John -
As always, sage advice. Wasn't sure about the 100, but given your thoughts
on cracking, that makes good sense.
Actually, on the A90 I didn't put in the unleaded seats on my rebuild - my
feeling was that a 6.9:1 compression and single springs on the valves meant
that the chance of valve seat recession on the old girl was basically nil.
I did, however, put in new exhaust valves and I still toss in some lead
substitute from time to time.
That being said, with the BJ8 running pretty high compression and higher
revs.... I am contemplating otherwise.... ? I will be doing a complete
rebuild on it.
Alan
'52 A90
'53 BN1
'64 BJ8
On Tue, Feb 3, 2009 at 4:56 PM, John Harper <ah@jharper.demon.co.uk> wrote:
>
> Alan
>
> I have a different point of view on this subject. One which might well
> start a new debate.
>
> Something like 20 years ago there was a great panic amongst UK users of
> older cars when leaded fuel was being withdrawn. Many owners of 100s asked
> me what they should do and my suggestion was; do nothing.
>
> Around that time the Vintage Sports Car Club published a 25 page document
> which was the result of significant testing that they had carried out on
> older engines.
>
> It was titled
>
> Valve Seat Recession
> Use of Unleaded Gasoline in Older Engines
>
> It was very comprehensive but the conclusion was that for low compression,
> medium tuned engines such at the 100 the recession due to the lack of lead
> on the valve seat was likely to be virtually non existent.
>
> Based on this many 100s that I know are still running around without
> changing valve seats without problems 20 years later.
>
> Now to this I can add another point. As we know the 100 head is very prone
> to cracking across valve seats. One owner I know added seats to a perfectly
> good head but soon found the a fatal crack had appeared beyond the valve
> insert. The conclusion was that as the seat has to fitted by interference
> fit, the extra pressure had caused the crack.
>
> My final point is that if one does get seat recession then all that is
> happening is that the part of the head that is being ground away would have
> to be cut away later when fitting a seat; so why bother.
>
> Regards
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