Porsche Crankcase sealant

Sealant tubes and tub

What to use and why

Crankcase sealant and which ones to use is a hotly debated topic among many Porsche forum posters. Over the years I have had many engines in front of me sealed with all kinds of different sealants. Often sealants have been recommended by “how-to” books or by “online experts” as better than the factory sealants. More often than not I have found these other sealants to be very poor in nature. These incorrect sealants often cause failures, resulting in catastrophic engine damage.

RTV Sealant left in engine case
RTV sealant contaminating the oil thermostat in a 911 engine. This caused a blockage in the main gallery resulting in main bearing failure. Pieces of RTV can been seen in the oil passageways

General Sealing Properties

First, before you apply any sealant, the gasket or sealing surface needs to be clean, dry, and flat. The purpose of a gasket is to seal up any differences in the two mating surfaces. The air-cooled Porsche models have two types of surfaces, one that uses a mechanical gasket and one that uses a chemical gasket. Whether the gasket is a mechanical or chemical one, the two surfaces need to be in good condition. A mechanical gasket is made of paper, composite, copper, or a cork type gasket. Where there is no mechanical gasket, they use a chemical type gasket. Remember that paper gasket areas are designed to be used without any sealants and the thickness of the gasket is part of the design dimensions.

RTV Silicone remnants left on the crankcase through bolt flanges. Silicone had also entered the oil system and was found through out the engine. Crankcase through bolts do not need any sealers as oil is controlled by O-rings.

The Factory Recommendations

Porsche has used three different types of crankcase sealant over the years. The first sealant used was the “Curil K”, this sealant was the only one available at the time of its use. In the 1970’s Porsche switched to the “Loctite 573” sealant and finally in 1978 settled on “Loctite 574”

Curil K Sealant

Curil K is an artificial resin-based sealant, that dries by solvent evaporation in about 5-10 minutes. It is a messy sealant that sticks to everything and excess sealant must be keep from dripping into the engine case. The sealant must also be kept away from any oil passageways as it can restrict or block them. The other issue is clean up, this type of sealant must be cleaned using a solvent based cleaner. All traces of the sealant must be removed before new sealant can be applied.

Curil K sealant is now called Curil K2

Properties of Loctite 573 Sealant

573 Loctite is a thixotropic and anaerobic type of sealer/adhesive. This means that the viscosity of the sealer will change under pressure and only sealant that is contained within the joint flanges will cure. Being anaerobic means that any sealant that is pushed outside of the joint flange will not harden. This means that it cannot restrict any oil passageways that it may end up in. Excess sealant will dissolve into the oil and come out at the first oil change.

The sealant does not need to be removed during service. When the two flanges are separated for service, only oil and debris need be removed. When applying the new bead of 573 it will dissolve any remaining cured sealant, essentially re-animating it.

Because of its viscosity being thixotropic, it is able to allow for a very close metal on metal bond. Only filling those areas of imperfection in the joint. This results in very stable dimension between to the two flanges. When the case halves are joined using loctite 573 it is not only sealing the joint, but also chemically bonding the two halves together. This makes the case assembly as if it is one solid piece, which helps the case maintain stability.

Loctite 573 sealant has excellent chemical resistance to engine oil. After 1000 hrs. @125° C in engine oil it is still at 100% of its original strength. It also has good resistance to gasoline and water/glycol.

Loctite 573
Loctite 573

574 Sealant

Five Seven Four has all the same basic properties of 573. However, the 574 compound is more resistant to heat aging than 573. The compression and lap shear strength of the 574 is considerably higher than the 573. This makes for a better adhesion of the two flanges. 574 Loctite can be told apart from 573 by its color. 574 is orange in color and 573 is Green in color.

Loctite 574 Flange sealant
Loctite 574 Flange Sealant

Other sealants people use.

For some reason people feel the need to not use the factory specified sealer. Over the years I have seen the effects of different sealers being used and none of them have been good. Recently I looked at the major online forums to get together a list of sealants that people using these forums are using. There are also a number of how-to manuals that list the same sealants.

  • Permatex Aviation Sealant
  • Three Bond 1211
  • Three Bond 1104
  • Yamabond (Basically the same as three bond 1211)
  • Dirko
  • Dirko HT

People are using other sealants, but these are the major ones. Other than the Permatex all of these sealants are RTV type compounds.

Permatex Aviation Sealant

This sealant is very similar to the Curil K sealant, both are artificial resin-based sealants. This sealant will somewhat dry outside the flange joint. There should be no excess sealant in the engine. All traces of sealant must be removed before re-application during service. Requires solvent type chemicals to remove. Basically, Porsche abandoned this type of sealant back in the 1970’s.

Permatex Aviation Gasket cement

Three Bond 1211

Three Bond 1211 is an RTV-silicone liquid and is white in color. The only good thing about this sealant is the long working time of about 30 minutes. However, using this type of sealant will result in multiple issues. Firstly, any sealant that is pushed outside of the joint will harden. Pieces of sealant can break off inside the engine and become lodged in small passageways. If you block a passageway, you can have damage from oil starvation.

More Problems with RTV Type Sealants

Next, RTV sealants tend to create a shimming effect on the flange joint. This is because the sealant begins to harden as soon as it hits the air. This will result in the joint thickness being larger as there is no metal on metal contact. This has a direct effect on bearing clearances in the 911 and 356/912 engine cases. A change in bearing crush and bearing clearance can result in lower oil pressures.

