I had some fun at the machine shop today.

[BillM]

OK, gotchya now, seems there's a few explanations here but here is a version of mine, the shop I worked in while with the Air Force we rebuilt F-111 parts, one item was the aileron hydraulic control units which had pistons in them with no rubber o-rings it was an all metal design with a clearance of only .0002" the fit was so close the oil bypass was very minimal, there was no way you could put these together at room temps the pistons had to be frozen in co2 and the cylinders had to be warmed up to assemble them, most aircraft hydraulic pumps are made the same way also, the finish of parts like this are ground very smooth usually to an 8 or better finish. In the case of a couple ground and finished flat blocks it would be the same and it's a combination of fit and molecular cohesion.

Maybe on the day they were assembled. I worked on the F-111D at Cannon AFB, NM, in 1974 and the first half of '75. I spent a fair amount of my time wiping hydraulic fluid off the sides of the aircraft I was working on with a rag and bucket benzine, back when OSHA was just a pup.

IIRC, they were the only aircraft in the USAF inventory that had an allowable hydraulic fluid leakage.

Bill