Head to Block Parallel flow vs series flow cooling

[Barth C.]

I recently rebuilt a 1966 L72 427 engine which had been sitting disassembled for many years. My problem is the motor overheats. It's an old subject discussed many times, but I've searched the archives extensively looking for information and am running out of ideas.

It does not heat up rapidly but heats up slowly and steadily. When the temp guage gets to 250 I get nervous and shut it down. It starts to boil over soon there after.

I've check everything I've seen in the archives.
- initial timing at 10 degrees BTDC
- new Robert Shaw high flow 180 degree thermostat
- rebuilt water pump with new impeller
- connected the vacuum advance to the full time port
- new Dewitt direct fit radiator with new 13 lb radiator cap
- checked the radiator for the presense of exhaust gases

Before assembly I had the block and heads cleaned and checked for cracks. The intake, heads, block, cam, carb, exhaust are all stock.

I thought perhaps my fan was not pulling sufficient air (stock fan with replacement clutch) so I took the car out on a long stretch of straight flat road running 55mph with the heater blowing very hot air. Temperature slowly but steadily climbs to the 250 mark. When I shut it down it starts to boil over within a couple minutes.

I saw a posting on another forum talking about parallel cooled blocks using head gaskets intended for series cooled blocks causing an overheating problem. I'm wondering if this is related to the issue I am having. In addition to the two oblong water passages at the back of the engine, it looks like the L72 has several large and small coolent passages between the block and heads. The head gasket I used restricks the size of the three large passages between the block and head along the valley side of the head. Please see attached pictures (note the head picture is of the head on my car; the gasket and block are representative pictures from the vendor and CF magazine).

Could these small holes in the head gasket be causing my overheating problem?
thanks for any information or ideas

[Joe L.]

Re: Head to Block Parallel flow vs series flow cooling

I recently rebuilt a 1966 L72 427 engine which had been sitting disassembled for many years. My problem is the motor overheats. It's an old subject discussed many times, but I've searched the archives extensively looking for information and am running out of ideas.

It does not heat up rapidly but heats up slowly and steadily. When the temp guage gets to 250 I get nervous and shut it down. It starts to boil over soon there after.

I've check everything I've seen in the archives.
- initial timing at 10 degrees BTDC
- new Robert Shaw high flow 180 degree thermostat
- rebuilt water pump with new impeller
- connected the vacuum advance to the full time port
- new Dewitt direct fit radiator with new 13 lb radiator cap
- checked the radiator for the presense of exhaust gases

Before assembly I had the block and heads cleaned and checked for cracks. The intake, heads, block, cam, carb, exhaust are all stock.

I thought perhaps my fan was not pulling sufficient air (stock fan with replacement clutch) so I took the car out on a long stretch of straight flat road running 55mph with the heater blowing very hot air. Temperature slowly but steadily climbs to the 250 mark. When I shut it down it starts to boil over within a couple minutes.

I saw a posting on another forum talking about parallel cooled blocks using head gaskets intended for series cooled blocks causing an overheating problem. I'm wondering if this is related to the issue I am having. In addition to the two oblong water passages at the back of the engine, it looks like the L72 has several large and small coolent passages between the block and heads. The head gasket I used restricks the size of the three large passages between the block and head along the valley side of the head. Please see attached pictures (note the head picture is of the head on my car; the gasket and block are representative pictures from the vendor and CF magazine).

Could these small holes in the head gasket be causing my overheating problem?
thanks for any information or ideas

Barth-----


These look to be the correct gaskets for your application. However, just to confirm, what is the Fel-Pro or GM part number?

Another question: do you know the history of this block? Is it possible that it was ever "filled" for drag racing purposes? You should have been able to tell this if you had the side freeze plugs removed.

[Larry B.]

Re: Head to Block Parallel flow vs series flow cooling

You said the initial was set at 10 degrees but what does the dist do. Does it ping. Is the damper tdc with the piston. Is it lean. IF you are not a total purest I can tell you how to cool it.

