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Kevin, Your exactly right about the different metal expansion rates when cold. The very reason to have a pre-heat done. Ive seen bent rods due to the oil in the lifters so thick that the lifter didn't move and bent the rod. This is an extreme case for sure. But it shows you should have a pre-heat when the temp gets down to about 32 deg. As to how long to let a propane pre-heat go I don't think that there is a general rule of thumb for this. You have to look at how the FBO is doing the pre-heat. What type of pre-heater is he using. I've seen quite a variety of systems from electric/kerosene torpedo's ducted with two hoses to the airscoop, to the "Flame" portable propane heater, which is very good unit easy to use, and finally the herman nelson big units army/airforce surplus. They all put out heat at diferent rates so its real hard to judge how long to let them run. The FBO may be your best source for how long to let it run. If I was to venture a guess, I would let them pre-heat about 30 min checking often on progress. Yes you want to be there while there doing the pre-heat. You have the time while getting flight plan completed so thats the time used to make sure no damage is being done to your airplane. One thing to check is that the cowl flaps are open allowing the heated air to move someplace. The air should move as if the plane is in flight in and out. Hope this helps alittle. Tony G.
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