Ford 302 Alloy Head Replacement.
by: Mike Osborne
from: Fortification - May 1996

Fortification is the Club Magazine of the GT40 Enthusiasts Club
Copyright: GT40 Enthusiasts Club 1996

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One evening driving home in my GT40 from Bath in February this year I had the misfortune to change from 4th to 3rd instead of 4th to 5th with the result that although I only revved the engine to 4,500rpm I bent nearly all the inlet valves in my cast iron headed engine. The reason was that the heads which I bought second hand had in fact been fitted with the wrong valve springs which when exceeding 4,500rpm bounced and as a result made contact with the pistons. I have learnt two lessons from this, one is to fit a proper gear selector gate to prevent such errors and the other is that if buying second hand goods always check them over before use as even with the best will in the world mistakes happen.

A week after wrecking the top end of the engine and with the wife beginning to sense I was not entirely happy I went to Real Steel's shop in London with a view to purchasing the necessary replacement parts. Having checked the price of the parts I soon came to the conclusion that it would not be that much dearer to replace the cylinder heads complete. I therefore bought a pair of Edelbrook performer alloy heads and matching cam.

I think I was extremely lucky that the pistons and bottom end of the engine were undamaged, this proves to some extent the strength of the engine. Anyone wishing to fit large valve heads will no doubt know that the valve pockets in the standard pistons are too small and the valves will foul the piston top, this includes dished, flat top and raised pistons. Once the heads have been removed it is possible to machine the piston valve pockets while in situ though great care is required to prevent any swarf from entering the engine oil ways. A minimum piston to valve clearance of .80thou is required for the inlet and .110thou for the exhaust, the point of minimum clearance is 5deg-20deg after top dead centre for the inlet and the exhaust minimum clearance is 20deg-5deg before top dead centre. A clearance around the valves of 10-15thou is also required but in practice this should be greater to produce a good gas flow.

The heads I purchased were of the Edelbrook aluminium 6025 series with 2.020" inlet valves and 1.600" exhaust valves, the cam was the Edelbrook PM7122 high revving unit which is a straight swap for the standard unit unless you have worn bearings it is satisfactory to install the cam using the original shells, Ford did however fit some engines with undersize bearings though I understand this is somewhat rare and in my case did not arise. If you do have the smaller shells then they must be removed and a standard set installed.

The next item is the installation of the hydraulic cam followers which should be well lubricated prior to insertion. Note - don't prime the followers before installing as this will result in bent push rods. The push rods which are required for use with this cam are the slightly shorter type which are also available from Real Steel and feature a central oil gallery, these also have removable hardened tips.

Rocker studs of 3/8" diameter should be used along with 5/16" valve guide plates. Double seal 2 ring positive stop oil control rings should be fitted to the valve stems. The valve lift using the PM7122 cam is slightly below the .575thou maximum when using 1:6 ratio steel roller tipped rocker gear. It is also recommended that the cylinder head bolts be replaced with ARP 7/16" diam. studs and nuts, these should be sealed into the block where they penetrate the water jacket to prevent leakage, make sure the ground washers are fitted to prevent galling, these studs can be far more accurately torqued to the 70ft lbs required. The heads are also machined to take the larger 1/2" studs if you so wish though the block will obviously need re- threading.

If the pistons fitted are of the slightly dished type then a compression ratio of 9.2 - 9.5 to 1 will be obtained when using the alloy heads as the combustion chamber is only 60cc, this I feel is adequate given the relatively low octane rates in use. Spark plugs are of the 14mm x 3/4" reach gasketed variety with a heat range of 5-8. Use competition head gasket (Fel-Pro 1011-1) to get a good seal as the expansion differential is much greater than the standard heads and cast iron block. Thought should also be given to replacing the timing chain set with Edelbrook 7800 series or similar which withstand the extra pressures produced. I've fitted an MSD6AL electronic ignition system and magnetic breakerless distributor which is set at 32deg advance at 3000rpm to match the power curve of the engine which used to be a real pain to start. Starting now is always first turn even in the cold or damp, I think this is probably the best bit of kit I've fitted to date.

If all of the above is carried out and the distributor recalibrated then the engine should produce in the region of 370 bhp with 3301bs of torque at 4,000rpm, the only draw back is the slightly lumpy tick over with very little power below 1,500rpm, the engine will however rev freely to over 6000rpm without complaint. After running the car for some 6 weeks and covering some 1,000 miles I checked it over and took it to Castle Combe circuit to get an idea of the performance characteristics after the changes, to say it is better is an understatement, once the power comes in the pulling power is tremendous and continuous up to the rev limit of 6000rpm that I've set.

One final word of caution that you should be aware off is that with the wider overlap of the valves and the better gas flow the whole unit is extremely noisy, I registered 117 decibels by the pits at Castle Combe, needless to say we were only allowed to do 9 laps before being refused further use of the circuit which is limited to 102 decibels during practice and 108 decibels during racing conditions. Anyone got a repackable exhaust system to reduce the noise level during competition?

Mike Osborne
Club Secretay

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