Horns & Torque Tubes.

[Ben Ayalon]

Lifted from delphi.

========================

Piper is ready to produce new torque tubes. They produced a run of four of which Denny Haskins and myself received two last week for our analysis and fit. Both are of higher quality than those in the past. These new tubes are being drilled in a new indexing machine and are very accurate. The holes in the tube are still .010 undersize and require reaming during installation. Because all the horns and tubes were match drilled during piper production years, no two are drilled the same, so there is still a small alignment error with the holes in the horn and the holes in the new production tubes requiring reaming to fit. This error is very minor compared to past new tubes. In theory, when the new horns and new tubes are drilled in the new indexing equipment, little or no alignment error should exist.

Piper’s price hasn’t changed. About $306+Shipping

Piper should have new production tubes available shortly with new horns to follow by January.

Denny has built up both new tubes as complete assemblies for exchange. New tube, good horn, new bearings and hardware, painted outside and sealed inside with rustproof primer, rebuilt trim spool and gage. One is sold and the other is still available.

I read a lot of theories about horn cracks being caused by removal or over torque. Both are very unlikely. The horn is a slip fit on the tube. The horn is approximately .001 larger than the tube. The horns aren’t cracking in areas consistent with over torque. Over torque would cause the horn to crack in the thinnest weakest areas of the forging.

The horns are cracking inside at the intersection of the two bores where the material is the thickest which would be indicative of stress during operation. Assuming that twins or late singles are not affected is only wishful thinking. All horns are the same part number. The same horn was used on all Comanches, 180, 250, 260, 400, and all twins. Only the tube thickness changed on 400’s and twins. Inspection is the only way to know for sure.

We have a pile of used tubes. Most were rejected for rust and pitting on the inside. Inspection inside is just as important. A small mirror on a stick and a flashlight will allow for internal inspection. Lightly honing the inside to remove surface rust will reveal any pitting.

Scientific recordkeeping of horn failures is a good idea, but only if all horns are removed for inspection. Inspecting a random few and looking for trends to avoid inspecting the masses is a pretty big gamble when you consider the consequences of continued flight with a cracked horn.

Eric Paul, eric@avrecords.com