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Replacing L98 heads, which is best?

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Old 11-14-2017, 12:29 PM   #1
polo91
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Default Replacing L98 heads, which is best?

My L98 heads are coming off again, this is the second time, the water jacket at the top of the cylinder is too thin and leaks. This problem has plagued me for the last couple of years. The car is a six speed 91 roadster that I drive every day with 160K on the clock and it runs perfectly (other than having to add coolant periodically). When I pulled the heads last year every thing was in tip top condition, with no wear on the bores so it is ready for another 100K+ miles. Since I don't have much confidence in the GM heads what do you suggest? Although the car is nor raced it is driven briskly and I don't want to make any other mods. So far the car is stock except for no cats and air pump. Increased performance is not an issue, I want a dependable bullet proof heads.
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Old 11-14-2017, 01:05 PM   #2
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Is your head and deck flat, maybe all you need is a high quality head gasket and the problem is solved. The stock gaskets in my opinion are not very good, run a coated gasket or even a MLS gasket and the problem should go away.
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Old 11-14-2017, 01:30 PM   #3
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I think its time to check if everything is true and square, then upgrade the gasket.

I'd have the heads worked over, ported & polished, new valvetrain, and just put in a good gasket and leave it be.
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Old 11-14-2017, 02:59 PM   #4
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not sure I understand
Quote:
the water jacket at the top of the cylinder is too thin and leaks
however, , if the heads are within spec flat, and both surfaces clean and prepped, a set of premium fel-pro head gaskets should do the job. I would also spring for a set of ARP head bolts. torque to spec. I typically do 30, 50, 70, and a double check @ 70. reusing the stock heads would save a bunch of money.
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Old 11-14-2017, 03:04 PM   #5
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Quote:
Originally Posted by bjankuski View Post
Is your head and deck flat, maybe all you need is a high quality head gasket and the problem is solved. The stock gaskets in my opinion are not very good, run a coated gasket or even a MLS gasket and the problem should go away.
I'll try the mls gaskets, it's worth a shot, would save a bundle. Any brand of gasket you like?
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Old 11-14-2017, 03:15 PM   #6
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Quote:
Originally Posted by vader86 View Post
I think its time to check if everything is true and square, then upgrade the gasket.

I'd have the heads worked over, ported & polished, new valvetrain, and just put in a good gasket and leave it be.
Earlier this year the heads were fully serviced by a good shop that I have done business with for over thirty years. I didn't have them ported or polished but everything is true and square. I also had a little aluminum added to one spot where the last leak appeared on #7. Going to give new mls gaskets a try, thanks.
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Old 11-14-2017, 03:40 PM   #7
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Quote:
Originally Posted by Joe C View Post
not sure I understand however, , if the heads are within spec flat, and both surfaces clean and prepped, a set of premium fel-pro head gaskets should do the job. I would also spring for a set of ARP head bolts. torque to spec. I typically do 30, 50, 70, and a double check @ 70. reusing the stock heads would save a bunch of money.
If upgrading head bolts and gaskets does the trick I will be more than happy. Off this topic, I am 70 years old, driven Corvettes since I was 19, this is my fifth one. I could easy spring for a much newer one but I love this car, to a guy my age this (and also the C5) look like what corvettes are supposed to look like, and the owner can maintain a C4 for sure.
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Old 11-14-2017, 05:13 PM   #8
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Originally Posted by polo91 View Post
If upgrading head bolts and gaskets does the trick I will be more than happy. Off this topic, I am 70 years old, driven Corvettes since I was 19, this is my fifth one. I could easy spring for a much newer one but I love this car, to a guy my age this (and also the C5) look like what corvettes are supposed to look like, and the owner can maintain a C4 for sure.
about 18 months ago, I did a valve job on my (iron head) 85 - prepped the block and heads, fel-pro head gaskets, ARP head bolts, etc.. everything's fine - zero issues so far (approx. 8K miles). one thing I might add is to clean, prep, and recondition all threaded bolt holes with thread chasers prior to assembly, and make sure the block and head surfaces are carefully prepped. not sure what type of head gaskets you're using, but on my 85 I used permatorque gaskets, p/n 7733. no need to re-torque and the 7733's are a bit more forgiving on surface prep. if you're ****, and follow everything to the letter, you should not have any further problems. http://www.felpro.com/find-my-part/f...rolet~Corvette .

off topic - same here. turning 70 next month, 9 corvettes, and love my 85. I've had everything from C3's to C6's, but the C4 is my favorite - I've had four, and my 85 is by far my favorite - 22 years now - family owned and driven now over 160K miles.


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Old 11-14-2017, 07:41 PM   #9
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double check to make sure they are perfectly flat..if its the same one giving you issue theres a reason why you.. need to find it before you throw it back together
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Old 11-14-2017, 08:32 PM   #10
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double check to make sure they are perfectly flat..if its the same one giving you issue theres a reason why you.. need to find it before you throw it back together
what is the flatness spec for L98 aluminum heads? I seem to recall .004" on iron heads (inline 4 or V8 applications).

edit: just read that for aluminum heads, no more than .002 in any direction. can anyone verify that?

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Old 11-14-2017, 08:42 PM   #11
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Quote:
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about 18 months ago, I did a valve job on my (iron head) 85 - prepped the block and heads, fel-pro head gaskets, ARP head bolts, etc, everything's fine - zero issues so far (approx. 8K miles). one thing I might add is to clean, prep, and recondition all threaded bolt holes with thread chasers prior to assembly, and make sure the block and head surfaces are carefully prepped. not sure what type of head gaskets you're using, but on my 85 I used permatorque gaskets, p/n 7733. no need to re-torque and the 7733's are a bit more forgiving on surface prep. if you're ****, and follow everything to the letter, you should not have any further problems. http://www.felpro.com/find-my-part/f...rolet~Corvette

off topic - same here. turning 70 next month, 9 corvettes, and love my 85. I've had everything from C3's to C6's, but the C4 is my favorite - I've had four, and my 85 is by far my favorite - I've owned it 22 years now - family owned and driven now over 160K miles.

