Exhaust valve guide loose

[Keith.owen]

A curios tale............

I took the head off of my VH to better set the timing.

I removed the valves and put it into a bowl of degreaser. Imagine my horror when I removed it and upon turning the head upside down to drain the liquid out of the top fins heard a 'clank'. Upon further examination I discovered that the valve guide was moving up and down in the head (although not loose enough to come out).

Interestingly enough, I never noticed this when I ground in the valves! Perhaps this explains the 'valve noise' that I had previously commented on (only the spring pressure would have held it in place to a certain degree)?!

Anyway, it was taken to the doctors (Owlesbury Crankshaft Services) who will fit new guides (they might have to fabricate an oversize one for the exhaust - since the new one I gave them might be a loose fit) and re-seat the valves etc.

They said it will take about a week, so with any luck I'll be back on the road soon>

Keith

[david.anderson]

Keith
Has someone enlarged the exhaust port up and reduced the seating depth. I have seen a couple of loose exhaust guides on iron heads and most alloy heads are loose.
While the bike was probably fitted with cast iron guides, when bronze guides are used it is easy to spin them in a lathe and knurl the guide up in size.
David

[Keith.owen]

Hi David,
I don't know about the exhaust port and the head is currently at the 'doctors'.

However, interestingly enough, the guide doesn't come out completely it appears the bottom ring is sufficiently large to prevent this.

I have a set of new guides that I supplied to the engineers, but they will turn up a new guide if this should prove necessary.

On the subject of choice of materials is there any benefit to be gained in choice of different materials?

BR,

Keith

[david.anderson]

Keith
it is normal to use cast iron guides in an iron head.
A bronze guide is technically superior in that it aids in conducting the heat away from the valve.
Bronze is also first choice for an alloy head as it better matches the expansion rate of the head. Where cast iron guides are used in an alloy head the interference fit must be greater (which in RH4 Norton Commando heads resulted in cracks eminating from the guide boss).
David