[Fot] Zinc Additive. now use of BRONZE GUIDES

[Randall]

> I discussed with highly regarded performance engine shop.
> Whenever they encounter bronze guides they clearance them.
> If it doesnt get done, there is a danger of seizing...

I agree.

The problem is that the bronze has a higher rate of thermal expansion than
the cast iron head, plus is softer/weaker.  So as the guide & head heat up,
the bronze tries to expand faster than the cast iron hole in the head.
Since the hole is stronger, the bronze expands inward into the valve stem
bore and closes up the clearance.  The only solution is extra clearance
(which means they leak oil and don't guide the valve as well when cold).

And since the guide also carries heat away from the valve, the problem gets
worst at sustained full power.

I believe the bronze wears slower in service (if it gets adequate
lubrication), but since you have to ream them oversize, the advantage is
lost.  In effect, you have to install them pre-worn.

-- Randall