68 Aluminum Heads and Overheating

**Joe L.** · May 26, 1999, 02:26 PM

Re: 68 Aluminum Heads and Overheating

Tom----

Several comments:

1) You mention that you have checked out the gauge, temp sender, wiring, etc., and you consider them to be reasonably accurate. How did you check out these components? In my opinion, the best way to verify the accuracy of a temperature sending system is to install another system of known accuracy and compare the readings which they each generate "in situ". If you check the components of your system individually and/or "off the car", you have no means of knowing if the components will measure accurately when combined together as a system or, more importantly, on the car in real world operating conditions. I would install a SYSTEM of known accuracy in the opposite side cylinder head or, better yet, on the same side cylinder head if another "port" is available. This might be going a little too far, but I, personally, enjoy this type of academic exercise. It's the scientist in me.

I might also add here, at the risk of starting a controversy or "panic", that recently I learned of the POSSIBILITY that aluminum head engines MAY have used a different and unique temperature sender. I just so happens that I am researching this possibility at this very time. I'll post when I get to the bottom of it.

2) Notwithstanding what I've suggested above regarding the accuracy of the temperature measuring system, the long and the short of it is that many 60s and 70s Corvettes RUN HOT. This is ESPECIALLY true of big blocks. Tales of HOT RUNNING big blocks are LEGION. I can tell you that this has been true since the days the cars were new. I was there and involved with the cars then and in the 30+ years since. For Corvettes, especially big blocks, to operate at "peace of mind" temperature readings, EVERY SINGLE ASPECT of the cooling system has to be perfect and I mean PERFECT.

3) The use of incorrect head gaskets could be a source of your problem. However, the "overlap" that you mention seeing in the oil drain back galley does not necessarily signify a problem. There is no way, however, for you to verify that the correct head gaskets are installed without removing the cylinder heads. Then, you'll need new gaskets, anyway. GM gaskets for aluminum cylinder heads are different than gaskets for cast iron heads. Some premium aftermaket gaskets are "dual use". However, I must tell you that I am not certain that the difference in GM gaskets has anything to do with cooling passage differences. As far as I know, all Mark IV big blocks, with the possible exception of the aluminum block, use the same cooling passage configuration.

**Joe L.** · May 26, 1999, 02:26 PM

Re: 68 Aluminum Heads and Overheating

Tom----

Several comments:

1) You mention that you have checked out the gauge, temp sender, wiring, etc., and you consider them to be reasonably accurate. How did you check out these components? In my opinion, the best way to verify the accuracy of a temperature sending system is to install another system of known accuracy and compare the readings which they each generate "in situ". If you check the components of your system individually and/or "off the car", you have no means of knowing if the components will measure accurately when combined together as a system or, more importantly, on the car in real world operating conditions. I would install a SYSTEM of known accuracy in the opposite side cylinder head or, better yet, on the same side cylinder head if another "port" is available. This might be going a little too far, but I, personally, enjoy this type of academic exercise. It's the scientist in me.

I might also add here, at the risk of starting a controversy or "panic", that recently I learned of the POSSIBILITY that aluminum head engines MAY have used a different and unique temperature sender. I just so happens that I am researching this possibility at this very time. I'll post when I get to the bottom of it.

2) Notwithstanding what I've suggested above regarding the accuracy of the temperature measuring system, the long and the short of it is that many 60s and 70s Corvettes RUN HOT. This is ESPECIALLY true of big blocks. Tales of HOT RUNNING big blocks are LEGION. I can tell you that this has been true since the days the cars were new. I was there and involved with the cars then and in the 30+ years since. For Corvettes, especially big blocks, to operate at "peace of mind" temperature readings, EVERY SINGLE ASPECT of the cooling system has to be perfect and I mean PERFECT.

3) The use of incorrect head gaskets could be a source of your problem. However, the "overlap" that you mention seeing in the oil drain back galley does not necessarily signify a problem. There is no way, however, for you to verify that the correct head gaskets are installed without removing the cylinder heads. Then, you'll need new gaskets, anyway. GM gaskets for aluminum cylinder heads are different than gaskets for cast iron heads. Some premium aftermaket gaskets are "dual use". However, I must tell you that I am not certain that the difference in GM gaskets has anything to do with cooling passage differences. As far as I know, all Mark IV big blocks, with the possible exception of the aluminum block, use the same cooling passage configuration.