[Barth C.]

Re: Head to Block Parallel flow vs series flow cooling

FEL-PRO 1057 is the head gasket.

I had the casting plugs out when the block was cleaned. I didn't notice anything special but not sure I would know what I was looking for. I do not know the history of the block except it had thrown a rod and one of the cylinders had been sleeved. I had a machinist bore it out .030, install cam bearings, and install the casting plugs. He didn't mention anything about the presence of filling. He said the sleeve had been correctly installed and was a permanent fix.

There is no pinging that I can hear but the car has sidepipes so I'm not sure I would hear it. The damper is original and appeared tdc with the piston when assembled. Before I do anything major like removing a head I plan to pull the plugs and double check tdc.

"Purist" is a relative term. The car had been customized back in the 70's and I've spent a lot of time making it correct. The block is not original to the car but the block and other major engine components are correct. I would hate to do anything that makes the engine compartment look incorrect, but I want to drive and enjoy the car.

again thanks for any information

[Stuart F.]

Re: Head to Block Parallel flow vs series flow cooling

Just one small comment: About a year ago, I had a major overheat that pushed lots of coolant out of the overflow. This happened shortly after I had changed the coolant and replaced the thermosat with a Robertshaw "type". This piece was an "Mister Gasket" item (made in China) purchased at my local Auto Zone store. I had driven about 20 miles to a Corvette show and, once there, had to idle a lot in line at the entrance. As expected, my engine did heat up some (about 210*), but nothing unusual. On my return trip, the engine seemed to be running fine and on approach to my exit I punched it to get around several slower cars in order to be in my exit lane. At the stop light all hell broke loose and I was dumping coolant all over - or so it seemed. Everything checked out ok once I got home (another 2 miles). As it turned out, my new thermostat had stuck closed after my engine cooled down at the show and did not reopen. Having an SHP small block, I had enough coolant circulation in the engine with the water pump by-pass to keep the engine at a normal temp at a low cruising speed, but once I accelerated up to a higher speed the engine over heated.

Bottom line: check your thermostat. These imported stuff just aren't up to the task. At least pull it out and do a stove top test in a pot of water with a good meat thermometer (don't tell ma ma).

Stu Fox

[Timothy B.]

Re: Head to Block Parallel flow vs series flow cooling

Barth,

Can you explain what a Dewitt direct fit radiator is compared to the original type radiator. IMO, if the car overheats at speed there is a problem with heat transfer in the radiator but Stu's right about the thermostat so think about that. What's the V/A control part #, how much engine vacuum at idle? Does the lower radiator hose have a wire to keep from sucking closed?

Find a shop that can double check that radiator cap to make sure it holds 15psi., repro caps are junk...If the car has a copper radiator find a nice AC stainless rc-15 cap, the ones that had AC stamped into them which is before they were outsourced. Also check archives and double check your temperature gauge for correct temp readings.

You have to start with simple things first.

[Duke W.]

Re: Head to Block Parallel flow vs series flow cooling

Converting to full time vacuum advance is the way to go, but did you check the VAC for proper function? The OE VAC is stamped 360 12, and the proper replacement is a NAPA VC1765 or equivalent in another brand, which are stamped B20 or B26.

A timing light and Mity Vac is best, but a quicky check is to remove the signal line from the VAC at idle and plug it with your finger. Engine revs should drop noticably and the engine will run very rough. If not, the VAC is dead or it still has ported advance. Then remove your finger from the signal line. If it's full time advance engine revs will increase, but it will be rough due to leaning out from the vacuum leak. If it's ported there will be little or no change compared to plugged.

You should also visually check the centrifugal advance for freedom of movement and determine the curve with a dial back timing light. Some guys glob grease onto the mechanism, which is a very bad idea. It should be clean other than the parts wiped down with WD-40 to provide corrosion protection. Grease is way to sticky and will gum up the works.

