Erbe had a lot of problems with a cylinder maybe he will chim in
400 deg is not a lot of
if they warp, something wrong happened. 400F isn't a lot of heat.
400 deg is not a lot of heat for steel, stainless, steel, titanium for some metals. 400 deg is too much heat for aluminum. Anything over 300 deg. aluminum's temper will be changed. I have had a lot of thin light-weight aluminum parts ruined when they are powdered coated. Warping is not the problem but changing the temper is a huge problem on aluminum parts.
We had to have a batch of 25 jet ski heads re-heat treated after they were powdered coated. They were so soft that the combustion chambers would stretch (volume of the head would increase) after 5 gallons of fuel. Head gaskets would blow in less than one week end of riding.
A customer recently brought us a set of Harley heads that he had powdered coated. Instant head gasket problems. We would machine the head gasket surface and a few weeks later the gasket would blow. We check the top of the cylinders and they were flat. We would check the head gasket surface and they would have waves in them. The customer replaced the heads .........no more head gasket problems.
Another customer had an aluminum four stroke head powder coated. One valve seat fell out of the head and two others were starting to move. The head had not had a problem before power coating.
Another customer had an aluminum swing arm powder coated and the swing arm started bending like it was made of rubber.
Another customer that makes cast aluminum patio furniture had a large batch powder coated. It turned it all soft and would bend when you sat it it.
I can keep the list going but I do not have time to list all of the different aluminum parts I have seen the heat treat ruined when it was powder coated.
I have seen some aluminum parts powdered coated that did not seem to have a problem but this is the exception not the normal pattern of powdered coated aluminum parts.
The problem with power coating shops is they are not metallurgist and do not understand the effects that heat and the amount of time a object spends at certain temperatures has on the heat treat of the part. Different metals respond to temperature differently than others. Some metals melt at temperatures under 500 deg.
Are you willing to take the chance of ruining parts that are discontinued or hard to find ruined by a powder coat shop that says it will not hurt your aluminum cases?