**Tom B.** · May 27, 1999, 12:09 AM

Re: 68 Aluminum Heads and Overheating

Thanks Joe. At the risk of embarrassing myself I'll explain my scientific method of checking my temperature system. Even though I mentioned the 68 is well restored, it still had the original main (dash) wiring harness which was now nothing more than a hodge-podge of splices. To reference Richard Newton in his book: "All the smoke was getting out". To cut to it, I spent this winter re-wiring the interior and sent my alternator, fuel, and temp gauges in to have them checked, cleaned, and/or calibrated. In the meantime I borrowed a set of known working 69 gauges and checked out my connections and wiring to their source. It is misleading for me to state that I checked out my temp sending unit in any other way. I had noted on a card the old gauge readings, borrowed gauge readings, and refinished gauge readings and all were what I call "identical". There have been other functional ways that I have "monitored" the temp gauge accuracy, for instance with a pair of leather gloves I can tell when the thermostat has opened and there is a flow through the upper radiator hose, check the gauge and it's about where 180 to 190 is. The times it has spit coolant out the overflow hose, check the gauge and it's about where 230 is. I know I did nothing more than that and felt that was what I meant when I said I checked them out for "all practical purposes".

I am still interested in your information, or anyone's, about the head gaskets I described. From what you stated I'm assuming it's confirming my observation that they are replacements, at the least that function and work, but is my description enough to know what they are and how restrictive they might be? The car hasn't been undrivable and I've yet to have to pull over and let it cool down. I'm hoping summer doesn't make a liar out of me. My experience with this rings true in your comment to me that "every single aspect" of the cooling system has to be perfect. That's exactly what I would like to accomplish.

**Tom B.** · May 27, 1999, 12:09 AM

Re: 68 Aluminum Heads and Overheating

Thanks Joe. At the risk of embarrassing myself I'll explain my scientific method of checking my temperature system. Even though I mentioned the 68 is well restored, it still had the original main (dash) wiring harness which was now nothing more than a hodge-podge of splices. To reference Richard Newton in his book: "All the smoke was getting out". To cut to it, I spent this winter re-wiring the interior and sent my alternator, fuel, and temp gauges in to have them checked, cleaned, and/or calibrated. In the meantime I borrowed a set of known working 69 gauges and checked out my connections and wiring to their source. It is misleading for me to state that I checked out my temp sending unit in any other way. I had noted on a card the old gauge readings, borrowed gauge readings, and refinished gauge readings and all were what I call "identical". There have been other functional ways that I have "monitored" the temp gauge accuracy, for instance with a pair of leather gloves I can tell when the thermostat has opened and there is a flow through the upper radiator hose, check the gauge and it's about where 180 to 190 is. The times it has spit coolant out the overflow hose, check the gauge and it's about where 230 is. I know I did nothing more than that and felt that was what I meant when I said I checked them out for "all practical purposes".

I am still interested in your information, or anyone's, about the head gaskets I described. From what you stated I'm assuming it's confirming my observation that they are replacements, at the least that function and work, but is my description enough to know what they are and how restrictive they might be? The car hasn't been undrivable and I've yet to have to pull over and let it cool down. I'm hoping summer doesn't make a liar out of me. My experience with this rings true in your comment to me that "every single aspect" of the cooling system has to be perfect. That's exactly what I would like to accomplish.

**Joe L.** · May 27, 1999, 12:59 AM

Re: 68 Aluminum Heads and Overheating

Tom----

No, I did not mean to imply that the head gaskets were necessarily aftermarket. They may well be GM. I've seen GM head gaskets that "overhang" in the area you mentioned. This "overhang" in the oil drain back passages in no way "restricts" coolant flow or the engines cooling ability. And, you need not infer that because this "overhang" exists that there is also similar restriction with respect to the coolant passages.

One other very important point is this: if your engine tends to "overheat" at idle and slow speed conditions, this is when a cooling system is "taxed" the most with respect to the need for a multitude of things being RIGHT. The fan shroud has to be RIGHT; the fan shroud seals have to be RIGHT, the fan blade has to be RIGHT; the fan clutch has to be RIGHT AND ABSOLUTELY PERFECT; the radiator has to be RIGHT; the hoses have to be RIGHT; the front spoiler and air intake openings have to be RIGHT.

So, if your car primarily has temperature problems at idle or slow speed, that's relatively "normal" and you will need to make sure that each and every one of these things is RIGHT. Then your car will be marginal as far as slow speed cooling is concerned.

