I'm replacing the seatpost with a longer one (see pic). Few questions:

Which parts of the bolt/fixing area should be greased, and which parts to avoid grease? (I read somewhere that certain parts should not get greased, but can't find that source any more)

Since the bike is brand new, is it OK not to apply more/new grease to the inserted part of the seatpost, since there is still the original grease inside the seat tube?

I don't have grease with me, only the TriFlow Superior lube, is it OK?

Thanks in advance.

I'd grease the saddle rail clamping bolt threads to prevent corrosion but the res of that clamp really doesn't need anything unless it creaks. As to the post itself, absolutely grease the part that inserts into the seat tube and cover it well. Don't rely on the grease already in the seat tube. I would use real grease, not oil. You can get a small tube at any hardware, auto parts or the automotive section of any big box store. Any type will do.

The basic rule of thumb is grease any metallic seatpost, but not CF. I'm not aware of any requirement to grease the fixing bolts or rails, but it probably wouldn't hurt anything. You grease the post to prevent corrosion and problems removing it later; there is a lot of area for corrosion to develop and once it starts, especially if the metal of the post and the frame are dissimilar, it can literally weld the seatpost to the frame in a fairly short period of time (e.g., a year or three), depending on moisture conditions. Virtually any grease will do and general-purpose greases are sold everywhere. If you're too cheap or busy to go buy a $3 tub of grease, then I'd just depend on what was there at factory assembly and skip the Tri-Flow or any other spray lubricant which contains solvents and will probably wash any existing grease away while not doing anything after a month or two. Excess grease is not really an issue except for the mess. But a thin coating of the entire post where it contacts the frame is ideal.

- Mark

Thanks for the replies and suggestions. I'm going to buy some new grease.

I've always used Park assembly compound (SAC-2) for where the seatpost goes into the seatube. Haven't had a need but you could also use on the saddle rails where they meet the clamp.

Haven't greased bolts on newly shipped seatpost clamps. Beware that probably the torque ratings shown are based on threads as they come from the factory -- greasing them up effectively changes the torque rating you should really use.

I use White Lithium Grease (it comes in a tube) I put it on everything... metal to metal.

Go to the auto parts store and get yourself a tub of boat trailer wheel bearing grease. It is waterproof, you can use it on practically anything on a bike which needs grease (including wheel bearings!) and a tub is almost a lifetime supply.

Most new seatposts have the clamp bolt pre-greased. But I always check anyway and add grease if there is none. Especially on single-bolt posts, you want that bolt really tight or it will come loose and ruin the "micro-adjust" ridges on the clamping surfaces. Grease allows getting the bolt tighter without stripping the threads, torque ratings be darned.

hmm.. I always thought grease would make it easier to strip the threads as less force is needed to thread a greased external thread into the receiving internal thread the same distance as an ungreased bolt. I thought most torque ratings are provided with "dry" assumption. Rule of thumb is that for wet threads (eg. grease or lubed) then reduce torque value by up to 40%.

It's also thick & sticky, not ideal for a bicycle.

One problem with threaded fittings is the galling of the threads. Especially with a soft metal like AL. Grease will help reduce this possibility. Additionally I always felt that a greased bolt steel less twisting stress and therefore less chance to fracture. Andy

Marine grease is great for almost anything on a bike. Not too thick or sticky. After using about every specialty and non-specialty bike grease under the sun I've settled on Marine grease as well. Cheap, holds up to moisture and lubricates as well as anything and you only need one grease for bearings, screws, seatposts, saddle rails, pedal threads, bar stems, etc. Only thing I wouldn't use it for is most cassette bodies as it's a little too viscous.

Yes, and the wrenching force goes into stretching the bolt rather than overcoming the friction of the threads; the stretch is what keeps the joint from loosening.
Many high-performance cars have their connecting rod bolts tightened by measuring the stretch rather than the torque, because of frictional effect uncertainty.

Galling is not so much of a problem with dissimilar metals like steel vs aluminum, but is a big problem with stainless bolts vs stainless threads (e.g. nuts or Helicoils) so greasing or other anti-galling treatment is a must.