The Ford Barn

JamesK44 · 12-07-2025, 12:32 PM

I have a 1940 coupe and the heater core has a small leak.What are the pros and cons on using KSeal to try to stop a leaking heater core? As far as I know no sealer has been used before. I’m concerned about engine water passes and the core itself due to the size of the core openings. Any advice would be appreciated. Thanks in advance and remember Pearl Harbor today.

tubman · 12-07-2025, 01:48 PM

I think it depends on how long the heater core has been in there. If it's an original or of unknown age, I'd remove it and repair or replace it. If it's a fresh restoration, I might chance the stop leak (only because the car has a rubber floor mat)..

Kube · 12-07-2025, 02:01 PM

Sealers designed for radiators work best under pressure. Your '40 runs zero pounds so the sealer stays in suspension longer and most will simply settle at the lowest spot in the system.
You may get lucky and it may work. It may.
Me? The '40 Fords often ran on the warm side. I'd not do anything that may make that worse.
I'd pull the heater and have a shop solder the leak.

Bored&Stroked · 12-09-2025, 10:33 PM

I'm with Kube - the last thing you need is a situation where you plug up your radiator - trying to fix a small leak in the heater core. Pull the core and get it repaired or replaced.

JamesK44 · 12-10-2025, 10:31 AM

Thank you all for your reply to using Kseal to repair a small leak on a heater core. You convinced me that I need to remove the heater and take the core to a repair shop. The issue of low pressure and the sealant settling before it actually got to the heater core was of concern and Kube, in my opinion nailed the answer Thanks to all.

19Fordy · 12-10-2025, 12:49 PM

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James, you may be able to solder the repair yourself. Check out the videos on You Tube.

mfirth · 12-10-2025, 05:15 PM

19Fordy makes a good point too. If the leak is easy to access you can do it. I'd use 50/50 solder and the area MUST be clean.

corvette8n · 12-11-2025, 08:54 AM

My shop teacher always said whatever you want to solder, needs to be physically and chemically clean.

leon bee · 12-11-2025, 11:03 AM

Muriatic acid can be your friend for cleaning old stuff for soldering.

Don T · 12-11-2025, 05:59 PM

Slightly of topic but just a note about sealers. I could not find the old go to, Bars Leaks in my local store so I grabbed a alternate of the shelf. It could best be described as 1/4" long thick hairs and the product did stop the rad leak. However when the rad pressure cap did its normal thing by lifting off its seat to drop rad pressure these solid little hairs settled on the seating surface of the rad. As these little guys were quite hard they prevented the cap from sealing to the rad. It was like running without a rad cap, even if the coolant was left low the little guys did their thing. The only way I could get rid of them completely was to pull the rad and the water pumps. As I had thrown the package they came in out I have no idea what the brand name was.

51woodie · 12-12-2025, 12:09 PM

When I bought my '46 Coupe ten years ago, it's general condition was very good. However, being an untrusting soul and longtime mechanic with lots of experience in leaking radiators and heaters, I pulled both and sent them out for cleaning and pressures testing. I also replaced the rad and heater hoses, as they were "baked". It took about fur hours all in, to r&r, refill the coolant and leak check. Better to do a proper fix, than one that may fail in the short term

petehoovie · 12-12-2025, 12:29 PM

Quote:

Originally Posted by 51woodie

When I bought my '46 Coupe ten years ago, it's general condition was very good. However, being an untrusting soul and longtime mechanic with lots of experience in leaking radiators and heaters, I pulled both and sent them out for cleaning and pressures testing. I also replaced the rad and heater hoses, as they were "baked". It took about fur hours all in, to r&r, refill the coolant and leak check. Better to do a proper fix, than one that may fail in the short term

aussie merc · 12-12-2025, 08:46 PM

for my 2 cents do it right do it once sealers are are just a get you home option often the leak is a warning of bigger issuse that you will need to fix later anyway and often with something of that age you find the air side of the core is clogged or corroded and requires cleaning for it to work effectively .Any good rad shop will be able to repair or even replace the core and tanks if its needed and you think its worthwhile when the cold starts to bite

12-07-2025, 12:32 PM	#1
JamesK44 Junior Member Join Date: Dec 2025 Posts: 3	Heater core leak I have a 1940 coupe and the heater core has a small leak.What are the pros and cons on using KSeal to try to stop a leaking heater core? As far as I know no sealer has been used before. I’m concerned about engine water passes and the core itself due to the size of the core openings. Any advice would be appreciated. Thanks in advance and remember Pearl Harbor today.