**Joe L.** · May 27, 1999, 12:59 AM

Re: 68 Aluminum Heads and Overheating

Tom----

No, I did not mean to imply that the head gaskets were necessarily aftermarket. They may well be GM. I've seen GM head gaskets that "overhang" in the area you mentioned. This "overhang" in the oil drain back passages in no way "restricts" coolant flow or the engines cooling ability. And, you need not infer that because this "overhang" exists that there is also similar restriction with respect to the coolant passages.

One other very important point is this: if your engine tends to "overheat" at idle and slow speed conditions, this is when a cooling system is "taxed" the most with respect to the need for a multitude of things being RIGHT. The fan shroud has to be RIGHT; the fan shroud seals have to be RIGHT, the fan blade has to be RIGHT; the fan clutch has to be RIGHT AND ABSOLUTELY PERFECT; the radiator has to be RIGHT; the hoses have to be RIGHT; the front spoiler and air intake openings have to be RIGHT.

So, if your car primarily has temperature problems at idle or slow speed, that's relatively "normal" and you will need to make sure that each and every one of these things is RIGHT. Then your car will be marginal as far as slow speed cooling is concerned.

**Tom B.** · May 27, 1999, 01:50 AM

Re: 68 Aluminum Heads and Overheating

Joe,

Thanks again for some of the most informative stuff. My first 'Vette was a 69 small block I had in the mid 70's, back when all the world was different. I didn't have another for 20 years, now the 68 is my 4th. The reason I mention this is until now I never had one that ran hot temperatures. The other recent two could sit at idle in the garage, any day of the year, until the tank ran dry and never come close to running hot. Maybe they would peak at 210. If I let the 68 idle, it's slow but in 15 minutes it'll be at 220+ and I either need to drive it or shut it down. What I have to go by and compared to the idle/cooling of the others, maybe I'm expecting too much. Which is why I keep asking the "normal question". Nobody likes to admit having a leaky basement or an "overheating" engine. I think I can improve on all that you mention. Most of what I have gathered from the postings already doesn't define an overwhelming single culprit, but rather a "fine tuning" of all the components of my cooling system. Thanks again.

**Tom B.** · May 27, 1999, 01:50 AM

Re: 68 Aluminum Heads and Overheating

Joe,

Thanks again for some of the most informative stuff. My first 'Vette was a 69 small block I had in the mid 70's, back when all the world was different. I didn't have another for 20 years, now the 68 is my 4th. The reason I mention this is until now I never had one that ran hot temperatures. The other recent two could sit at idle in the garage, any day of the year, until the tank ran dry and never come close to running hot. Maybe they would peak at 210. If I let the 68 idle, it's slow but in 15 minutes it'll be at 220+ and I either need to drive it or shut it down. What I have to go by and compared to the idle/cooling of the others, maybe I'm expecting too much. Which is why I keep asking the "normal question". Nobody likes to admit having a leaky basement or an "overheating" engine. I think I can improve on all that you mention. Most of what I have gathered from the postings already doesn't define an overwhelming single culprit, but rather a "fine tuning" of all the components of my cooling system. Thanks again.

**Wayne M.** · May 27, 1999, 04:23 PM

Re: 68 Aluminum Heads &amp; Overheating

Tom, last night, I was in the process of posting a response when Windows 98 crashed/froze for the umpteenth time (thank you, Bill Gates). As Joe says, alum. head gaskets are different, being of composite (sandwich) construction. When you look at the gasket corner reinforcement strip visible in the upper left corner of the oil drain-back galley, is it steel ? You might be able to check its edge with a dentist's mirror. If not composite, it's probably wrong. I'm not familiar with what GM currently sells for these heads, but I believe they originally used a Victorcore (red) compostion type # 3969865 for '68 L88 and L89's (opinions welcome). If it's blue, it could be a Fel-Pro (aftermarket) product.

All Mark IV's have the same coolant passages on passenger and high performance heads, with some minor variations. The Mark IV BLOCKS have more variation with respect to the holes on the exhaust side, but these are blocked off anyway with stock gaskets. The 3 head holes on the intake side are from 0.8" (ends) to 0.4" (middle) in diameter, but the 3 stock RESTRICTION holes in the gasket are only 0.17". Drilling these holes larger, and/or adding holes on the exhaust side may seem, in principle, to increase flow/cooling, but (with the stock waterpump) change the flow pattern, bypassing and overheating certain parts of the engine. Of course, you need to remove a head to discover any mods to the gasket coolant holes.

