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Re: Leaded vs. unleaded

Subject: Re: Leaded vs. unleaded
From: Scott Fisher <>
Date: Thu, 20 May 1999 17:47:41 -0700
Rick Sinclair writes:

> O.K., call me ignorant, but remember, I am only 14.

That may be so, but you've found the cure for ignorance: ask questions! 
I know a lot of people a lot older than 14 who still haven't figured
THAT out!

>  I would like to know the pro's and con's of using
> unleaded fuel/gas/petrol ( Whatever you want to call it) in a 
> car that was designed for leaded.

Short answer: Don't worry, your '68 should already have the right
equipment in it (hardened valve seats, primarily).  If you're having a
valve job done, the machine shop should automatically install new
hardened valve seats these days because unleaded gas is the only choice.

Long answer: in 1993, I pulled the cylinder head off my street-driven
'71 MGB because of a blown head gasket.  On inspection, it turned out
that one of the valves had receded a fair bit into the head, so I
decided to take the head to Kaeding Engineering in Campbell, CA (no
affiliation except I used to give them piles of money a long time ago),
the shop that did all the machine work on my MGB racer.  Their chief
machinist at the time used to work on the crew of Terry Visger, SCCA
National champ in an MGB back in the '70s; the Kaeding name should also
be familiar to followers of USAC sprint car races.  Neat shop.

The machinist explained that the key problem with early unleaded gas was
that the combustion temperatures were higher than with leaded gas, and
this was causing erosion of the valve seats -- but only in cars without
hardened valve seats.  He found that even the stock M.G. heads had been
fitted with hardened seats at the factory, as performance items, and
that they never had problems with that.  He also said that some time in
the late Seventies, the oil companies figured out how to adjust the fuel
additives so that the combustion temperatures were reasonable and that
since then, valve recession with unleaded gas was not appreciably worse
than it was with leaded gas.  Since he a) repairs valves for a living
and b) used to be with the country's most successful M.G. race team, I
believed him.

A quick way to tell whether your (non-running) M.G. needs a valve job:

Pull the rocker shaft and lay a straightedge along the tops of the
valves.  If there's more than a little daylight under more than a couple
of valves, you need a valve job.  The valves should all be the same
height; any that are taller than the rest have receded into the head and
need to have the valve seats replaced, and this isn't something where
you replace just one -- replace all eight.

Best of luck getting your '68 fixed up and out of the garage!

--Scott Fisher

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