Bronze valve guides in cast iron heads

[Alan Whitaker]

Hi Guy,
Can you advise me on fitting bronze guides into cast iron heads, I have guides that were part of the original cast head.
I have read with interest your last article on replacing worn guides, I have to replace worn guides and the following was where I was stuck.
On cast heads the old guide is machined out for the new guide as there is a difference in expansion of cast heads and bronze should any special clearances or procedure be used.

Many Thanks for all your knowledge shared

Alan

[Guy Croft]

Hi Alan

sorry for delay, due to site server tech problems on Sunday morning.

Good point. Since the coefficient of thermal expansion of cast iron is about half that of copper alloy (bronze) the bronze guide tries to expand on OD and cannot to the extent that it would in an alloy head. In an alloy head the head expands with the guide at a similar rate, so a very tight fitting guide in a cast iron head could potentially grab the valve if the OD or bore fit is too tight.
I'd go for 0.001" between guide and head (which is plenty if done accurately) and certainly 0.001-0.0015 guide bore to valve.

Come back to me if you are concerned about this,

GC

[Alan Whitaker]

Many thanks for that guy.
It sounds ok to me and my local machine said doing the job is no problem to them.

Alan