Another factor to consider is the chemical resistance. The chemical is not as good as the anaerobic sealers. When exposed to engine oil, changes can be seen within 24 hours. Also, silicone type sealants are not fuel compatible at all, so any fuel entering the joint area can result in early failure

A big problem with RTV sealants is they will not stop the case halves from moving. Unlike the loctite 573 or 574 products, RTV sealants will not make the two case halves as one. With RTV sealants there is room for flex. On a high-performance engine that develops high torque, this can result in main tunnel misalignment.

RTV sealants require a lot of work to clean up at service intervals. Solvents will not easily remove RTV sealants. You need to mechanically scape off this type of sealant. Remnants of old sealants can become trapped in small passageways resulting in failures after a repair.

ThreeBond 1211
ThreeBond 1211

Three Bond 1104

Listed as a synthetic rubber liquid gasket system designed for larger surface gaps. This sealant will dry to a slightly tacky condition. It also has all the same drawbacks of the Three Bond 1211 when used in the Porsche applications. The one bonus of this sealant is it lists a resistance to gasoline though I would still not use anywhere in an air-cooled Porsche application. When you order three bond 1104 in the USA, you will be shipped Three Bond 1194. This is due to 1104’s lead content.

Three Bond 1194
ThreeBond 1194


I could not find a true technical data sheet on this product. The only sheets I could find were the MSDS sheets. These sheets list it as a room temperature vulcanizing silicone sealant. Yamabond is interchangeable with the Three Bond 1211. All the same draw backs of the 1211 apply to this product and I would not use in a Porsche application.

Yamabond RTV sealant
Yamabond RTV Sealant

Dirko and Dirko HT

Dirko and Dirko HT are both RTV Silicone sealants. The difference between the two sealants is that the HT is listed as a high-temperature version, more suited to exhaust systems. Because both sealants are still RTV based sealants they will suffer the same issues and drawbacks that the Three Bond 1211 does. Do not use either of these sealants in Porsche engine applications.

Dirko RTV Sealant by ElRing


Hands down the Loctite 574 is the best sealant for the job. It offers superior strength and resistance to engine oil. Any loctite outside of the joint does not dry. So you can wash away any excess. You can get a tighter metal on metal joint that will maintain correct main bearing clearances. It makes for faster clean up and repair times during scheduled services. It will form a permeant bond between the 2 case halves making for a more stable motor. Finally, it has excellent heat aging properties, meaning that it will outlast the RTV type sealants.

There is nothing wrong with looking for something better or just in case 574 is not available. We can see that Porsche was able to transition from Curil K, to 573, and finally settling on the 574 product.

The Future of Loctite

In the spirit of looking for something better, in 2016 Loctite came out with Loctite 518. This is still an anaerobic type sealer and is in the same class as the 574 sealers. It has higher temperature ratings and looks like it would be a good substitute for the 574 if required.

I have attached all of the links to each of the individual Technical Data Sheets for all of the sealants I have spoken about in this article. I encourage you to read them for yourself and compare the differences. The factory puts in a huge amount of time and research when designing the product. The factory specifications are rarely wrong and if used correctly will guarantee results. The only reason why I would move away from a factory product would be because it is no longer available.

573 Loctite Technical Data Sheet Click Here, 574 Loctite Technical Data Sheet Click Here, 518 Loctite Technical Data Sheet Click Here

Curil K Technical Data Sheet Click Here, Permatex Technical data Sheet Click Here, Three Bond 1211 Technical Data Sheet Click Here

Three Bond 1104 Technical Datasheet Click Here, Yamabond MSDS Sheet Click Here, Dirko Technical Data Sheet Click Here


7 thoughts on “Porsche Crankcase sealant”

  1. Henry Smith use Three bond product some place on old 911 case, he have build many Porsche Engine. Ivan Boger, Norway.

    1. Ivan,
      just because one aftermarket builder uses threebond sealer does not make it the right choice.Porsche has spent millions of dollars in development and countless amount of hours in product testing to choose the right sealer for their application. In thirty years of building Porsche engines as a factory trained technician i have never seen an assembly failure due to the use of loctite 574 case sealer. I have also never seen an engine failure due to the use of loctite 574 case sealer. In those thirty years i have seen numerous engine failures due to use of silicone sealers including Threebond. Every true Porsche engine builder that i know and respect all use the factory sealant, loctite 574. I know who Henry is and as far as i know he has not had any formal training in the Porsche product. this does not mean that he knows nothing but the fact that he is using a product that i know is not right for the application calls into question every other thing he does. It is ultimately up to you as the engine builder to use whatever you think is right for you. My biggest suggestion to you if you plan on building an engine that you expect to last for many years to come then use the product it was designed to be built with.

  2. Kurt,
    What’s the best approach to seal the number eight bearing on a magnesium case engine?

    1. Tim,
      I assume you are talking about when installing the number 8 O-Ring when assembling the case? If so this Oring should be assemblied dry and without any crankcase sealant at all. There is also a pressure relief groove in the number 8 bearing that needs to be keep clear of any sealant as if it becomes blocked or restricted it can cause the Oring to leak. The only thing you could add to the oring would be a small amount of silcone grease to help the oring move in the groove so that it will settle in to a relaxed position. This is also something we cover extensively in our engine rebuild course. https://klassikats.com/index.php/course/engine-rebuilding-for-porsche-2-0l-2-2l-2-4l-and-2-7l/

  3. Stewart Harper

    Im finding it difficult to source 574 for my aircooled case halves,so 518 would be a good alternative i assume?

    1. Stewart, yes 518 will work just fine. make sure that everything is clean and dry and that the case halves are completely ready for assembly before applying the sealant to the case.

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