About two years ago I bought a pair of #7 heads at a flea market for $75.
I forget the whole story (I never believe stories), but is was something like he had them rebuilt, bolted them on, drove around the block, and suffered a catastrophic bottom-end failure.
They had more miles on them than "around the block," but conceivably less than a thousand.
They have new valves, seats, and springs, brass or bronze guides, and have been decked.

About a year ago I picked up a running '68 HC 350 for $350 to use until I "get around" to building my 425 . I'll get to it one day. Really.

I pulled the bearing caps, PlastiGaged it, they looked good, but one rod bearing was a little wide, so I replaced it. I put in a HV oil pump I had lying around, and a cam I got from Mark. I did NOT pull the heads, because I didn't want to get into the whole "head gasket thickness" issue.

It took a few weeks to get it dialed in, but I did, and it runs well, though the idle is a hair rougher than I'd like. I did a compression test, and found that #8 was a bit low (about 90 if I recall, wet or dry), so I did a leakdown test, and found it had a leaky exhaust valve.
Eh. It's a low-buck deal, and ran fine, so I didn't sweat it.

Well, it's the winter, so I pulled out those rebuilt heads I'd bought and checked them out.
They do look very good.
I cc'd them, and found one has 63cc and the other has 60cc.
I measured their thickness (head sealing surface to rocker pivot bolt surface, which is also the valve cover gasket surface), and found the 63cc head to be 3.840", while the 60cc head is 3.813", for a height difference of 0.027".

Assuming a head gasket bore of 4.126" (I know, it may be 4.250") and a dish volume of 13.4cc (I've measured pistons with that volume in the past), and a deck height of 0.0235" (based on what I've seen in the past), that means that the 63cc head will give me about 9.43:1 with a nominal 0.017" shim gasket and 8.9:1 with a roughly 0.043" Fel-Pro, while the 60cc head will give 9.73:1 or 9.17:1.

Now, here's my question: Assuming that the deck clearance height on my engine is roughly the same from one side to the other when I pull the heads, it would seem to me that to best balance the engine, I should use a thicker gasket on one side, and a thinner gasket on the other side. Seeing as how the thickness difference between the Fel-Pro and the shims is pretty much exactly the same as the height difference between the heads, should I install one of each?

Or am I just crazy?

- Eric

 

That's unrelated to the answer to your main question...

To the first order, the engine doesn't care if the combustion volume is in the head casting or in the head gasket, it's just volume. Yeah, there are second-order differences, but not enough to matter. Some head gaskets really are sold individually.

Of course, I'd just have the second head milled to match the first, but that's just me...

 

Beat me to it, seems like the best way to go and relatively inexpensive, probably less than 2 sets of gaskets.

 

Yeah, but in fairness to Eric, that's why I pointed out that some head gaskets are sold singly.

Of course, I suspect buying two single head gaskets is like buying two one-way plane tickets instead of a round trip (though without the TSA full body cavity search. )

 

There are some that enjoy the cavity search....

 

I can definitely get Fel-Pros singly (4.25" 8171s are at my local NAPA for less than $20), and I believe I can get steel shims singly as well, so that's really not the issue.

I could have the other head milled to match, but that will begin to introduce complications.

The bottom line looks like a simple job just got complicated.
I need to get the old heads off so I can see what I've got - if by coincidence the deck clearance height on one side is much higher than the other (unlikely, unless, of course, I just got the head milled, in which case it's inevitable), and only then can I know how I'm going to address the problem.

My concern here is that I don't much want to lose compression, or put together an engine with slightly more compression on one side than the other, but I also don't want to accidentally increase compression, as it's a bit too high for 93 octane already.

In theory, a pair of heads that have been slightly decked will increase compression over stock heads that have never been off, all other things being equal, but if that's the case, and the heads on there are the generally agreed stock 68cc, then the current compression is 8.96:1, which doesn't seem like it should require extra octane, so I am hoping that I find some carbon in there to explain it when I pull the heads.

If I've got just under 9:1 now, and I need to add octane booster to 93 octane fuel, and I raise it closer to 10:1 with new heads, I'll be in trouble.

Oh, and there's a reason why I avoid flying...

- Eric

 

93 octane is good through 11.2:1 compression under normal temp conditions. I don't think that is going to be an issue at 9-10:1.

 

I COMPLETELY disagree with this statement. In fact, I find it ridiculous. Find me a single cam grinder or engine builder who thinks running an 11 to 1 iron headed engine on 93 octane is a good idea and I'll be surprised.

I do however think that there is no reason that you should need octane booster on a 9 to 1 engine, unless the cam is very conservative.

 

I'm running 10.5:1 in my BBC 292/292 @544 lift, 109 LA, with 20* initial and 34-36 total mechanical with no issues on 93. I keep my water temp to around 185-190.

 

Gotta ask ... why not just sort out that one faulty valve and call it done?

 

Because to get one head off (necessary to do one valve), I need to pull the intake and replace the intake gasket (a PIA and a few bucks).
Since I have a 100,000 mile-plus engine with one bad valve, the odds are good that there are others that are not at their best either, --and--
Since I already have a set of rebuilt heads sitting around, I MAW just replace both of them.

