Nige
Senior Member
Ken, we have had exactly the same situation occasionally with 789C/D hoist cylinders and I have no idea why. 95%+ of the cylinders it's no problem at all, the head cracks and screws right off by hand. Just occasionally one comes along to give you grief - and they all came off the same fleet of trucks doing exactly the same work, there appears to be no rhyme or reason to it. The gland/head either moves about 2 turns with difficulty and then stops and will not go either in or out, or does not want to move at all right from the get-go.
If a head moves the 2 turns with difficulty we know from experience that the internal thread in the hollow rod is goofed, so now if a head does not go easy within half a turn of it cracking it is stop time. In either case (non-moving or 1/2 turn) in order to salvage the more expensive rod #24 in your diagram with its internal thread undamaged we send the whole assembly out to a local machine shop, have them put it in a lathe and machine the head/gland until it's so thin it literally falls out. It's the only solution I have been able to come up with so far. We actually had to send one to the machine shop yesterday for exactly that issue. After removing the head they also clean up the internal thread in the rod using a new head as a go/no-go gauge until the head will screw on and off by hand right until it bottoms. So far we have had no "repeat offenders" after doing this refurb process so maybe the root cause is quality issues in the internal thread in the rod..?
If a head moves the 2 turns with difficulty we know from experience that the internal thread in the hollow rod is goofed, so now if a head does not go easy within half a turn of it cracking it is stop time. In either case (non-moving or 1/2 turn) in order to salvage the more expensive rod #24 in your diagram with its internal thread undamaged we send the whole assembly out to a local machine shop, have them put it in a lathe and machine the head/gland until it's so thin it literally falls out. It's the only solution I have been able to come up with so far. We actually had to send one to the machine shop yesterday for exactly that issue. After removing the head they also clean up the internal thread in the rod using a new head as a go/no-go gauge until the head will screw on and off by hand right until it bottoms. So far we have had no "repeat offenders" after doing this refurb process so maybe the root cause is quality issues in the internal thread in the rod..?
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