something that can be detected after the engine has been painted, and will NCRS judges typically deduct for this? Thanks very much for your assistance. Mike
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Re: C-1 Head Gasket
An NCRS Judge MIGHT be able to detect the difference, but for what? One point typically? I'd run with the latest technology in gaskets, I have been know to go out an flog the engine on an old car, and I want it to hold together. (1965 Corvette FI car class winner in smallblock class at Supercar Showdown some years back-still own)Bill Clupper #618 -
Re: C-1 Head Gasket
Mike,
Fairly certain that the steel shim gaskets were/are approximately .0020" compressed. 1964-65 SHP smallblocks used 2 x .0020" gaskets to drop CR by about 1/4 point, to address detonation complaints.
I would absolutely use modern composition gaskets, WITH the steel cylinder rings when installing heads. Comp gasket are available WITHOUT the steel ring, which are around .0025" compressed. Most composition gaskets with the sealing ring measure somewhere around .0025-.0035". Extra thick gaskets are available up to .0048" compressed. I would build in as much compression as I can, using the thinnest composition head gasket possible. If the heads and decks were never milled, then you're looking at upwards of .0045" piston to deck plus gasket clearance. I agree with the crowd advocating the smallest quench possible, to combat detonation. Chevrolet engineering likes at least .0035" piston to head clearance. This is likely conservative, and I would prefer closer to .0028".
Detonation issues can be addressed later, if necessary, by modifying the distributor's advance map.
JoeComment
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Re: C-1 Head Gasket
Quench clearance like "rod ratio" is overblown hype!
Chevrolet's minimum quench clearance recommendation for a RACING ENGINE is .035-.040", but most production engines were built with quench clearance much higher than that - up to about .060".
According to Taylor in "The Internal Combustion Engine in Theory and Practice" which is STILL the IC engine design bible over thirty years after the first edition was published, increased resistance to detonation is lost once the quench clearance exceeds .005 times bore, which is .020" on a 4" bore engine - way below Chevrolet's minimum recommendation.
Head gasket thickness is an important variable in compression ratio and a mere .010" increase can reduce CR about a quarter point. When building a restortion engine for maximum performance, one should manage the CR to get the highest possible number commensurate with the cam and available commercial fuels, which is up to 9.75:1 with the base engine cam and up to 10.25 with the Duntov and L-79 cams and 10.5 with the 30-30 and LT-1 cams. The only way to achieve these targets is to accurately measure deck height of the completed short block, have accurate volumes for the head chambers and pistons, then use a compression ratio calculator to select a suitable gasket thickness to achieve the target range.
This process must actually start by measuring deck clearance BEFORE the short block is disassembled so you can establish the actual deck height using nominal rod length and piston compression height. The measurements will also tell you if the decks are parallel to the crankshaft and if one deck is higher than the other, which is not unusual.
Also understand that the advertised CR for vintage engines is about as realistic as the advertised HP ratings. Most production engines went out the door with a half point lower compression than advertised, due to high block decks and double gasketing in the case of early SHP/FI 327s.
If decks and head surfaces measure flat with a .0015" feeler gage and machinists bar, there is no reason not to use a shim type gasket if a thin gasket is necessary to achieve maximum target CR. Shim gaskets are available as thin as .015" and most have organic coatings, which aid sealing.
The 3830711 shim gasket went into production circa '66 or '67. GM specifies the compressed thickness as .022", but actual measurement indicate a range of .025-026". Evidence that the previous OE gasket was thinner is the lowering of the specified CR for base engines from 10.5 to 10.25. The 462 heads also had about one cc more volume than 461s, which drops CR about 0.15, everything else the same. I believe the earlier gasket was thinner, on the order of .018", but at this point it's tough to find never opened OE engines to measure.
Head gasket compressed thickness is easy to accurately measure with the heads installed by slipping the appropriate feeler gage between the head and block near the corners of the engine. Also, at least on 327s, the corner of the head gaskets have a small hole and they are visible. I don't think this is in the JGs, but it's easy for a judge to measure head gasket thickness and observe the holes.
