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MG MGB Technical - Unleaded Gas

I am new to the '70's MG's. Have a MGB-GT '74 and the manual says unleaded gas usage could lead to engine problems. What have folks done to avoid this now that all the gas is unleaded? And, is this really a concern" The manual also states that the car is designed to use 91 octane, what about the unleaded plus gas at 89 octane?
Thanks for the help, guys!
JOHN
JW Colson

John: Theres plenty in the archives on this old chestnut. First and foremost, use the highest octane you can find. I am not sure if the 91 quoted was based on the old rating system, or is the newer rating. Too low will cause engine pinging that will damage your engines pistons and bearings, so it is best to use the right octane. Super Shell or Sunoco 94 is what I use, but my 69 has a high compression engine that called for 98 octane, old rating.
The engine damage referred to in the manual is probably valve seat recession. The lead was used to "lubricate" the exhaust valves. Without it the heat generated can cause the exhaust valve head and valve seat to weld themselves together momentarily, which results in microscopic metal loss that over time results in valve seat slowly being eaten away. Some writers claim that engines that used lead originally have a "lead memory" that lasts for some, unspecified period. Many owners report that they travelled many tens of thousands of miles without nay problems. Very hard driving is known to cause more rapid wear.
Most owners, myself included, will not run out and have the cylinder head reworked to install new hardened seats and stellite valves, but will wait until other engine work is being done, and then have the head done too.
About every other tank I use lead substitute to prolong the inevitable.
Andrew Blackley

One more thought regarding this:

If any valve work has been done on the car since the mid 1980's, it has probably had hardened seats installed.

If by some miracle it still has the original valves and seats after 30 years, then as Andrew said, there's probably enough lead impregnated into the seats to carry you for awhile. Otherwise the seats would already be destroyed.

The one exception is if the car has extremely low original mileage, thus original valve seats and not much lead in them (i.e. been stored in a barn for 28 years).

As to the octane, find a high speed road or highway next to a wall or construction barrier. Get going at about 3500 RPM in high gear, open the window nearest the wall and stand on the throttle. If you can hear any pinging echoing off the wall, then use higher octane. Otherwise, you're okay.

Mostly, drive and enjoy

- Matt K.,
another '74 owner.
Matt Kulka

FWIW, back before I knew about valve recession and whatnot, I had the head on my '69 reworked. They put in new guides, new seals, and cut the valve seats -- we didn't replace them. I'm convinced that any "lead memory" would have been cut away at that time, but 25,000 miles later I'm having no valve seat recession. I usually run 87 octane (with the timing suitably adjusted) with no additive, apart from an occasional dose of Techron or some other cleaner (usually with every oil change). My attitude has been that I won't worry about it until I start seeing problems....

And, as Andrew says, this has been discussed ad infinitum in the archives. Probably worth a read if you want to take the time....

HTH!
Rob Edwards

Matt makes a good point -- I don't know what was done to this car before I got it. It may well have hardened seats. I do know that it sat for many years before I acquired it, and at that time had about 91,000 miles on the clock.
Rob Edwards

This thread was discussed on 24/06/2003

MG MGB Technical index

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