If Mark V gaskets are used, certain block and head coolant holes are blocked off and others exposed, also affecting flow pattern. However, this mis-match would be evident to most mechanics.

I don't wish the job of head removal on anyone, but if all else fails.....

**Wayne M.** · May 27, 1999, 04:23 PM

Re: 68 Aluminum Heads &amp; Overheating

Tom, last night, I was in the process of posting a response when Windows 98 crashed/froze for the umpteenth time (thank you, Bill Gates). As Joe says, alum. head gaskets are different, being of composite (sandwich) construction. When you look at the gasket corner reinforcement strip visible in the upper left corner of the oil drain-back galley, is it steel ? You might be able to check its edge with a dentist's mirror. If not composite, it's probably wrong. I'm not familiar with what GM currently sells for these heads, but I believe they originally used a Victorcore (red) compostion type # 3969865 for '68 L88 and L89's (opinions welcome). If it's blue, it could be a Fel-Pro (aftermarket) product.

All Mark IV's have the same coolant passages on passenger and high performance heads, with some minor variations. The Mark IV BLOCKS have more variation with respect to the holes on the exhaust side, but these are blocked off anyway with stock gaskets. The 3 head holes on the intake side are from 0.8" (ends) to 0.4" (middle) in diameter, but the 3 stock RESTRICTION holes in the gasket are only 0.17". Drilling these holes larger, and/or adding holes on the exhaust side may seem, in principle, to increase flow/cooling, but (with the stock waterpump) change the flow pattern, bypassing and overheating certain parts of the engine. Of course, you need to remove a head to discover any mods to the gasket coolant holes.

If Mark V gaskets are used, certain block and head coolant holes are blocked off and others exposed, also affecting flow pattern. However, this mis-match would be evident to most mechanics.

I don't wish the job of head removal on anyone, but if all else fails.....

**GDaina** · May 27, 1999, 09:42 PM

Experienced same problem...................

years ago, in traffic the temp gague would climb. I ended up replacing the fan clutch unit. The factory unit will cost you around $110.00 or and Eaton aftermaket that is so close to original for around $35.00.

Another thing that will help is to remove the front license frame. Don't know if your state requires front plates, but Ohio does, and for years ran w/out the fron plate. Had it handy in case I was stopped and would pop it on the dash. Worked like a charm, and temp NEVER climbed past 180 degrees.

BTW, what is your production number on your 68. I also have an L-89, and looking for an owner with a Sept-Oct build date.

**GDaina** · May 27, 1999, 09:42 PM

Experienced same problem...................

years ago, in traffic the temp gague would climb. I ended up replacing the fan clutch unit. The factory unit will cost you around $110.00 or and Eaton aftermaket that is so close to original for around $35.00.

Another thing that will help is to remove the front license frame. Don't know if your state requires front plates, but Ohio does, and for years ran w/out the fron plate. Had it handy in case I was stopped and would pop it on the dash. Worked like a charm, and temp NEVER climbed past 180 degrees.

BTW, what is your production number on your 68. I also have an L-89, and looking for an owner with a Sept-Oct build date.

**Tom B.** · May 28, 1999, 04:01 AM

Re: Experienced same problem...................

GDaina,

Sorry I didn't check the message board until now. Thanks for some updated info. I still have the front license plate frame on, it's required here, buuuuut what I mentioned to Joe was that I was planning to fine tune all components of my cooling system since we really haven't defined a single overwhelming culprit. I am considering EVERYTHING that gets posted. Thanks for reminding me to include the front LP bracket.

The build date of my car is December 20, 1967 and has a few interesting aspects. If you want, E-mail me at stingray@interl.net. We'll talk.

**Tom B.** · May 28, 1999, 04:01 AM

Re: Experienced same problem...................

GDaina,

Sorry I didn't check the message board until now. Thanks for some updated info. I still have the front license plate frame on, it's required here, buuuuut what I mentioned to Joe was that I was planning to fine tune all components of my cooling system since we really haven't defined a single overwhelming culprit. I am considering EVERYTHING that gets posted. Thanks for reminding me to include the front LP bracket.

The build date of my car is December 20, 1967 and has a few interesting aspects. If you want, E-mail me at stingray@interl.net. We'll talk.

68 Aluminum Heads and Overheating

68 Aluminum Heads and Overheating

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