Exhaust Valve Guide Eaten!

[robjameson]

While refitting my rocker boxes having re-torqued my head bolts today, one of the threads pulled. Thought i'd better remove the head in order to get a helicoil put in.

On removing the silencer, there was a suspicious rattle, and on upending it some bits fell out, which looked suspiciously like a valve guide.

Off with the head and exhaust valve out and this is what I found.

I suspect that the issue has been caused by a problem I had with timing earlier in the year, and I have overheated the guide, mixed with what looks like a slightly bent valve.

Good job I bought a spare VH last weekend I suppose!

[admin]

was that Mr MItchells VH, perchance ?
The one, that a certain Paul Jamesone (any relation ? ), warned me off ...

[robjameson]

It certainly was John's VH!

Dad warned me off it as well, but being quite persistent I went to have a look with him and it was actually very good in the flesh, just goes to show that you need to have a look at these things and not just go off the photos.

It is a 1955 machine but at some point has had a replacement frame from the factory, so has fittings for half and full width hubs. Needs work but one day will be a nice enough bike.

[nevhunter]

What sort of material is it? The alloy heads normally run bronze guides don't they? Nev

[andrew.chapman]

If the valve is bent then that is probably the cause of the broken guide. I would be looking for the cause of the valve/piston contact - incorrect valve timing, over-revving or a sticking valve are most likely.

[robjameson]

Turns out it wasn't a bent valve, it was caused by the guide coming loose and burrowing its way down the soft alloy head, and then effectively splaying out the valve guide at the base of the valve stem.

All got repaired by S.E.P Kegworth early this year and no issues since.