Head gasket bore size and impact on ring sealing - Team Camaro Tech
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post #1 of 15 (permalink) Old Jan 19th, 05, 11:19 AM Thread Starter
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I am upgrading to Cometic MLS head gaskets. I am .030 over on the 350" 4.0 cylinder bore size.

I currently am using the Fel-Pro #1003 head gasket which has a 4.166" diameter on the bores. Cometics sells a number of diameters and it was recommended that I use the #C5325-040 with a bore size of 4.065, just a tad bigger than the actual cylinder bore of 4.03. However, they also sell sizes similar to the Fel-Pro diameter, including a 4.165 (very close to Fel-pro's 4.166, and many sizes between their 4.065 and the 4.166.

My dilemma is that I just read an article that discusses how the choice of the gasket bore size compared to the actual cylinde bore dimensions, can impact piston ring sealing greatly due to its influence on cylinder roundness when it is torqued down. They also talk about how using different head bolts can also have the same effect. They also warn against a gasket bore size that sits on the ridge of the cylinder bore walls as that can cause hot spots and gasket failure.

After readling all this, I do not know what would be the best gasket size for my 4.030 engine. Any guideline on the gasket bore size relative to the actual bore?

1973 RS Z28: 401ci Dart Little M, Littlefield 6-71 supercharger @14lbs boost, Callies Magnum crank, Lunati rods, JE pistons, 7.8:1, AFR 227 Comp heads, T&D 1.6 shaft rockers. Reed solid FT cam (240/[email protected]/.542 lift-112 LSA), QFT 750 carbs
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post #2 of 15 (permalink) Old Jan 19th, 05, 11:26 PM
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i asked a similar question here a while back and the general consensus was that it really doesn't have a noticeable effect,but that was about a "larger than the bore" gasket.one smaller than the bore is definately asking for trouble.
worthy of further research,but a tough one to prove.
head studs are always better than head bolts,but you can't pull the heads,with the engine in the car.
if you can find a 4.060 gasket that would be about right.
Eric

86 camaro,383,topline vortecs,scat crank,wiseco forged 9.5:1,6"scat rods,comp xs282s,performer rpm,750 proform vac sec,msd in cap, hooker coated 1 5/8"headers,3"flowmaster,700r4,2800 stall,3.42 10 bolt posi,hotchkiss lca's,aje tube k member & a-arms
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post #3 of 15 (permalink) Old Jan 21st, 05, 06:57 PM
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My machinist was pretty insistent that he do the final torque plate honing with the exact same part # head gasket that I was planning on using on the engine.

Of course I didn't follow his advice. Motor runs great anyway.


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post #4 of 15 (permalink) Old Jan 22nd, 05, 04:38 PM
 
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All I'm going to say here is that I used Mr. Gasket, 4" bore, .020" thick, steel shim headgaskets on my about 9.5 to 1 CR., 406 SB after I "file/stone ground-fit" them by each intake valve for like a year as my daily driver.

I mocked up my motor and reached up inside with a scribe and circled each bore to mark where any overhang was and then relieved them by hand! I also had to use one of my old 406's headgasket's for a template when I drilled steam holes in my heads and shim headgaskets.

I used either dauber Copper-Coat or spray can aluminum paint but it's been years so I don't remember which??

I was almost anal with respect to retorquing them after heat cycling them at least three times over a couple of days AND I never had a problem!

No smooth heads or decks either, just stock finished that I cleaned up real good!!

pdq67



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post #5 of 15 (permalink) Old Jan 23rd, 05, 08:06 AM
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Mike,
I ordered my Cometics with the bore size the same as my cylinder. There is not "ring" on the Cometics unlike most off the shelf gaskets. The whole gasket is made of "multi layer steel".

They are great gaskets. I thought I suggested them to you when you were having that engine built. We are talking about your blown 350, correct? I take it you blew a head gasket?

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post #6 of 15 (permalink) Old Jan 23rd, 05, 08:08 AM
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Royce do they have a website?

