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Thread: Cylinder Head Studs for Magnesium Engine Cases

  1. #1

    Cylinder Head Studs for Magnesium Engine Cases

    I am sure that everyone is aware of the arguments surrounding the use of Dilavar and other aftermarket studs.

    There are also arguments about Timeserts versus Case Savers and the debate just keeps on going around and around.

    Standard steel studs seem to be unpopular, Dilavar has a bad reputation for failure and the other studs available are all materials that are more at home in the Hot Section of Fighter Aircraft Engines than in basic IC engines.

    It seems that the common opinion is that the more exotic, hence expensive, the better the stud.

    I have been concerned for some time that many of the ‘high end’ studs that are strongly promoted increase clamping forces when a Nikasil cylinder expands and do cause a significant increase in clamping force and increase the risk of pulling studs out of Magnesium cases.

    There also seems an opinion that the extra clamping produced by cylinder expansion can be a good thing and prevents head leakage.

    I don't agree with this idea, the head needs to be clamped correctly when cold so they seal effectively. The clamping doesn't need to be increased as the engine heats up as combustion pressures don’t change much from hot to cold.

    To try to come to a sensible conclusion some basic estimates to see what increase in preload is developed by some of the different studs available seems a reasonable idea.

    The properties of the stud that influence pull out forces are quite straightforward and easily summarised:

    a. The difference in the Coefficient of Expansion between the stud and the cylinder.
    b. The Elastic Modulus of the material used for the manufacture of the stud
    c. The cross-sectional area of the stud

    To keep the calculations simple I assumed that the cylinder and head are infinitely stiff (not quite true but a reasonable first approximation).

    This assumption means that the numbers produced shouldn’t be used for any real modelling but are valid to rank the various studs in terms of their potential to damage a Magnesium Engine Case.

    I have also assumed a Delta T of 120 degK

    Standard Steel studs have a shank diameter of 7mm. The Elastic Modulus of the steel used for this type of stud is around 205GPa and the Coefficient of Expansion is approximately 10.8 ppm/degK.

    This produces an increase in clamping force of around 2500 lbs per stud.

    Early Dilavar studs also have a shank diameter of 7mm, the Elastic Modulus of Dilavar is around 195GPa and the expansion is 18ppm/degK. This results in an increase in clamping force of just less than 500 lbs which is much safer than the steel stud.

    Aftermarket studs vary in material between Inconel 718 and 17-4PH. These steels have similar expansion to a conventional steel and an Elastic Modulus which is around 198GPa.

    Many of these studs have an larger diameter – 9.2mm is typical- This increase is always justified by a claim of improved performance but seems very similar to the core diameter needed for thread rolling which also has a strong impact on the cost of manufacture due to a reduction in machining cost.

    A large diameter stud of 9.0mm would see the pull out force increase to around 4200lbs – a 65% increase from the steel stud and a 8-fold increase from Dilavar.

    I am not sure this type of stud is a great idea for early and increasingly difficult to replace Mag cases even when fitted with Time Serts, Big Serts or Case Savers – although these parts will help.

    I don’t think I would install this type of stud in a 1969 911S case as they are becoming too hard to find.

    The high clamping force will also have a detrimental effect on the long term performance of the engine case as the increased clamping force will tend to promote stress relaxation in the case which will reduce the effective strength of the threads in the magnesium.

    Dilavar seems to have, at least in some circles, become unpopular due to cracking issues. This seems to have been solved and the current 993 studs don’t seem to fail but at a cost of around $60 each ($1600 per engine set by the time you add nuts and washers) they should be good.

    There are two alternative strategies.

    The first is to use a stud with a smaller cross-sectional area. This will reduce the influence of expansion on the increase in pull out force. It will weaken the stud BUT on early Mag engines we only use 24lbsft so why do we need materials with a strength of 200ksi.

    Using a 5mm diameter shank would reduce the increase in pull out force to around 1100lbs – still more than Dilavar but much less than either standard studs or the larger diameter ‘special’ studs.

    The other way is to use a material with a lower Elastic Modulus.

    Titanium Alloys have a Modulus of around 115GPa which is around 50% of the steel used for the standard stud.

    A 7mm diameter Ti Stud will generate an increased pull out force of about1200lbs, still a sensible figure.

    A suitable Ti alloy has a tensile strength of around 180ksi which is more than adequate and is available in grades that will function well even in Salt Water environments and would be resistant to crevice corrosion.

    By using a nut made from a different material the affect of galling that can occur with Ti fasteners can also be eliminated.

    As we are currently building a 1969 S motor for an FIA spec rally car we have decided to make a few sets of Ti studs and test out how they perform. The cost should be much lower than 993 Dilavar studs. They should be ready early in the New Year.

  2. #2
    You are going to use 7mm diameter shanks on the Titanium studs? Thanks for the calculations. Would you use Titanium for intake and exhaust or only the bottom set of studs?

    Max_911S_fahrer, on Flickr
    1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
    Early 911S Registry Member #425

  3. #3
    Max,

    Yes we will use a 7mm shank as this seems to be a sensible compromise between strength and stiffness.

    I don't like the idea of mixed materials between upper and lower studs and never really understood the reason for Porsche using Steel upper and Dilavar Lower studs on the SC Motors.

    I am not sure at equilibrium there is enough difference in temperature between the two sets of studs.

    We will use Ti for all of the studs.

  4. #4
    Chris, what I have done a few times, and realize I'm NOT the engineer you are, but what I have done is to use the shielding tin as used on the 3.2 engines. My take being it helps shield the studs from as much cooling air therefore allowing the suds to be hotter and hopefully expand more. It has worked on a couple of occasions, but maybe it was just a fluke.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  5. #5
    In one of the threads recently someone had a picture of a fiberglass jacketed stud that had corroded between the metal and fiberglass and then cracked. Sounds like a baffle works better.

    Max_911S_fahrer, on Flickr
    1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
    Early 911S Registry Member #425

  6. #6
    Senior Member 210bhp's Avatar
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    Chris

    Will this be an MFI engine or conversion to carbs?

    Regards
    Mike
    RS#1551
    67S
    73E (home after 25 years) and sold again
    Early S reg. #681

  7. #7
    The engine needs to run an MFI. We have just finished refurbishing the throttle bodies and will make an effort to rebuild the pump.

    There is a Diesel Injection Specialist in Coventry with a test bench that will run the pump for us and I am just keen to try to do the job in-house.

    Keeping the studs warm seems like a good idea as the temperature will still be reasonably low but I will carry on with the Ti studs as I think we will just have a bit more margin for error.

    We also have some re-designed rocker shafts and seals for this engine which could be useful in stopping leaks from the carriers.

  8. #8
    This is an interesting development. Very cool.
    Kenik
    - 1969 911S
    - 1965/66 911
    - S Reg #760
    - RGruppe #389

  9. #9
    Member swisscheese's Avatar
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    Bumping an old thread here, re. the current status / views of Dilavar or steel studs in Magnesium cases. Any new insights on this? Any furher experiences with the 993 TT studs?
    911S 1973
    Early 911S Registry #176

  10. #10
    My advice would be OEM steel studs with time certs for a street motor magnesium case application. Steel studs do not break and do not pull from a mag case when time certs are properly installed.
    Tom Butler
    1973 RSR Clone
    1970 911E Restoration in Progress

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