I also have the same question as Tim. What's the difference between the DeWitts "direct fit" and OE replacement. I'm sure Tom can answer the question for you.

Most replacement fan clutches are set for engines with 195 thermostats, so they tighten at a higher temperature than early clutches that were set up for 180 ...stats.. This might allow the engine run 210-220 in low speed stop and go traffic, but not 250.

Duke

[Michael H.]

Re: Head to Block Parallel flow vs series flow cooling

Converting to full time vacuum advance is the way to go, but did you check the VAC for proper function? The OE VAC is stamped 360 12, and the proper replacement is a NAPA VC1765 or equivalent in another brand, which are stamped B20 or B26.


Duke

I have to guess that there is something else wrong. I have an almost identical car/setup with no vacuum advance connected at all and my car doesn't over heat.
A new 66 425 HP car didn't have any vacuum advance at idle and they didn't over heat.

If setting the timing a little higher or using direct manifold vacuum seems to help the over heating problem a little, it's just covering up some other problem.

Some engine rebuilders still think a "rough cylinder wall to help seat the rings" is the way to go. It isn't and it creates a lot of heat.

I would also be looking for a vacuum leak or some other issue.

One more possibility... today's E10 gasoline seems to run about one or two jet sizes lean. (closer to two) Lean fuel burn is hotter and will increase coolant temperature.

[Duke W.]

Re: Head to Block Parallel flow vs series flow cooling

It depends on how you define "overheating". Big blocks have always had a reputation for "running hot". Most owners get nervous when the temperature climbs over 210, and every owner who has converted ported vacuum advance to full time reported lower coolant temperatures in heavy stop and go traffic after the conversion, which greatly improved their peace of mind. It also improves fuel economy in most urban driving conditions.

There have been many cases of hot running and overheating due to dead VACs and gummed up centrifugal advance mechanisms.

Yes, there may be other issues here, but checking simple things first like the spark advance map, fan clutch function, and radiator performance is wise thing to do before tearing into the engine.

When it comes to "overheating" and many other problems, the first thing to do is apply Occam's Razor.

Duke

[Michael H.]

Re: Head to Block Parallel flow vs series flow cooling

It depends on how you define "overheating". Big blocks have always had a reputation for "running hot". Most owners get nervous when the temperature climbs over 210, and every owner who has converted ported vacuum advance to full time reported lower coolant temperatures in heavy stop and go traffic after the conversion, which greatly improved their peace of mind. It also improves fuel economy in most urban driving conditions.

There have been many cases of hot running and overheating due to dead VACs and gummed up centrifugal advance mechanisms.

Yes, there may be other issues here, but checking simple things first like the spark advance map, fan clutch function, and radiator performance is wise thing to do before tearing into the engine.

When it comes to "overheating" and many other problems, the first thing to do is apply Occam's Razor.

Duke

I agree with much of that but it still doesn't address the real problem. A properly operating vacuum advance certainly can't hurt.. and it may cover up other issues, but it most likely isn't the root cause of the problem.

I'll have to go back and read his original posts but I think he stated that he already covered most of the typical causes of over heating??

[Joe L.]

Re: Head to Block Parallel flow vs series flow cooling

Barth,

Can you explain what a Dewitt direct fit radiator is compared to the original type radiator. IMO, if the car overheats at speed there is a problem with heat transfer in the radiator but Stu's right about the thermostat so think about that. What's the V/A control part #, how much engine vacuum at idle? Does the lower radiator hose have a wire to keep from sucking closed?

Find a shop that can double check that radiator cap to make sure it holds 15psi., repro caps are junk...If the car has a copper radiator find a nice AC stainless rc-15 cap, the ones that had AC stamped into them which is before they were outsourced. Also check archives and double check your temperature gauge for correct temp readings.

You have to start with simple things first.

Tim-----


A DeWitts Direct Fit radiator is a radiator which will replace the original radiator with no modifications whatsoever and, in most cases, will appear very similar to the original radiator. In the case of this application, the original radiator is a copper/brass unit with 26" core. The Direct Fit is an aluminum radiator of the same size and configuration as the original. It has about 20-30% more cooling capacity than the original radiator.