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post #7 of 15 (permalink) Old Jan 23rd, 05, 08:51 AM Thread Starter
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Thanks Royce,

I talked to Cometics again a few days ago..they said that about .030" bigger than the bore is plenty safe..allowing .015" on each side.

Yes, I remember you recommending the Cometics. I don't think I have a blown head gasket as cylinder compression checks out fine and no water in cylinder. Still do not know why front cylinders just started to run lean, but I think it is the intake gasket/ vacuum leak. Haven't pulled it apart yet to diagnose, but will soon. I am going through the whole motor while it is out even though I have just about 800 miles on her. Good chance to throw in the stronger gaskets as I am runnning far more boost than originally planned. The motor seems to prefer the higher boost as my static compression is quite low at 7.6:1. I am 9.6% underdriven which is supposed to give me 10 pounds on a 350ci, but I saw 14-15 pounds with the crappy foam edelbrock filters I had on there, but is down to 12-14 once I put on the thick K&N filters/ I want to limit it to 12 lbs. I was told that my Littelefield race blower will make much more boost than the average street blower as it has much closer tolerances faciltated by the teflon strip and the hard anodized case with some serious extra ribbing on the underside so the case can't flex...good for 27+ pounds of boost. Didn't think much of at first, but now I beleive it. However, I honestly beleive a 6-71 is too big for a 355 street motor. Yes it makes a ton of boost upstairs, but I have had smaller blowers before, such as the original B&M 162ci (discontinued) and a 4-71 and they seem to make boost quicker down lower and they idle better because they don't pulse the air flow as severely as a big old 6-71...so more practical for the street, while not giving up much on peak horsepower. I think the B&M/Holley 250ci blower is perfect for a street 355 engine. A 6-71 displaces 411ci of air each rotation. A 6-71 just looks cooler is all. Now I digress.

1973 RS Z28: 401ci Dart Little M, Littlefield 6-71 supercharger @14lbs boost, Callies Magnum crank, Lunati rods, JE pistons, 7.8:1, AFR 227 Comp heads, T&D 1.6 shaft rockers. Reed solid FT cam (240/[email protected]/.542 lift-112 LSA), QFT 750 carbs
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post #8 of 15 (permalink) Old Jan 23rd, 05, 09:28 AM
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Mike,
With only 800 miles I hope all is well and it should be.

A 6-71 is not to big for a 355ci engine. It's all about the combination. Are you still running those Edelbrock carbs? I seriously suggest you rethink that if you are. You will notice a huge difference. My Hampton blower is hard anodized has extra ribbing etc... as well. The tolerances are tight but he doesn't use teflon. For the boost levels any street blower it at there is no need for telfon, that is for all out race blowers making BIG boost. No way the bennefits will ever bee seen with pump gas. Judging from your 1/4 mile experience your blower makes plenty of power down low as well as up top. The bigger blower is definitely an advantage over the smaller ones. More power with less heat, while the smaller ones do make power they are boost limited due to having to spin them too fast. It all comes down to what you want to do with the car, for just cruising around on the street the smaller blower is fine but, if you want to make big power the bigger blower is the way to go.

As you found out the boost is somewhat a misleading measurement. You can have two 355ci engines and just change the heads or intake and the same blower (with the same drive ratio) will make different boost on each engine. This is why I don't really like using "boost" as a measurement (but that's all we have). The actual "flow" is what matters. When you changed air filters you increased the flow but the "boost" went down. I bet if you were on a dyno, even though you made less "boost", you would see you were making more power.

Keep us posted on your progress, please.

Royce (NO XQSSS) Bradley

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post #9 of 15 (permalink) Old Jan 23rd, 05, 10:23 AM Thread Starter
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Royce,

Don't get me wrong, engine ran great and I have more power than I can use. I will be back at the track in 3-4 months with full slicks to make sure that the car runs all day at me ET goal.

Engine builder keeps reminding me of the carbs, too..I have always used Holleys and know I would make a bit more hp with Holleys or Demons, but I am knocking down 17mg and still getting better. My 2 750 edelbrocks are jetted as rich as possible with bigger custom jets that Edelbrock does not sell over the counter. I am happy so far, but if I need to go even a tad richer on the WOT ratio, that means Holleys or Demons.