12-07-2025, 01:48 PM	#2
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 12,133	Re: Heater core leak I think it depends on how long the heater core has been in there. If it's an original or of unknown age, I'd remove it and repair or replace it. If it's a fresh restoration, I might chance the stop leak (only because the car has a rubber floor mat)..

12-07-2025, 02:01 PM	#3
Kube Super Moderator Join Date: May 2010 Location: Wisconsin, USA Posts: 10,541	Re: Heater core leak Sealers designed for radiators work best under pressure. Your '40 runs zero pounds so the sealer stays in suspension longer and most will simply settle at the lowest spot in the system. You may get lucky and it may work. It may. Me? The '40 Fords often ran on the warm side. I'd not do anything that may make that worse. I'd pull the heater and have a shop solder the leak. __________________ "I can explain it for you. However, I can't understand it for you".

12-09-2025, 10:33 PM	#4
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,906	Re: Heater core leak I'm with Kube - the last thing you need is a situation where you plug up your radiator - trying to fix a small leak in the heater core. Pull the core and get it repaired or replaced.

12-10-2025, 10:31 AM	#5
JamesK44 Junior Member Join Date: Dec 2025 Posts: 3	Re: Heater core leak Thank you all for your reply to using Kseal to repair a small leak on a heater core. You convinced me that I need to remove the heater and take the core to a repair shop. The issue of low pressure and the sealant settling before it actually got to the heater core was of concern and Kube, in my opinion nailed the answer Thanks to all.

12-10-2025, 12:49 PM	#6
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 11,916	Re: Heater core leak Sponsored Links (Register now to hide all advertisements) James, you may be able to solder the repair yourself. Check out the videos on You Tube.

12-10-2025, 05:15 PM	#7
mfirth Senior Member Join Date: Aug 2017 Location: ohio Posts: 1,165	Re: Heater core leak 19Fordy makes a good point too. If the leak is easy to access you can do it. I'd use 50/50 solder and the area MUST be clean.

12-11-2025, 08:54 AM	#8
corvette8n Senior Member Join Date: Jul 2010 Location: 36 miles north of Albany NY Posts: 3,323	Re: Heater core leak My shop teacher always said whatever you want to solder, needs to be physically and chemically clean.

12-11-2025, 11:03 AM	#9
leon bee Senior Member Join Date: May 2017 Location: Arkansas Posts: 768	Re: Heater core leak Muriatic acid can be your friend for cleaning old stuff for soldering.

12-11-2025, 05:59 PM	#10
Don T Senior Member Join Date: Mar 2017 Location: Alberta Canada Posts: 108	Re: Heater core leak Slightly of topic but just a note about sealers. I could not find the old go to, Bars Leaks in my local store so I grabbed a alternate of the shelf. It could best be described as 1/4" long thick hairs and the product did stop the rad leak. However when the rad pressure cap did its normal thing by lifting off its seat to drop rad pressure these solid little hairs settled on the seating surface of the rad. As these little guys were quite hard they prevented the cap from sealing to the rad. It was like running without a rad cap, even if the coolant was left low the little guys did their thing. The only way I could get rid of them completely was to pull the rad and the water pumps. As I had thrown the package they came in out I have no idea what the brand name was.

12-12-2025, 08:46 PM	#13
aussie merc Senior Member Join Date: Sep 2016 Location: sydney australia Posts: 1,156	Re: Heater core leak for my 2 cents do it right do it once sealers are are just a get you home option often the leak is a warning of bigger issuse that you will need to fix later anyway and often with something of that age you find the air side of the core is clogged or corroded and requires cleaning for it to work effectively .Any good rad shop will be able to repair or even replace the core and tanks if its needed and you think its worthwhile when the cold starts to bite