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MG MGB Technical - Stainless exhaust valves

A friend told me if I use stainless exhaust valves I can use unleaded petrol with out having to do any other modifications.
Is this correct?
Thanks
TS KING

Not really, the 'unleaded' modification is more about cutting out the old seats and using hardened inserts, as it is the seats which recess into the head (but only under extreme conditions, see below). This usually *is* accompanied by a valve and guide change, but I don't know about stainless.

If there is nothing wrong with the head then I wouldn't advise having the job done just for the sake of it, people have had problems with sticking valves afterwards if the wrong combination of materials is used.

I've been using unleaded now for over ten years with no 'modifications' and no sign of valve seat recession yet. I have had to retard the ignition slightly to prevent the pinking that occurs more readily with unleaded, even higher octanes like Shell V-Power 99, but that will still happen if you have valves and seats replaced unless you also have combustion chamber modifications.

I do use Castrol Valvemaster as a precaution, but the original tests of these additives only showed recession where a new engine was used at full power for long periods. Some years ago some apparently BMW research was being circulated that stated that if an engine had done more than 40k on unleaded, and hadn't had the seats recut or valves replaced, there was enough lead leached into them to protect them for 'the rest of the normal service life of the engine' whatever that was.

America has had unleaded for a lot longer than Europe, but I haven't seen more than a couple of (unsubstantiated) claims of valve seat recession anywhere.
Paul Hunt 2010

I drove my TF-1500 over 60,000 miles after rebuild. I did not install hardened seats. Pulled the head to install new guides and seal. There was no visible signs of seat recession at all.
Apparently seat recession occurs rapidly if you run under load at more then 3/4 throttle.
As a teenager I worked at a marina in the mid to late 40's and the only fuel available was white gas without lead. Only inboards thqat were constantly run at high speeds had mild seat recession.Then lead was added to control spark knock with higher compressions. Now we are going back to no lead fuels .
Progress??????
Sandy
SANDY SANDERS

The stainless exhaust valves will cope better under arduous conditions, you need, as Paul said, appropriate lead free compatible ex seat inserts. An engine that has been run for along period on leaded fule does retain a 'lead memory' which protects the seat until it is recut or lapped.

Peter
P Burgess

Stainless valves will last about twice as long as regular valves. But unless you put a lot of hard miles on the engine, they're probably not a good "investment". Regular valves with the hardened seats are good enough for most uses.
JM Morris

I think you will find ex valves for Bs are 21/4N stainless with Cr stem as standard and the inlets are EN52 standard. After market ex valves should be checked with a magnet and at least the valve head should be non-magnetic. Non stainless head valves are only suitable for low duty exhausts, I think the B series gets hot enough to warrant the originals quality valves being fitted.

Peter
P Burgess

This thread was discussed between 29/06/2010 and 03/07/2010

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