Sunday, May 30, 2010

Factory New Limits and other Nonsense

Pure nonsense! I can give you an example of how one engine manufacturer's cylinder barrel new limit is the same as maximum service limit. What... new limit=service limit!

But there is a story behind this so here goes...

Many years ago one of the engine manufacturer's had a problem with excessive cylinder barrel wear on a high-performance airplane model. There was a meeting of the user group for that airplane and there the factory rep assured all of the owners that they would take care of the problem under warranty. They would boroscope the cylinders and any cylinders that had excessive barrel wear (no cross-hatch left i.e. bore polished) would be removed and re-honed.

Fair enough, but some of the owners were concerned that they were going to get back cylinders that were not to new limits and close to being worn out. The factory rep assured them that "any cylinder not in new limits would be replaced with new". More than fair.

So everyone's happy and the factory rep goes to my shop and says "John, we want you to do all of the cylinder work on this warranty problem." OK, I say, "but if the barrel is already worn out and I hone it, it will be larger. I can't hone it smaller so I know they won't be within factory new limits."

John, the factory rep says, you don't understand. You are going to hone it to factory new at +5 oversize. But Mr. factory rep,  you don't have a +5 oversize. We do now!

So here is how this works:
Mininum new is 5.000 inches
Maximum service limit is 5.005

Mininum new for 5 over is 5.005
Maximum service limit for 5 over is 5.010

So you see these cylinders were factory new but at +5 oversize, or were they service limit for standard size? Take your pick as they are both!

Now it's not all that bad because maximum service limit is not a wear limit as such.  It doesn't mean that the cylinder barrel will stop working at that size. It is a repair limit or the limit of size during repair in which the engine will perform satisfactory during its anticipated life. All things being equal we want it be as close to new size as possible but there is way too much emphasis by users on "new limits".

Now that we're talking "new limits" or 'factory new limits" there is one more gotcha. We got into trouble above by not having a thorough understanding of new limit. Now we're in trouble because of imprecise thinking; marketeers in this industry take advantage of it (you).

If you specify "Lycoming new limt" or "Continental new limit" we can all agree on what this limit is as it is published in their "Table of Limits". But when we say "factory new limits" then who's limits are these? You might incorrectly assume that it is Lycoming or Continental. There are shops that work to their own limits and to them "factory" referes to themselves.

1 comment:

  1. very good tuttorial. John would you please tell me how many factory rebuilts can be performed to still have strengh on the cilynder, in other words, before running with drill into the atmosphere. Thank you. Pablo