Trials Australia Forums

So at some stage in the distant past... Somebody swapped out the cylinder studs on my 158 Bultaco.

And then they put them in the wrong way. But only two of them went in the the wrong way and I don't know which two but it's a moot point cos the two rusted ones won't come out. They are saying, "Any more pressure smart ass and we are gunna snap off inside this casing and that will teach you....."

So this is how it will go back together.

Some with the curse thread in the casings, which are the shiny refurbished ones...


And two with the rusty patina which have the fine threads in the block. And are threatening to snap.

I also straightened the rusty one after I took this photo.





The rusty ones have no intention of coming out.

Bummer. A moot point as you say, but i'd be thinking the fine threads down into the alloy case material and the coarse threads up for the nuts?

Yeah. That's the way I went and that was the way it worked. It must have taken some serious detirmination to wind those M8mm fine pitch thread into 5/16 hole. . And that's where they stay.

I.

In the vehicles I have worked on it has always been course thread into the alloy. It appears to be due to the depth of thread contact ie difference between major and minor dia. of the thread pattern and the tapping limitations of the older alloys. Finer threads just seem to weld themselves solid. If they can be removed it usually needs a helicoil insert to repair it.

18ss wrote:In the vehicles I have worked on it has always been course thread into the alloy. It appears to be due to the depth of thread contact ie difference between major and minor dia. of the thread pattern and the tapping limitations of the older alloys. Finer threads just seem to weld themselves solid. If they can be removed it usually needs a helicoil insert to repair it.

The nuts I ordered were for the fine thread so I guess in Bultaco land the course thread goes into the block.

I.

Yes the course thread end goes into the cases on a Bultaco. The head nuts are a fine thread. Graham.

I wonder if I heated the casings just a bit. Works on nearly everything else.
Jeez I'd hate to snap one off tho.