[Joe L.]

Re: Head to Block Parallel flow vs series flow cooling

FEL-PRO 1057 is the head gasket.

I had the casting plugs out when the block was cleaned. I didn't notice anything special but not sure I would know what I was looking for. I do not know the history of the block except it had thrown a rod and one of the cylinders had been sleeved. I had a machinist bore it out .030, install cam bearings, and install the casting plugs. He didn't mention anything about the presence of filling. He said the sleeve had been correctly installed and was a permanent fix.

There is no pinging that I can hear but the car has sidepipes so I'm not sure I would hear it. The damper is original and appeared tdc with the piston when assembled. Before I do anything major like removing a head I plan to pull the plugs and double check tdc.

"Purist" is a relative term. The car had been customized back in the 70's and I've spent a lot of time making it correct. The block is not original to the car but the block and other major engine components are correct. I would hate to do anything that makes the engine compartment look incorrect, but I want to drive and enjoy the car.

again thanks for any information

Barth-----


The Fel-Pro 1057 should work just fine for your application. It's actually designed for a larger bore size than you have, but it should work just fine for your 4.280" bore.

If the block had been "filled", it would have been readily apparent with the side freeze plugs removed. Basically, if it had been "filled", it would have appeared "solid" behind the freeze plug. These are not all that common but they're out there. It's used by drag racers to "stabilize" and strengthen the block. Cooling is not an issue since these engines run for only short periods of time.

[Barth C.]

Re: Head to Block Parallel flow vs series flow cooling

I'm going back and verifying everything is working properly.

I friend suggested I do a cooling system pressure check to determine if I have a leak inside the engine. I borrowed a Stant cooling system pressure tester but unfortunately it does not properly fit the DeWitt radiator neck which appears slightly narrower than OEM.

Has anyone else experience this issue with DeWitt radiators. Or can anyone recommend another method of pressure testing the cooling system without attaching a pump in place of the radiator cap?

thanks

[Dick W.]

Re: Head to Block Parallel flow vs series flow cooling

I'm going back and verifying everything is working properly.

I friend suggested I do a cooling system pressure check to determine if I have a leak inside the engine. I borrowed a Stant cooling system pressure tester but unfortunately it does not properly fit the DeWitt radiator neck which appears slightly narrower than OEM.

Has anyone else experience this issue with DeWitt radiators. Or can anyone recommend another method of pressure testing the cooling system without attaching a pump in place of the radiator cap?

thanks

Never had any problems with his radiators. Are you using the correct adapter, my kit has either three or four adapters

[Steven B.]

Re: Head to Block Parallel flow vs series flow cooling

Just one small comment: About a year ago, I had a major overheat that pushed lots of coolant out of the overflow. This happened shortly after I had changed the coolant and replaced the thermosat with a Robertshaw "type". This piece was an "Mister Gasket" item (made in China) purchased at my local Auto Zone store. I had driven about 20 miles to a Corvette show and, once there, had to idle a lot in line at the entrance. As expected, my engine did heat up some (about 210*), but nothing unusual. On my return trip, the engine seemed to be running fine and on approach to my exit I punched it to get around several slower cars in order to be in my exit lane. At the stop light all hell broke loose and I was dumping coolant all over - or so it seemed. Everything checked out ok once I got home (another 2 miles). As it turned out, my new thermostat had stuck closed after my engine cooled down at the show and did not reopen. Having an SHP small block, I had enough coolant circulation in the engine with the water pump by-pass to keep the engine at a normal temp at a low cruising speed, but once I accelerated up to a higher speed the engine over heated.

Bottom line: check your thermostat. These imported stuff just aren't up to the task. At least pull it out and do a stove top test in a pot of water with a good meat thermometer (don't tell ma ma).

Stu Fox

EXACT same thing happened to me in UP of MI.