D-902 Rocker Arm

[JB on the Palouse]

I have a RTV900 that cracked the head. We rebuilt the whole engine and put a new head on (a head that came already assembled with valves). Got it all up and running and it seemed to run fine. After about an hour of use or so, it "popped" and upon inspection it had broken the #1 exhaust rocker arm. I thought it was weird, but ordered another. Checked to see that everything was moving the way it needed, double checked timing and valve adjustment, got it all back together and it ran great. However, again after just short use, it did the same thing and broke the exact same rocker arm.

This time I pulled the head and looked everything over. All valves appeared to seat exactly as they should. There did not appear to be any restrictions (that I could tell). I double and triple checked my clearances and put it all back together. It fired right up and ran nice and smooth. As soon as we took it out to do a little work it did the same thing AGAIN! I have previously talked to 2 Kubota mechanics and followed instructions for set up and inspection. They both verified that it appeared I had done everything they way it was needed. But, now I am just completely stumped and a bit frustrated. Do I need to look at another head? Can it be something else? I appreciate any insight. We use this machine on a school campus, and it has been frustrating not having it in service.

 

[North Idaho Wolfman]

Are the caps on top of the valve stems?

If the caps are all on, then I would think it's an issue with the valve or valve guide binding up when it gets hot.
If there is a warranty with the head I would have them replace it.
If there is not or it's too much of a pain.
You could pull that valve and inspect the stem for signs of heat or wear.
And inspect the valve guide.

Or take it to a good machine shop and have them check it.

 

[JB on the Palouse]

Are the caps on top of the valve stems?

If the caps are all on, then I would think it's an issue with the valve or valve guide binding up when it gets hot.
If there is a warranty with the head I would have them replace it.
If there is not or it's too much of a pain.
You could pull that valve and inspect the stem for signs of heat or wear.
And inspect the valve guide.

Or take it to a good machine shop and have them check it.

Yes, the caps are all on. When I pulled the head back off, I pulled each valve and inspected and they all "appeared" to be true and in good shape. I believe I will end up taking it in to a shop...just wanted to see if anyone else had experienced this or had an idea. Appreciate the response.

 

[BX25D Rookie]

It's the valve stem binding/seizing in the valve guide when hot, or the valve spring achieving coil bind.
I built a race car motor years ago. When assembling the cylinder heads, on one valve spring where the valve spring seats in the machined pocket surrounding the valve guide, I neglected to ensure the bottom ending coil on that valve spring seated completely in the machined pocket in the cylinder head.
The last lower coil tail end at the bottom end of the spring sat up on the as cast surface, instead of being located fully in the machined pocket.

When the lifter/pushrod/rocker arm tried to open that valve, the valve spring was "coil bound" and
it pulled the rocker arm stud up out of the cylinder head. (press fit on the rocker stud)
Check that the valve spring "pockets" where the valve springs seat on the machined surfaces are all
at the same height from the head gasket surface. If those machined surfaces where the bottom of the valve springs sit are incorrectly machined (not deep enough) that can and will cause coil bind.
Re-machining or cylinder head replacement will be the cure if that is the issue.

A valve guide sitting "high" can also cause valvetrain bind/breakage. When the valve opens, the bottom surface of the valve "cap" can hit an excessively high valve guide, which can snap a rocker arm or a broken valve stem and/or valve cap keeper failure.