Why are my Head Gaskets Leaking?

[Cuda Cody]

I'm building a stock 1970 426 Hemi engine and the head gaskets are sweating (leaking might make it sound like more then it is) and I can see the antifreeze right at the head gasket.

Here's what has been done.  The engine was built to stock specs and the machine shop that does the work has done a few of these Hemi's for me.  Stock 1970 block with new mopar performance heads.  We broke the engine in on the dyno and it runs and makes perfect and makes great power.  After I have all my engines broken in on the dyno I always run them on my run stand for about 20 minutes then let them cool off.  I do this over and over for about 5 or 6 times to see if any leaks are going to show up before I go on to the next step of painting the engine.  Well, this time I noticed the head gaskets were showing small amounts of green antifreeze on both heads.  It only leaks when it's running and at full operating temperature.

I took the heads off and put new head gaskets on.  Put it back on my run stand and the head gaskets still leaked a small bit.  Took the engine back to the machine shop and they pressure tested it.  It help 70 PSI for 24 hours.  So I put it back on my run stand and put the leak stopped in the antifreeze (the $35 bottle that can mix with antifreeze).  I've run it 3 times for 20 minutes and the leak seems to be a little smaller or less then originally, but I still see a green tint around the head gasket. 

First photo shows the engine cold before start up.  The rest of the photos are after it run for 20 minutes.

Why does this only happen when it's hot and what could be causing this? 

[1 Wild R/T]

Pretty common leak for Hemi's...  I personally have known of four that leaked there..... Many repairs have been attempted.... The final answer Loctite 518...

https://www.amazon.com/dp/B000FONONG/ref=asc_df_B000FONONG5172538/?tag=hyprod-20&creative=395033&creativeASIN=B000FONONG&linkCode=df0&hvadid=192217280775&hvpos=1o1&hvnetw=g&hvrand=14764572926818076141&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032443&hvtargid=pla-306605440538

Once you use 518 you'll find many uses for it, it's a different kind of sealant, LOCTITE 518 cures when confined in the absence of air between close-fitting metal surfaces. It is an anaerobic gasketing material designed for use on rigid iron, aluminum, and steel flanged mating surfaces. Fills gaps up to 0.25mm and forms a flexible, chemical-resistant seal that will not tear or decay. Parts disassemble easily, even after extended service.

[Cuda Cody]

   Thank you for the fast reply.  I've never used that loctite on head gaskets before.... can you explain a bit more about the proper way to use the sealant?  Do I take the heads off and get a new set of gaskets?  Then put it on the gaskets (both sides?) around all the openings? 

My buddy has build a bunch of engine in the 80's said he always used Indian Head Shellac.  Does that do the same thing as the loctite 518?

[1 Wild R/T]

Indian Head is old tech, it's basically a glue, use it & have to disassemble it the future & you'll bee scraping forever....

518 isn't new but the first time I used it was in the mid 80's when Ford was supplying it for a TSB... They had a common head gasket leak on Tempo's.... An 16" long area of the head that wasn't well supported was leaking allot of oil, design defect, they should have had a row of small bolts... The 518 100% solved the issue... Back then a little .2 oz tube was over $20... So for Ford to use it you know they had a hard to solve problem...

518 spread on exposed metal will stay soft & jelly like for months, take away the air & it sets up like a rubber film similar to the material a ballon is made of.. It will bond to many surfaces but if you have to take it apart it will easily peel off....

I don't use thermostat or water pump gaskets anymore... Any close fitting metals I grab the 518... I have it in a caulking tube.....

In your application I would put a small bead around the water passages and the oil passage on both sides of the gasket but stay back from the combustion chamber...

[RUNCHARGER]

I can't tell what headgaskets you're using. I have always used the stock steel shim headgaskets and coppercoated both sides of them when using iron heads and block, no leaks in 37 years. There isn't much iron between the water passages and outside edge of the casting at that location and it's pretty finicky.
I haven't used the 518 for that application but if Wild says it's good I'm sure it is.

[Cuda Cody]

Thanks guys for the help!     Glad I asked. 

I've been using the $200 Mopar Hemi gaskets.  Are there better ones I should be using?  Looks like I'm going to take the heads off 1 more time to get this thing perfect.  So if there is a better gasket I would love to know now before I order a set.

[RUNCHARGER]

This is what I use but I don't know what thickness you need or the deck finishes you have.

http://www.manciniracing.com/mophemsteels.html

[HP_Cuda]

Cody

This seems to be happening because both surfaces are not mating up 100% due to variances in the head surface and block surface. I have used 518 before but it was used to hold a bearing in place - it worked great but you need to ensure there is an absence of oxygen for it to dry, they sell it in small amounts and it's not cheap.

If you are pulling it apart again, I'd check your head surfaces and the block surface. You may need to deck them to move past this point. I would never use the leak seal products because they will clog up your radiator and generally cause you more pain than what they are worth.


B

[1 Wild R/T]

Cody

This seems to be happening because both surfaces are not mating up 100% due to variances in the head surface and block surface. I have used 518 before but it was used to hold a bearing in place If it was used to hold a bearing in place then you used to wrong product... Loctite 618,620 are bearing retaining products it worked great but you need to ensure there is an absence of oxygen for it to dry, they sell it in small amounts and it's not cheap. It's a heck of a lot cheaper than it use to be...

If you are pulling it apart again, I'd check your head surfaces and the block surface. You may need to deck them to move past this point. It's a common leak area on Hemi's freshly decked, freshly milled, still they leak.... I would never use the leak seal products because they will clog up your radiator and generally cause you more pain than what they are worth.


B

[Cuda Cody]

The block was milled and checked when built.  We even double checked the heads as that's what I thought it was.  This is the main reason I put them on the run stand after I have them broke in on the dyno.  Finding little things like this is much easier to handle now instead of in the car.  And I'm like you when it comes to using the stop leaks stuff.  I don't use it in a car, but on my run stand that uses a ford radiator I'm okay seeing what it will do.  Din't solve the problem so I will be pulling the engine apart again to use the loctite on the gasket.  I'll update this thread once I have the new gasket in place.

[JS29]

This may sound silly but check the lingth of your bolts, heard of it happening before!!!      

[cudabob496]

why not alumaseal leak stop in coolant??????

[RUNCHARGER]

The dowels can hold a head up too but I think he just has to treat the gaskets. The surfaces will tell the story, there's probably a 1/8" of sealing area in that spot at times

[Cuda Cody]

I tried it on the run stand and it helped, but did not solve it 100%.   

why not alumaseal leak stop in coolant??????

[HP_Cuda]

I found out it was loctite 609 green anerobic retaining compound. Dang fading memory.

Cody I guess you could try to try to seal that area but here's another idea - MLS gaskets do the job really well dunno if you already tried this route?

https://www.summitracing.com/search/part-type/head-gaskets/make/chrysler/engine-family/mopar-hemi-early?ibanner=MobileSwitchNo

Cody

This seems to be happening because both surfaces are not mating up 100% due to variances in the head surface and block surface. I have used 518 before but it was used to hold a bearing in place If it was used to hold a bearing in place then you used to wrong product... Loctite 618,620 are bearing retaining products it worked great but you need to ensure there is an absence of oxygen for it to dry, they sell it in small amounts and it's not cheap. It's a heck of a lot cheaper than it use to be...

If you are pulling it apart again, I'd check your head surfaces and the block surface. You may need to deck them to move past this point. It's a common leak area on Hemi's freshly decked, freshly milled, still they leak.... I would never use the leak seal products because they will clog up your radiator and generally cause you more pain than what they are worth.


B