My kind of guy. I have mine nine years and don't plan to part with even if I got another.
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Old 11-14-2017, 08:51 PM   #12
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Quote:
Originally Posted by cuisinartvette View Post
double check to make sure they are perfectly flat..if its the same one giving you issue theres a reason why you.. need to find it before you throw it back together
It will be right before assembly, the tolerance on the gasket at the top of the cylinder bores are very close. I sure wish there was a little more meat on the water jacket.
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Old 11-14-2017, 08:59 PM   #13
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Guys I appreciate the replies, you have renewed my confidence in the GM castings and may have saved me a bundle. It will be a couple of weeks before the job is finished and I get a couple hundred miles on the car, but I will tell you how it turns out. Thanks again.
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Old 11-14-2017, 09:46 PM   #14
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Your weld job at least in that area made the material T0 IE. not heat treated dead soft. I would pick a gsket that does not have a wire inside it as it may notch the head in that area. I would think a gasket with a wide flat sealing area would be the best bet. Metal gaskets will require something stiff and smooth and the T0 section is a fail there. Do what you think is right but might want to run this past the person at the shop that did the heads. It has been close to 35 years since I have done any engine building but that is not to say I don't know what has to happen or the traps you can fall into. I have learned most of them the hard way by being humbled when they did go wrong.
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Old 11-16-2017, 07:01 PM   #15
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Quote:
Originally Posted by polo91 View Post
I'll try the mls gaskets, it's worth a shot, would save a bundle. Any brand of gasket you like?
Cometic seems to be the choice of a number of engine builders.


I would STRONGLY recommend making Very sure that the head surfaces are absolutely flat. Check the block surfaces as well, but the aluminum heads are normally what warp.

The suggestion about ARP head bolts is also a good one.
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Old 11-18-2017, 01:25 PM   #16
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FelPro head gaskets on the old L-98 will work well. Proven so many times to be drop dead reliable.


No need for the expensive Cometics for a stock engine.
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Old 11-18-2017, 02:33 PM   #17
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Polo91 in your situation I would consider a coolant sealer. It reads like a fairly minor leak. Myself I never cared for coolant additive but I discovered the GM additive when trying to drain my GEN II LT1 block and had to punch through the hardened sealer after removing the block plug.

You may want to read up on this sealer but with 160k mi it ain't gonna make things worse. Here it is on Amazon for less than ten bucks:
Amazon Amazon

Hope that link works and good luck.
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Old 11-18-2017, 04:52 PM   #18
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Polo91 in your situation I would consider a coolant sealer. It reads like a fairly minor leak. Myself I never cared for coolant additive but I discovered the GM additive when trying to drain my GEN II LT1 block and had to punch through the hardened sealer after removing the block plug.

You may want to read up on this sealer but with 160k mi it ain't gonna make things worse. Here it is on Amazon for less than ten bucks: https://www.amazon.com/Genuine-GM-36.../dp/B000QIH3C4

Hope that link works and good luck.
thought I read on one of my FSM's (early L98 cars) to add the GM coolant sealant tablets with some engine or cylinder head repair procedures - i'll be damned if I can find it now. I seem to recall that GM used the same coolant sealer on the assembly line years ago as SOP. FWIW, I added three tablets (AC Delco cooling system seal tabs, P/N 12378255) to my 85's cooling system after doing a valve job. it's been about 8K miles now - not noticing any issues - heater seems to be functioning just fine, coolant color looks normal, and of course, no noticeable coolant usage. I check my coolant every week or two, and can't remember the last time I added anything to the overflow reservoir.
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Old 11-19-2017, 12:25 AM   #19
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Quote:
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thought I read on one of my FSM's (early L98 cars) to add the GM coolant sealant tablets with some engine or cylinder head repair procedures - i'll be damned if I can find it now. I seem to recall that GM used the same coolant sealer on the assembly line years ago as SOP. FWIW, I added three tablets (AC Delco cooling system seal tabs, P/N 12378255) to my 85's cooling system after doing a valve job. it's been about 8K miles now - not noticing any issues - heater seems to be functioning just fine, coolant color looks normal, and of course, no noticeable coolant usage. I check my coolant every week or two, and can't remember the last time I added anything to the overflow reservoir.
I think you are correct in that GM used the sealant from the factory. But I see the need to prove it as the sealant is a GM/ACDelco product. User need to research for himself.
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Old 11-19-2017, 12:44 AM   #20
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Quote:
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I think you are correct in that GM used the sealant from the factory. But I see the need to prove it as the sealant is a GM/ACDelco product. User need to research for himself.
GM 12378255 or 10-108 both on package. Have to crush them before adding is what I was told and easiest way is in a zip lock sandwich bag. I am thinking it was 1 tab per gallon and they come on a card of 5 as a single part. the local GM dealer had them on the shelf but forget what I paid for them though sure it was pretty cheap like 10 bucks. I used them on my Pontiac v6 the somehow looked like it got oil in the coolant and was told needed a new intake manifold as it is composite or some such crap along with all new hoses coolant flush with detergents then neutralizer and and and until the bill would have come close to 1k. Had them dump coolant and rinse with hot tap water running the engine between doing it a couple of times. I added the tabs when I got the car home drove for 15 minutes and has been 2 years just fine. Same hoses manifold water pump gaskets etc. Dealer thought it was a hail mary deal and would not work but told them if coolant can't get under edge of gasket it can't corrode anything. Smart move now is change coolant every few years as corrosion inhibitors do wear out.
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