Alex Can I butt in on this thread and add a related question for you? Tomorrow I am going to reinstall my Hynautic H-50 pump at the lower helm. It just came back from Charman Enterprises where I got a new shaft seal and shaft installed. I didn't expect the shaft to be defective. The old fluid in the Hynautic "Reservalve" was red like ATF, but now I have added some Teleflex SeaStar oil which is supposed to meet Mil Spec H5606C and, since Teleflex acquired Hynautic, is the recommended oil for my system. While my lower helm pump has a new seal, the upper helm does not and I'd like to avoid a new leak there. Should I stick with ATF of would the H5606C be OK to use? Will the H5606C allow the seal to shrink? The old Hynautic documentation says to use Hynautic MCO-06 fluid or Mil-H-5606. It says ATF "may be used but will cause harder steering." Harder steering is not great, but I just don't want another leaking seal. Also, any idea why the shaft would have had a groove at the seal lip? That lower helm has had almost no use in the 18 year life of the pump and boat. TIA Jim Fidler "Fiddlesticks" ============================================== Basically, if your seals shrink when they're not supposed to they can leak while unexpected swelling can lead to excessive wear. The underlying reason for this swelling or shrinking is related to the presence or absence of Napthenic base stock in the finished product. I've posted explanations of this in the past so I won't bore you with it here. The important thing is that your system was designed with a specific fluid in mind and you will be best served to use only that fluid. Shrunkenly yours, Alex _______________________________________________

----- Original Message ----- From: "Jim Fidler" <fidlerjim@earthlink.net> [snip] > The old fluid in the Hynautic "Reservalve" was red like ATF, [snip] > Should I stick with ATF of would the H5606C be OK > to use? Will the H5606C allow the seal to shrink? > > The old Hynautic documentation says to use Hynautic > MCO-06 fluid or Mil-H-5606. It says ATF "may be > used but will cause harder steering." [snip] Hi Jim, Don't assume that it's ATF just because it's red! Some aircraft hydraulic oils (Texaco's for example) are red. If the manual calls for Mil-H-5606 then that's what you should use. FWIW: There are actually two "5606" classifications: 5606C (which may have been changed to 5606G) is referred to as a "Super Clean" product while 5606A is sold with cleanliness standards similar to ordinary automotive and industrial oils. IMHO the 5606C oil will cost you about twice what the 5606A will for no added benefit in your application. As to the harder steering with ATF statement: That's just because ATF is quite a bit thicker than 5606 (if I'm doing my sums correctly, ATF is about six or seven times the viscosity of 5606). Viscously yours, Alex