Smallfry
HE's Spartacus.
- Messages
- 2,257
- Location
- Kent UK
I don't know if this is the right section to post this, but as it is possibly a material problem, here goes ..........
I have a little Yanmar excavator or mini digger if you like. It overheated thanks to K Seal, so I had to strip it and take the head off.
Anyway, to cut a long story short, the engine is OHV with pushrods, exactly like a BMC A or B series engine, but smaller, with the rocker cover retained on exactly the same fashion, with hollow bolts with an internal thread that fixes on the two end rocker gear retaining studs, which are longer than needs be for this purpose.
This being a three cylinder engine, the rocker gear is retained by the two M8 end studs, with flanged nuts, and a central M8 bolt. On trying to tighten one of the end nuts, the thread on the stud stripped ...... you know how five minute jobs go.
I didn't have any HT setscrews that I could cut down, only some threaded rod (allthread) but I didn't think this would be up to the job, so didn't try it. I remembered that in my boxes of bits and pieces for the lathe, there were some long grub screws. I fished them out, and they were the right length, and the right thread, plus the benefit of of having a socket in one end for ease of installation with an allen key. I screwed them in with the key, only using the short end for leverage, until they bottomed out. Installed the rocker gear, set the valve clearances, and fitted the rocker cover, and called it a day. All good.
Didn't touch it the following day, but the day after that, I went to do some more reassembly work. While pushing on a breather hose, I found the rocker cover was loose. On lifting it, the retaining bolts, the rocker assy nuts, and most of the studs came along with it. They had both snapped off flush with mounting surface
Looks like brittle fractures to me. The nuts are done up to 26Nm, so not THAT tight. I would have thought the grub screws would take a lot more tension than this, or am I missing something ? Are they too strong and therefore brittle, or not meant to be used in this way ?
Only saving grace is that they didn't fail when the engine was running.
I have a little Yanmar excavator or mini digger if you like. It overheated thanks to K Seal, so I had to strip it and take the head off.
Anyway, to cut a long story short, the engine is OHV with pushrods, exactly like a BMC A or B series engine, but smaller, with the rocker cover retained on exactly the same fashion, with hollow bolts with an internal thread that fixes on the two end rocker gear retaining studs, which are longer than needs be for this purpose.
This being a three cylinder engine, the rocker gear is retained by the two M8 end studs, with flanged nuts, and a central M8 bolt. On trying to tighten one of the end nuts, the thread on the stud stripped ...... you know how five minute jobs go.
I didn't have any HT setscrews that I could cut down, only some threaded rod (allthread) but I didn't think this would be up to the job, so didn't try it. I remembered that in my boxes of bits and pieces for the lathe, there were some long grub screws. I fished them out, and they were the right length, and the right thread, plus the benefit of of having a socket in one end for ease of installation with an allen key. I screwed them in with the key, only using the short end for leverage, until they bottomed out. Installed the rocker gear, set the valve clearances, and fitted the rocker cover, and called it a day. All good.
Didn't touch it the following day, but the day after that, I went to do some more reassembly work. While pushing on a breather hose, I found the rocker cover was loose. On lifting it, the retaining bolts, the rocker assy nuts, and most of the studs came along with it. They had both snapped off flush with mounting surface
Looks like brittle fractures to me. The nuts are done up to 26Nm, so not THAT tight. I would have thought the grub screws would take a lot more tension than this, or am I missing something ? Are they too strong and therefore brittle, or not meant to be used in this way ?
Only saving grace is that they didn't fail when the engine was running.
