B21 BH (BT751) swing cylinder support - replace?

[joea99]

Swing cylinders leaking on the BH and finally getting around to resealing them. While pulling off the "support" (the top pivot point for the cylinders) the bolts were very reluctant to come off. Made sure there was no stress on the bar or cylinders to cause that. All four of them were arguing the whole way out.

Seems every one had "galled" in the support bar (blind holes). The part is 75586-14110, item 110 in the parts diagram of the swing cylinder.

The bolts are pretty much toast, I could not clean them up with the thread restore dies I have, it just tore them up worse. The threads in the support bar are more or less "gone" on two holes (still there but pretty much useless for holding IMHO), the other two are acceptable. None of them resisted the thread restore "tap" I ran all the way down.

Asking for opinions on reusing the support bar as is, finding some way to restore full threads, or just buy a new one (about $120). I realize it is pretty hard to judge something like that from my description, but don't think pictures are possible.

What I cannot really grasp is how those could be galled like that. The bottom threads are fine which says to me they galled on the way out. I get how that might happen if the bar was "cocked" while taking them out, as if the weight of the boom was on the cylinders, but the boom ends were already out and quite free to move about and I doubt the first one would have been affected in any case. If they were installed wrong, I'd expect them to be galled all the way down the hole. I've seen that sort of thing with steel into aluminum, but never steel on steel.

Thanks for any wisdom.

 

[TractorTech]

They might have galled when installed at the factory. If the bottom threads are good and the galled threads cleaned up with the tap, can you find longer bolts of the same grade to catch the rest of the threads?

You could also assemble back and keep an eye on it.

 

[joea99]

They might have galled when installed at the factory. If the bottom threads are good and the galled threads cleaned up with the tap, can you find longer bolts of the same grade to catch the rest of the threads?

You could also assemble back and keep an eye on it.

I had not thought of that. Now that I measure things up, those bolts only go in about the depth of the galling, so you may be right.

The hole depth is about 29mm, the thickness of the plate the support bolts up to is 12mm, the flat and lock washers about 6mm. So a bolt of about 45mm of length, below the head, should almost bottom out.

The bolt head has 6 lines, and a solid triangle, picture attached. Any idea what grade it is? I'm guessing grade 8 but the triangle throws me.

 

[TractorTech]

Grade 8. Not metric

 

[joea99]

Grade 8. Not metric

Wonder if that was the problem? Someone used the wrong bolts?
I get that the markings are "not metric", from what I found on the net. The BH tags says built in GA.

The restore tap I ran down the hole is definitely a 12x1.25 and it gave almost no resistance at all, I only needed a hand wrench in a couple of spots and very gentle pressure.

Metric pitch gauge is close at 1.25, inch is close at 20. Diameter is 12.56mm or .495 inch.

 

[TractorTech]

It sounds like it is the right bolt as the parts list shows:

HEX, BOLT​

Part # 75599-04525
Job Qty 4
-HEX.BOLT
Notes: 1/2-20UNF*1 1/4 (not noted is the hardness)

I didn't realize that the bolts came in horizontally from the back. I was thinking they sandwiched the piece from the top.
The backhoe is made by an American company that Kubota bought and it makes sense that the hardware is standard

 

[joea99]

It sounds like it is the right bolt as the parts list shows:

HEX, BOLT​

Part # 75599-04525
Job Qty 4
-HEX.BOLT
Notes: 1/2-20UNF*1 1/4 (not noted is the hardness)

I didn't realize that the bolts came in horizontally from the back. I was thinking they sandwiched the piece from the top.
The backhoe is made by an American company that Kubota bought and it makes sense that the hardware is standard

You the man, thanks much. I am so used to not seeing fastener specs I did not see listed and just assumed metric.

I suppose I can still try the longer bolt idea though, yes?

The 1/2x20 thread restore tap worked well to clean up the threads, one giving me a very hard time, but, seems good, in fact almost the entire length of the hole. That's when I realized why the 12x1.25 went in so easy. It was the wrong size. I need to wake up before I work.