Yes I do have a B&D valve reconditioning outfit, which I could use to fix the valve and seat for free, and I probably will, just for practice, but since the new heads in the barn already have new guides, valves, springs, rockers, pivots, and hardened seats, why not use them?

- Eric

 

I knew there was a reason

 

There's a reason for everything...

... Except some of the stuff my wife does.

- Eric

 

Pull them and see what you got. Rocket Racing sells the .028" head gaskets individually, pretty sure. You shouldn't need 93 with 9 to 1 compression, heck 87 should work fine. How many degrees of timing are you running? Might be time to buy an adjustable vacuum advance. I wish Flea Markets existed around here that had early 350 heads rebuilt for cheap, you lucky dog.

 

I agree.


It maxes out at at 35° at 3,000 RPM.

And I have ajustable vacuum and mechanical advances, all dialed in, to run smooth and fast on 95 octane, which I would expect if the compression were the nominal 10.25:1, but, as has been discussed, actual CRs tend to be lower than the official specification in regular production motors.

As I said, I hope a direct inspection of the combustion chambers will show that the compression is higher than calculated, and why (like, maybe the CCs are full or carbon, or maybe I got the only pair of heads that are actually 64cc).

- Eric

 

X2.
I'd cut the heads to make them equal. Then make sure there are no sharp edges etc in the chamber.
With a cam of decent duration and 9.5:1 you should have no problem running 93 octane. You might want to check for hot spots in the heads. You could have some cooling system issues.


Oldcutlass - how do you come up with 11.2:1? Is there a formula you used based on your BBC combo?
My bet is no. Remember a BBC has a totally different combustion chamber as well as a much shorter rod which effects piston dwell time. You're comparing apples to oranges.

 

Why does nobody ever compare anything to pears? I like pears. And plums too.

 

Now that was funny

 

So your saying that compression is different if you compare say a Mopar BB vs a Chevy, vs an Olds. I always thought compression is compression, maybe I'm wrong. I've tried to limit compression ratios to no more than 10.5 for 93 octane pump gas for this area of the country at my altitude and temps in the summer.

 

Which I plan to do now, once I've had a look inside the cylinders.


Yeah, the CC surfaces in the heads are the untouched cast surface, and I've considered giving them a quick brush with a coarse RolLock disc to take down any peaks (if I try polishing them, I'm going to get in deeper than I need to, and, of course, increase their volume).
I did notice that one of the valves has a tiny nick on the CC surface, and I plan to dress that with a file.

While on the subject of surface irregularities, when I cleaned the (very thin, light) layer of carbon off of the CCs of the second head, I noticed that all of the new seats were staked in four places each (I either didn't look that closely at the other head, or they weren't staked - I'll have to check them).
I know that this is the "old school" way of "making sure" that the seats stay put, but I was also under the impression that in the "modern world," it wasn't really done anymore.
What say you: Extra insurance, or the sign of a hack job?


I agree. The new heads are very clean-looking, with nice, clean, open cooling passages, and I'll have a look at the old ones when they're off, too.
The cooling system pressurizes properly, and keeps around 180° with a 180° thermostat, except in the very hottest of weather, uphill, on the highway, where it creeps a little.

Since the whole subject appears to be opened up now, if I'm going to get one head decked, I'm going to have to pull all of the valves.
I pulled one pair from one cylinder when I bought the heads, to be sure they looked good (they did; looked new, in fact), but hadn't planned on pulling more.
Now that they'll be coming out, I was always taught to lap them quickly before reinstalling them (enough to get an even grey ring of sealing surface). These have a factory-type "one angle" face.
Any arguments against lapping them?
Any arguments in favor of a three-angle valve job strong enough that they'd make me drive the heads 350 miles to where the valve machine is, learn to use it, do all the valves and seats, and drive back (bearing in mind that this is a stock motor that will probably never see more than 4,500 RPM, and will be pushing a 4,500 lb car through a TH400 and 3:08 gears)?

Thanks,

- Eric

ps: ever tried comparing kumquats and green tomatoes?

Also, Eric, I think Mark may mean that some combustion chamber shapes can go to higher compression levels with the same octane fuel, without pinging, than others can. I'm sure he'll explain.

 

A 3 angle valve job is a good idea, so is back cutting and or larger valves and bowl work. I say mill the head to match and leave it there. I am sure that 350 feels like a race motor compared to the 260 it replaced, as is.

 

Keep it up and we'll MAW Eric into building a 455...

 

Ohhhhh, no you won't. I'm far too cheap an SOB for that.

I believe in focusing the work on the intended use. In this case, a cheap engine in a cheap, fun car. No races, no craziness; just a big car with a little pep that doesn't drain the tank between stoplights.

It already runs well, so my chief focus is not to screw that up.

