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Old 03-28-2020, 03:20 PM   #1
pebo va
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Default Overheating Flathead

Last fall I had a 1936 flathead rebuilt stock by an experienced engine machine shop. It has run hot since it was installed. Sometimes it overheats in 3 miles and sometimes it will run 1-2 hours before overheating. It starts up easily, runs strong with good power just as it should. But when steam comes out the radiator cap I need to add 1-2 gallons of water to bring the level up. It is not running out the overflow and there are no thermostats. When the radiator is full the water moves nicely as seen at the cap. When overheated and steaming I can hear gurgling and boiling noises. The radiator has been flow tested twice. All good. The water pumps are news Fryers. The distributor is correctly timed. I have changed the head gaskets twice. No help. It appears that somewhere internally the water jacket is leaking into the exhaust system. I added a bottle of Moroso's Ceramic block seal: no help. On my last road test last week on a 55 degree day, I ran at 35-55 MPH over country roads and at about 90 minutes it started steaming. While running, I then measure the temperature at various points with a laser gun. Aluminum heads on both sides 148-150 degrees.Lower hoses about 160. Water pumps 206 and 204. Front of the block 204. Top tank on radiator 204. Exhaust manifold 290 and 300.
Before I go to the trouble of pulling the engine is there something I am missing?
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Old 03-28-2020, 03:25 PM   #2
flatjack9
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Default Re: Overheating Flathead

Any sign of water under the engine when parked? Remove the exhaust manifolds, run the engine, watch for water.
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Old 03-28-2020, 03:31 PM   #3
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Default Re: Overheating Flathead

You can purchase a cooling system test kit for about $40, may be helpful to help narrow your search and rule out an internal leak.
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Old 03-28-2020, 06:13 PM   #4
pebo va
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Default Re: Overheating Flathead

Thanks.
I used a test kit to see if there was any exhaust coming out the radiator top. Negative.
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Old 03-28-2020, 06:15 PM   #5
pebo va
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Default Re: Overheating Flathead

Great suggestion. Although a pain to do with pipes attached, easier than removing the engine.
Thanks.
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Old 03-28-2020, 06:28 PM   #6
Ken/Alabama
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Default Re: Overheating Flathead

Disconnect the upper hoses from the water pumps .Pour water in the necks on the pumps to bring the water level up to the top. Remove the belt so the pumps don’t turn .Crank it up and look for bubbles . Might be a leaky head gasket.
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Old 03-28-2020, 06:39 PM   #7
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Default Re: Overheating Flathead

how much was the eng bored was it over 060
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Old 03-28-2020, 06:53 PM   #8
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Default Re: Overheating Flathead

Does it have the correct head?
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Old 03-28-2020, 07:38 PM   #9
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Default Re: Overheating Flathead

I'm not an expert on Flatheads, but running without thermostats probably isn't helping. If the "coolant" flows too quickly through the engine (because t-stats removed) the heat isn't going to get pulled out of the block and heads.
Just an observation. Best of Luck. Mike


Any white smoke or sweet smell from exhaust pipes?
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Old 03-28-2020, 08:07 PM   #10
philipswanson
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Default Re: Overheating Flathead

Simple, pressure check system to see where the water is going.
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Old 03-28-2020, 09:36 PM   #11
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Default Re: Overheating Flathead

My 35 was doing the same thing. At first I did have a head gasket leak but that was corrected. The radiator was good looking radiator so just had it cleaned. Was not bad at hiway speeds, but when I slowed down thru town it would turn into a steam jenny. I took it to rad. shop to have it rodded out. Shop owner told me about a new core called HTX Special which I had installed. much more efficiant. Car runs 180 degree at all speeds. The new core has more cores and finer fins
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Old 03-28-2020, 09:37 PM   #12
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Default Re: Overheating Flathead

With your situation, (new rebuild and good radiator) and all possible checks reveal nothing that helps, quite often the block was ONLY hot tanked or shake and baked, NOT acid stripped.
Nothing will remove the rust and scale except acid stripping. Usually the most build up is around the base of the rear 2 cylinders.
These engines could run stone cold (figuratively speaking) from the factory.
They should do the same after rebuild. (if done right)
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Old 03-28-2020, 10:23 PM   #13
John R
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Default Re: Overheating Flathead

Some initial thoughts:

The radiator appears to be good. It is lowering the coolant temperature (top to bottom) by about 40 degrees. However, it would have been a bit more accurate to measure at the bottom tank of the radiator rather than on the lower hoses.


You stated "...the water jacket is leaking into the exhaust system." This is a very important point. Is this diagnosis is based on copious amounts of water coming out the tailpipe or something else? In a top-mounted water pump and unpressurized system, you'd expect to find some evidence of exhaust in the water but your test was negative.



The "gurgling and boiling noises" are not surprising given that it's already overheated and 1 - 2 Gallons low on water.
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Old 03-28-2020, 10:47 PM   #14
deuce_roadster
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Default Re: Overheating Flathead

Are the heads aluminum or cast iron? Have you retorqued the heads AT LEAST 3 times? Do you know how you are supposed to do that?
Good luck.
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Old 03-29-2020, 11:17 AM   #15
pebo va
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Default Re: Overheating Flathead

It was .030 over.
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Old 03-29-2020, 11:23 AM   #16
pebo va
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Default Re: Overheating Flathead

Yes, torqued in the correct order three times after cooling down each time.
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Old 03-29-2020, 11:32 AM   #17
pebo va
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Default Re: Overheating Flathead

My invoice only says block cleaned and magna-fluxed. I will ask about the acid stripped.
But where is the water going??
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Old 03-29-2020, 11:33 AM   #18
pebo va
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Default Re: Overheating Flathead

How do you pressure check the cooling system?
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Old 03-29-2020, 11:33 AM   #19
Kurt in NJ
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Default Re: Overheating Flathead

The radiator on my 36 would flow the diameter of the hoses---- but when I looked in with a borescope 25% of the tubes were clogged ----
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Old 03-29-2020, 01:42 PM   #20
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Default Re: Overheating Flathead

Guys, don’t overlook this symptom in the original post:
“Sometimes it overheats in 3 miles and sometimes it will run 1-2 hours before overheating.”
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