I have an elusive rear seal or pan (probably pan), the new problem with the lean front cylinders (intake gasket I think) and you may recall that on my maiden voyage I split the cast oil filter mount adapter in two at WOT spitting every drop of oil on the street in a second, so I want to check the main bearings, which I think are fine, but need piece of mind. I will also detail the block a little more, smoothing the block like you did. Even thinking of painting block a dull, dark gray so it looks like an aluminum block at first glance..seen it at a show and it looked nice.

Also, there is no way these big blowers like to idle less than 1,000 rpm. I was warned about this beforehand. I have a short intake duration cam and mine idles 900rpm at best. My B&M would idle at 700-750rpm with a bigger cam.

I always thought about where I would make power, not just about peak power. I could gain 50+hp with a simple $150 cam change and it would still be streetable, but that power would be up around 6500rpm. I spend 99.99% of my time below 3,500rpm and wanted a max rpm of 6,000rpm. But come to think of it now, I could really do without some of the lowend power I have now as I have too much. However, she cruises nicely at 1800rpm..nice on the highway at 70+rpm fo a long cruise and I can still hear when I get to my destination (I have 3" Flowmaster 40's).

I have heard the teflon debate for years. Some say there is more blow-by at low RPM when the rotors spin slower and that is where the tighter tolerances help out preventing leakage around the rotors...I guess it will remain an enigma to me.

This car is a real driver, which for me has it all. However, when my engine guy starts one of the pump gas 8-71 blown Canfield-headed, street-cammed 540ci engines he sells, I admit I get chills down my spine..1000hp & and 800+ ft lbs. Surges and thumps like a funny car..but I would probably kill myself with that thing on street tires..not to mention the 2-3mpg.

How is your new project coming along?

1973 RS Z28: 401ci Dart Little M, Littlefield 6-71 supercharger @14lbs boost, Callies Magnum crank, Lunati rods, JE pistons, 7.8:1, AFR 227 Comp heads, T&D 1.6 shaft rockers. Reed solid FT cam (240/[email protected]/.542 lift-112 LSA), QFT 750 carbs
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post #10 of 15 (permalink) Old Jan 23rd, 05, 10:29 AM Thread Starter
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http://www.cometic.com


very helpful tech support

1973 RS Z28: 401ci Dart Little M, Littlefield 6-71 supercharger @14lbs boost, Callies Magnum crank, Lunati rods, JE pistons, 7.8:1, AFR 227 Comp heads, T&D 1.6 shaft rockers. Reed solid FT cam (240/[email protected]/.542 lift-112 LSA), QFT 750 carbs
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post #11 of 15 (permalink) Old Jan 23rd, 05, 10:34 AM
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Thanks Torquer.

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post #12 of 15 (permalink) Old Jan 23rd, 05, 11:57 AM
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67 Plum,
Sorry about the link. I meant to post it and when I finished my long winded post I forgot.

Mike,
The mileage you are getting is awesome, I am no where near that. I also built mine to be on the ragged edge of pump gas and streetability. Like you mentioned with a roots blower low end power/torque is not an issue. The reason I suggested a carb change (and tried to talk you into Holleys when you were building it) is because the meter better in boost situations (the Edelbrocks could be your problem with the lean cylinders too). I think you will have a better running engine/car throughout the RPM range and make more power on top of that. It sure is hard to argue with 17mpg though. I did send you the sound clip of my car didn't I? That is the sound of about 10mog highway if I was careful and about 3-5mpg around town (man I need to get this thing back in a car).

The new project is coming along slow but, things are coming together now. My wheels were supposed to be here this past Friday. The tires are in but, the Budnik wheels aren't there yet (any day now). My front crossmember and suspension should be here any day now, Wilwood disc brakes all the way around are here (can't do anything with them). The wheels are the big hold up, I can't cut anything until I have them on hand to take measurements. So depending on which gets here first will determine if I start cutting the front or the back first. I still have to measure and order my roll cage and cut out the firewall. The doors fenders, hood, trunk and all interior and glass are off/out (bare shell). Not much else I can do until my parts start showing up. I will post some pictures soon. I still think I can finish the car before August, which is my goal.

