Valve seats work-harden in use.
An engine starts it's life on leaded gasoline. The lead helps preserve the seats until they work-harden. The engine can then be switched to unleaded gasoline and probably not have a problem with valve recession/seat wear.
Then someone grinds the seats, removing the work-hardened layer, and there's no more leaded gasoline. The seats begin wearing before they can work-harden.
Valve seat recession is a TOTAL CRAP-SHOOT. Some guys have problems, some guys don't. Seems to be worse if the engine has run at heavy throttle--towing a trailer, going up the mountain, running at WFO in a boat, etc. When GM was spending it's own money on engines that were likely to see unleaded gasoline, the exhaust seats were hardened (either by induction-hardening the parent metal, or by installing hardened inserts) beginning in '71 or '72.
I'm surprised that a shop will replace the valves, and NOT at least "touch up" the seats--and if they "touch up" the seats, that work-hardened layer is in trouble. But that's them, and I haven't examined the seats closely--or used marking compound to see how well the valve faces fit.
You're removing a wonderful steel-shim gasket, approximately 0.020 thick. The replacement gasket will almost certainly be twice as thick. Lower compression, more clearance between piston and head. Neither is good. But, ya do what ya gotta do.
Head Gasket Replacement
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Re: Head Gasket Replacement
It seems like the thickness for every head gasket on the market for this car is between 0.038-0.044 inches. Is the specs on the head gasket thickness in the service manual (I don’t have it with me right now)? For my case, the pros outweigh the cons since the head gasket was rusting and allowing coolant to escape. Also the compression will drop regardless since I’m removing the carbon buildup from the engine.
Update: They do have gaskets with a thickness of 0.027 inches, but come with a price:
Update: They do have gaskets with a thickness of 0.027 inches, but come with a price:
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Re: Head Gasket Replacement
I found this article or website that explains what Shurkey mentioned:
https://oldsjunction.classicoldsmobile. ... /ofhed.htm
https://oldsjunction.classicoldsmobile. ... /ofhed.htm
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Re: Head Gasket Replacement
Is there going to be a big difference in performance or fuel economy with thicker head gaskets?
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Re: Head Gasket Replacement
NicolasB wrote:It seems like the thickness for every head gasket on the market for this car is between 0.038-0.044 inches. Is the specs on the head gasket thickness in the service manual (I don’t have it with me right now)? For my case, the pros outweigh the cons since the head gasket was rusting and allowing coolant to escape. Also the compression will drop regardless since I’m removing the carbon buildup from the engine.
Update: They do have gaskets with a thickness of 0.027 inches, but come with a price:
So tired of vender keyboard monkeys putting out unresearched info. 403 head gaskets
will not interchange with anything else. Bruce Roe
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Re: Head Gasket Replacement
Is the 403 very different compared to the other ones listed?
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Re: Head Gasket Replacement
NicolasB wrote:Is the 403 very different compared to the other ones listed?
The 403 is mostly based on the 350, but the bore is even larger than a Cad 500.
The engine of choice here, because this largest small block runs, and runs, on
crap 87 unleaded. Bruce Roe
Re: Head Gasket Replacement
Milling the head to gain back compression ratio is only 1/2 the solution.
A. The real compression ratio was lower than what Oldsmobile advertised to begin with.
B. Milling the head does nothing to reduce the piston-to-head distance screwed-up by the double-thick head gasket.
C. Excessive piston-to-head (squish/quench) distance tends to increase the likelihood of detonation, tends to require additional spark advance due to the slow burn. Power and throttle response are down, unburned hydrocarbons increase, and a higher-octane fuel may be needed.
The only advantage Olds engines have in this regard is that the combustion chamber has very little quench/squish area, as a design factor.
The "fix" for all of this is to mill the block deck, not the cylinder head. If the block height is reduced so that the piston top is level with the block ("zero deck"), the .040 gasket gives nice quench/squish; and an increase in compression--possibly as much as was originally advertised.
A. The real compression ratio was lower than what Oldsmobile advertised to begin with.
B. Milling the head does nothing to reduce the piston-to-head distance screwed-up by the double-thick head gasket.
C. Excessive piston-to-head (squish/quench) distance tends to increase the likelihood of detonation, tends to require additional spark advance due to the slow burn. Power and throttle response are down, unburned hydrocarbons increase, and a higher-octane fuel may be needed.
The only advantage Olds engines have in this regard is that the combustion chamber has very little quench/squish area, as a design factor.
The "fix" for all of this is to mill the block deck, not the cylinder head. If the block height is reduced so that the piston top is level with the block ("zero deck"), the .040 gasket gives nice quench/squish; and an increase in compression--possibly as much as was originally advertised.
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Re: Head Gasket Replacement
Ah okay, I see. I don’t mind a slightly lower compression, I’m sure the car will run just fine, better with a working head gasket.
I bought a set of new valve lifters and pushrods, as well as the head gasket kit, but I noticed that two of the pushrods have a lump of metal near one of the ends. Would that affect performance or be harmful to the car when installed? Also, do you have to test the new lifters before installing?
I bought a set of new valve lifters and pushrods, as well as the head gasket kit, but I noticed that two of the pushrods have a lump of metal near one of the ends. Would that affect performance or be harmful to the car when installed? Also, do you have to test the new lifters before installing?
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Last edited by NicolasB on Sun Aug 01, 2021 9:28 am, edited 1 time in total.
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Re: Head Gasket Replacement
Here’s what I was referring to:
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