Stock 1275 Metro's ran 12 thou inlet and exhaust. ATF is automatic transmission fluid.

Thanks for your comments, havent check on the valve tolerance I will do that, what is recomended for this head in intake and exhaust ? Also what is atf ?

Daniel

As Dan stated around town local use does not seem to hurt the valves. The point where it's said the most wear happens is when you are cruising at a constant speed on the highway where the exhaust temps get their hottest. I run my "leaded" Mini's with just gas around town and on the highway (sometimes pulling a trailer) on the advice from an article i read i add a few ounces of atf to a full tank instead of the additive. I have done this for quite a few years and have checked the valve clearances regulary with no noticeable valve recession.

A big valve head normally refers  to 12G940 heads with 1.4" intake valves and 1.15" exhaust valves. The small valve version has a 1.3" intake valve and the same size exhaust valve. Cooper S head have 1.4" intake valves and 1.21" exhaust valves.

My car has the same big engine. It should ahve a compression ratio of about 10.5:1, so should need higher-octane fuel. Yes the valves are bigger (no idea what size), but probably do not have hardened seals. Unleaded fuel is OK, but expect eventual valve seat erosion and slowly diminishing valve clearance (assuming non-hardened seas). If the car is an occasional-use one (i.e. not a daily-driver) or used for racing, The advice I was given is not to worry about it, as the erosion is slow, but be aware of peformance. Using a lead-stubstitute is supposed t preent it, but unleass you know the engine's life history, you ahve no idea how long it was run on unleaded before you got it.

If you do adjust valve clearance, the experts (Kieth Calver et al) reccommend different settings for intake and exhaust to smooth out idle on what can be a slightly rough-idling cam. Sorry, I have no idea where i wrote tthat down! Note the valve dimensions in the Haynes book for 12H or 12A Mini engines are not the same as for 12HA.

Hi all its been a while since I posted here , my 61 austin mini has the metro engine ( vanden plas ) , 12HA83AA , if I remember correctly its big valve leaded head , what size ? Any one has opinion on running the engine with unleaded fuel , I have been using it with out any trouble until now.

Daniel