Cylinder Head Studs - A cautionary note.

[chris_seven]

I was idly looking at EBay yesterday when a set of Porsche 911 'Dilavar Style' Painted Cylinder Head Studs caught my eye.

They have a price of $14.99 each and are said to be made of A286 Steel and made in the USA.

They claim to have very similar expansion characteristics to Dilavar but unlike Dilavar they are Magnetic.

They wisely suggest that they are used in sets and not mixed with conventional steel studs.

As a Metallurgist with more years of experience than I am prepared to admit to having I cannot help myself but have to comment.

The company selling these studs clearly has a very poor understanding of Metallurgy 101.

A286 is in fact an Austenitic Precipitation Hardening Stainless Steel it is used commonly in the hot section of Jet Fighter Aircraft Engines and it has excellent corrosion and strength characteristics at temperatures around 1300 degF.

I can't imagine why it would it would be cheaper than Dilavar. It s normally produced in AMS Grades which means it would be 'flight certified' and eye wateringly expensive.

I am sure Porsche had a specialist German Alloy producer develop Dilavar due to the high cost of A286 which has been around for longer than Dilavar.

This, unhappily is not the only problem.

A286 does have similar expansion to Dilavar of this there is no doubt.

Both a 286 and Dilavar are both Austenitic Alloys and this means that they must both be non-magnetic.

There is no way that A286 with approximately 25% Nickel and 15% Chrome could ever be magnetic.

It is physically impossible to manufacture or treat an alloy with this composition to become magnetic as its basic crystal structure won't support ferromagnetism.

If they are made from A286 they can't be magnetic and if they are magnetic they can't be A286 - simply due to very basic metallurgical 'rules'

If the studs are magnetic and hence either ferritic or martensitic they will not be high expansion again for reasons of very basic crystallography.

There is another potential issue.

If we assume that the statement about the studs being magnetic is just an error and they are made from A286 I still have concerns over the properties of the material.

A286 can be purchased in very high strength conditions but at the highest strength is susceptible to corrosion pitting in the presence of chlorides. The corrosion pits then allow fatigue cracks to form and failure follows rapidly.

This is the main reason for Dilavar Failures.

The fact the studs are painted seems to hint that they may be a high strength 'temper'.

There was also a time a few years ago when some aftermarket 'Dilavar' studs found their way into the supply chain with disastrous results. I believe these aftermarket studs were also painted.

A286 is also used in marine environments but in order to meet NACE requirements it strength is limited to around 120ksi with a yield strength of 90ksi.

I believe that the stresses in the stud on an engine using Nikasil Cylinders would be approaching 90ksi and I would worry that if overtightened during assembly they could yield as the engine warms up.

I am sorry to bang on but Studs on 911 engines are, as we all know, are very important and the basic facts used in the description of these studs is inaccurate and on that basis and until some of the questions are answered I wouldn't touch them with a stick.

[SIMI BOB]

Chris,
Thanks for sharing your amazing knowledge of metals and metal processes with us. I always enjoy your posts.
Bob B

[Christian Guthrie]

Great post! Thanks for taking the time to share. Once again, this reinforces the old adage " you get what you pay for."

[LiveFromNY]

I was idly looking at EBay yesterday when a set of Porsche 911 'Dilavar Style' Painted Cylinder Head Studs caught my eye.

They have a price of $14.99 each and are said to be made of A286 Steel and made in the USA.

They claim to have very similar expansion characteristics to Dilavar but unlike Dilavar they are Magnetic.

They wisely suggest that they are used in sets and not mixed with conventional steel studs.

As a Metallurgist with more years of experience than I am prepared to admit to having I cannot help myself but have to comment.

The company selling these studs clearly has a very poor understanding of Metallurgy 101.

A286 is in fact an Austenitic Precipitation Hardening Stainless Steel it is used commonly in the hot section of Jet Fighter Aircraft Engines and it has excellent corrosion and strength characteristics at temperatures around 1300 degF.

I can't imagine why it would it would be cheaper than Dilavar. It s normally produced in AMS Grades which means it would be 'flight certified' and eye wateringly expensive.

I am sure Porsche had a specialist German Alloy producer develop Dilavar due to the high cost of A286 which has been around for longer than Dilavar.

This, unhappily is not the only problem.

A286 does have similar expansion to Dilavar of this there is no doubt.

Both a 286 and Dilavar are both Austenitic Alloys and this means that they must both be non-magnetic.

There is no way that A286 with approximately 25% Nickel and 15% Chrome could ever be magnetic.

It is physically impossible to manufacture or treat an alloy with this composition to become magnetic as its basic crystal structure won't support ferromagnetism.

If they are made from A286 they can't be magnetic and if they are magnetic they can't be A286 - simply due to very basic metallurgical 'rules'

If the studs are magnetic and hence either ferritic or martensitic they will not be high expansion again for reasons of very basic crystallography.

There is another potential issue.

If we assume that the statement about the studs being magnetic is just an error and they are made from A286 I still have concerns over the properties of the material.

A286 can be purchased in very high strength conditions but at the highest strength is susceptible to corrosion pitting in the presence of chlorides. The corrosion pits then allow fatigue cracks to form and failure follows rapidly.

This is the main reason for Dilavar Failures.

The fact the studs are painted seems to hint that they may be a high strength 'temper'.

There was also a time a few years ago when some aftermarket 'Dilavar' studs found their way into the supply chain with disastrous results. I believe these aftermarket studs were also painted.

A286 is also used in marine environments but in order to meet NACE requirements it strength is limited to around 120ksi with a yield strength of 90ksi.

I believe that the stresses in the stud on an engine using Nikasil Cylinders would be approaching 90ksi and I would worry that if overtightened during assembly they could yield as the engine warms up.

I am sorry to bang on but Studs on 911 engines are, as we all know, are very important and the basic facts used in the description of these studs is inaccurate and on that basis and until some of the questions are answered I wouldn't touch them with a stick.

I was just about to say the exact same thing.