- Eric

 

Static compression ratio is a fixed amount, but various factors determine the dynamic compression ratio, rod length is one, cam another. Also, chamber design, material, intake manifold, etc also affect it. And, with a big block of any make, you can run more cam and the cubes make up for driveability. Just because you can run 10.5 in a BBC does not mean you can run 10.5 in a SBO.

 

Thank you Jim.
As he does all too often he missed my point.
Not only do BBC chambers seem to have a greater cushion for detonation (wedge vs bathtub, that's pears to plums) I'd also bet the BBC's 1.53 rod ratio helps as well. A SBO is 1.77 which means it will dwell at the top longer, making it that much more prone to detonate.

Thanks.

 

Not trying to be a jerk, but this is really all basic stuff. I am far from a professional engine builder.

Back on topic, Eric, what cam are you currently running? Pardon me if I missed it.

 

You seem to forget that some heads are more efficient than others a BBC has a much more effective squish area than an Olds head, making theses comparisons is why everyone thinks you can build a 400HP SB olds on the cheap like a Chevy.
Sorry don't mean to dog you but you are comparing an apple and a banana and saying they taste the same.

 

It's a TQ-20 on a 106° centerline (recommended by Mark), but that is NOT important, as I just put it in last year, I am not going to change it, and it works well.

This is just about swapping a heads because of a bad valve. That's all.
I am not rebuilding this engine.

- Eric

 

I agree that quench and squish play a major factor in how much compression you can run. Piston top style, cam, and types of materials all also play a part in whether you will detonate and how much total timing you can run. My statement was that people are running higher compressions with the right combination of parts no matter what engine manufacturer on 93 pump gas. The same principles apply.

 

I disagree. Take a 460 BBF, a 454 BBC, a 455 BBO, and a 455 BBB. Completely different engine designs with different rod lengths, strokes, bores, and heads. It will take a different combination of parts to maximize performance. This is one of the biggest issues we have; guys apply what works on a 350 SBC to a 350 SBO, and bitch when it runs like crap.

 

No, I agree with you, it takes a different group of parts for different manufacturers, but you can still get them to run at higher compressions with the right parts.

 

1972 350 heads have 68-69 cc stock. Yours' are 60-63. Your heads must have been milled quite a bit to achieve that cc. When they go back on a block I would consider milling the intake to match the heads or you may run into leaking issues. Just an observation.

 

A fine observation. Yes, I'll need to compare heights and make sure the intake lines up completely.
The other alternative is to mill the intake side of the heads just a bit.
And, yes, the shorter one is milled just a bit into the cast-in numbers on the underside.

But, as with all things, I'll know when I know...

- Eric

 

Maybe..... But, you aren't going to get an iron-headed SBO to run on pump gas at 11.2 to 1 without a BIG cam and a fperfect tune, IMHO.

 

anyone want to set up a betting board on this one?

 

As to the 11:1 CR question. I have been very successful at building street motors for 93 octane pump gas with 11 or 11.5:1 CRs. Sometimes customers brings you a project and you end up with what the supplied parts allow. Its all in the camshaft. These builds have been iron headed and proved to be very very tolerant of the high CR. They've all turned out very spirited also. Two guys were actually scared of the builds once installed. One of them was a late 400 that was absolutely viscious. Drop the clutch and roast the tires at just about any rpm or speed. While I do normally try to stay in the 10.5:1 range as max for a pump gas motor I adjust accordingly when it ends up higher than desired.

The Olds combustion chamber is one of the most efficient and spark knock resistant that there is.

 

We all knock the Olds combustion chamber, I have. I ran nearly 60 degrees of part throttle timing with no signs of pinging. Of course this is on 8 to 1 motors. I have also noticed Edelbrock headed old and new chamber builds still run 34-36 to make max power. My iron head 8 to 1 SBO's ran best at 36 total at the track but still slow.

 

I respectfully disagree.
If you can run a lot of timing that means the combustion chamber is actually inefficient. Look at LS stuff, typically on a high compression application 34 degrees on pump gas is about it period. And that's with aluminum heads.
I did a 10.8:1 iron headed 455 with a somewhat lazy 238/246@.050 cam. 29 degrees of timing was about all it would take. At 32 it detonated violently. Not very knock resistant.
If you're running 40, 50 or even 60 degrees of timing anywhere in the rpm'load range then that means it's not very efficient because you're having to light the fire that much earlier.

I'm currently doing 2 long rod 455's. I expect them to tolerate less timing because of the longer rod. In essence that will make them "more efficient" but it's a band aid. Think about it, you have Oldsmobile engines with 1.70+ rod ratios that take 40, 50 and 60 degrees of timing. Absolutely not very efficient.

 

I think from an engineering standpoint, that is a difficult statement to defend or quantify.

 

If the Olds chamber wasn't efficient we would have seen fuel injection and the other electronic controls for emission compliance on Olds V8s. But they died running carburetors. Just because you can run a lot of timing doesn't mean you should be. The last pump gas 455 I had on the dyno liked 28° for best torque and HP. We ran it at 34° to begin with and then 36° before going ghe other way and never heard an audible spark knock. I've had a lot of builds that just did not need 30 or more degrees of total advance.