I have no information on OE 283 gaskets, but they were probably thinnner than the 711. Also, most 283s have a relatively low specified CR, 9.5:1, which is low for today's premium gasolines, some of which are only a couple of RON points behind leaded premiums from the past. My CR recommendation for 283s (and 350s) is the same as 327s based on installed cam, so a 270 should be up to 10.25:1 which will probably require domed pistons. The combination of "low" compression and the high overlap Duntov cam on 270s results in poor torque bandwidth.
Selecting a head gasket without measuring deck clearance, chamber volume, and having a reliable specification for piston volume is just guessing, and if you blindly use a composition gasket you will end up with lower than optimum compression, but I suppose one "advantage" is that you can use mid-grade or even regular unleaded fuel, but you are leaving a significant piece of torque bandwidth and top end power on the table.
And if you manage the CR to an appropriate value by judicious piston and head gasket seletion, the final quench clearance will be in the range of the vast majority of the tens of millions of small block engines built by GM.
DukeComment
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Re: C-1 Head Gasket
Duke,
You know me better'n that. Everything that you said about measured volumes/clearances, etc. goes without saying. I still prefer minimum quench clearances, though.
JoeComment
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Re: C-1 Head Gasket
I prefer minimum, too, and if building a racing engine I would deck the block to achieve it and maximize CR.
But when restoring a numbers matching block shooting for minimum quench clearance will require the thinnest available gasket, which may push CR to too high a level even if the decks are high, and the thinnest available shim gasket will not get you down to .040" quench clearance if the decks are higher than the 9.025" nominal dimension, which most are. You'd have to deck the block!
If one is restoring a numbers matching block, quench clearance is barely a secondary consideration. It's of little importance.
On a racing engine it's a big deal, but 99.9 percent of the participants on this forum aren't building racing engines. They're restoring vintage engines.
The trouble with hot rod magazines and Internet forums is that the "advice" is oriented toward building racing engines. "Smokey Yunick says..." Bill Jenkins says...", so people get into these mindless discussions about "rod ratio" and "quench clearance", which just aren't issues unless you are building an engine for professional racing like NASCAR, F1, or pro drag racing.
Engine system engineering considerations for road engines, especially if you don't have the luxury or decking the block, are quite different than racing engines or a custom road engine where preserving the original number and tool marks are not an issue.
DukeComment
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Re: C-1 Head Gasket
(composite (versus steel) head gasket, is this something that can be detected after the engine has been painted, and will NCRS judges typically deduct for this)
Yes you can tell weather steel or comp. loose points I dont know? but steel was used. I'm not as smart as these others guys about what they talk about, but you decideComment
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Re: C-1 Head Gasket
Thanks to everyone for the incredibly informative and detailed responses. Also, good to have you back, Roy. I understand you were on the mend for a couple of days. Your absence here was noted. MikeComment
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Re: C-1 Head Gasket
I've used steel on every engine I have ever rebuilt except one early Chrysler hemi and never had a failure. One thing that Smokey did say that I believe in is that most engines that are being built for street performance and some strip use don't need decked. Once you have the block cleaned you lay a long straight edge across the deck and check for any low or high spots. A flashlight from the back side works well. If it's flat it's flat and decking isn't going to make it enough flatter to actually gain anything. If you are cutting a block to set deck height for a specific final CR then that is different.
If you put a composite gasket on where you took off a thinner steel shim your CR will drop if that's all that you changed. I think that decking is a cash cow for a lot of shops and they over sell it.
TomComment
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i would not be afraid to use a steel shim head
gasket on any rebuild if you coat it with high temp aluminum paint. with the CR in the 10/11:1 range there should be no problems even if you do not deck the block.you can "cut" the heads a few .000 of they are off without a problem because any valve job will "open" the chambers to a few more CCs. i would use the "bare" ones not the one with a coating from the manufacturerComment
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steel shim head gaskets compress to
.003 more that their original material thickness,.018 gasket will compress to .021.Comment
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