Royce (NO XQSSS) Bradley

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post #13 of 15 (permalink) Old Jan 23rd, 05, 12:53 PM Thread Starter
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Royce,

I did not think the 750cfm edelbrocks would cause an issue with boost, just with the amount of gas a healthy DP carb could dump in a shot. On paper, a 355 adjusted for the increase volumetric efficiency of the blower cannot use much more than 1000cfm and I know a few blown SBCs making more hp than I with smaller carbs (600s or 650's). I will say that one of them did mention that his doesn't really make boost until 4000rpm. I don't think I have a problem with low end boost, just the the liitle blowers seem to build a bit quicker. Anyway, I don't want more lowend hp, for sure. I need to lean the primaries out just a hair, and I think I can get 18 or so mpg.

No, the carbs are not causing the sudden lean problem. It was carbureting just fine until my last time out. I have an air:fuel guage and I take plug readings every 2nd ride, so it was fine. I think I blew an intake manifold gasket as I never rechecked them after installing as I was too lazy to pull the blower to do it.

I am taliking to one guy in New Zealand with the same car and almsot same engine build that is running low 11's with a single 800 holley on his 6-71. He runs it hidden under that nice fake Ederele injecter scoop that Don Garlitis sells.

Actually, I would really like to go to fuel injection and was looking seriously at the $5000 EFI ederle-type hat system for BDS, but I heard from a few people that got sucked in that the electronics on it are NG.

Budniks? You are going 15" and not 17", right? I recall you were doing a backhalf for that full pro-street look.

1973 RS Z28: 401ci Dart Little M, Littlefield 6-71 supercharger @14lbs boost, Callies Magnum crank, Lunati rods, JE pistons, 7.8:1, AFR 227 Comp heads, T&D 1.6 shaft rockers. Reed solid FT cam (240/[email protected]/.542 lift-112 LSA), QFT 750 carbs
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post #14 of 15 (permalink) Old Jan 23rd, 05, 01:33 PM
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No problem Royce thats what i figrued.I had tried www.cometic.com it didnt work must have typed it wrong or something.

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post #15 of 15 (permalink) Old Jan 24th, 05, 06:08 AM
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Mike,
As you have already noticed, when dealing with blowers the advice you get from different people will be different. While I agree you don't "need" more than 1000CFM with your set-up, it doesn't hurt to "over carb" with a blower like a naryrally aspirated engine. I was shocked that 2 750 DP blower carbs were suggested to me. I did some research and was told (from a very reputable carb shop, that built my carbs) that they never suggest anything smaller than 2-750's on any blower 6-71 or larger. The harder you make the blower work the more heat you will produce. For a street cruiser that is not seriously trying to make all the possible power a singe 850 will work but, I know for sure he would be faster with more carb. The use of the car has a big efect on which way you go. I am usually talking about maximun performance not gas mileage,so we may be looking at it from two different points. If my car with a big blower only ran 11's I would be highly disappointed. You can easily run 11's with no blower. I also built my engine with that in mind. There are a lot of cars with basically stock engines and a blower, for them to run 11's is somewhat impressive. A car with good performance heads, healthy cam.and a big blower should be able to at least crack into the 10's. This is all with the mind set that you have tires and suspension to make use of the power.

Yes, I went with 15" Budnik Cobalts. I am not into the big rims. The other point is that my car has small wheel wells (came stock with 13" rims) so any wheel larger than a 15" would not look right, it's a fairly small car (about the size of a 63 Nova). Tire selection is also limited for the larger wheels (drag tires). I am a big fan of the fatties and skinnies and always have been. I really like that look. The car will be front and back halved, tubbed (but not huge tires) 325/50/15 ET Street Radials. Hopefully a mini tub will be enough and I plan to run ladder bars (only because I need all the roon for a back seat as possible) a four link offers more adjustability but, really eliminates any chance for keeping the back seat.

Royce (NO XQSSS) Bradley

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