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What fuel mix?


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I don't know about the GMC, but some of the older engines will run on pretty much anything, so if supply was bad poor quality fuel might be all thats available.

The problem is that unleaded burns hotter, and doesn't lubricate the valves, so burns them out. I'm not convinced this will be a problem for you ? In my classic car, I used to use (before unleaded conversion - hardened valve seats) the castrol valvemaster additive. Mainly because this product is so easy to use. Fill the tank up, note the number of litres, squeeze the bottle and you add fluid to the top chamber of the bottle until you reach the number of litres you just put in, add it to the tank. Dead easy

I hope that helps Jack






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I use this....




Costs about £6.00 for 500ml


Available from http://www.jeeparts.co.uk/Jeeparts/customer/home.php and many other Classic Motor Suppliers


SuperBlend Zero Lead 2000, the No.1 lead substitute, has been formulated to enable vehicles originally designed for leaded petrol to run safely on unleaded fuel. A standard 500-ml pack contains enough additive to treat 150 litres (over 30 gallons) of fuel.


SuperBlend Zero Lead 2000 is fully tested and endorsed by the Federation of British Historic Vehicle Clubs.



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I never use adds in the fuel... we have unleaded fuel with an octane rating of 95 over here.


A friend of mine develops engines for Ford, He told me one that it is not important to use any adds for our "leaded" engines while they don't make high revs... He told me that it is only important for cars that make high revs like a MG sportscar wich is running 6000revs...

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Hi, I use the castrol valvemaster in my jeep, and have not had any problems to date. I half suspect that the additive is not needed, as the engine will probably run on whatever you put in it, there was a war on you know! However, it does give me a little peace of mind, and the small extra cost is not too bad, if it does stop me needing to have the valve seats done in future.

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Without either fitting hardened valve seats or using an additive the valves will definitely batter their way through the cylinder head ,at higher revs they will just go through a bit quicker.

As Jack knows we use Broquet pellets in both a jeep and a CCKW, not cheap but they are fit and forget and have a long history as they were originally designed by a B rit engineer for Merlin engines fitted in Hurricanes supplied to Russia.

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