GT40 heads on a Tiger
I saw this old posting that I started back in 2013 about GT40 heads for my Tiger. I thought I would add the latest info on installing these heads.
The GT40 heads are readily available from Explorer or Mountaineers in the wrecking yards. These were used in the mid nineties on the 5.0 V8, but there are two variations: early and late. The early heads are clearly marked "GT40" and have three raised bars in the casting that looks like "III" on the end of the head. These are the correct head for the Tiger V8. The later heads are marked with "GT40P" and have four raised bars in the casting that looks like "IIII". The later heads have an improved exhaust flow, but the plug holes were moved and re-orientated such that the new plug position caused a position conflict and with the Tiger exhaust manifold/headers in addition to the exhaust mounting face were slightly angled relative to the exhaust manifold/headers.
The "III" heads have a casting number of F1ZZ-AA and have a much larger combustion chamber of 63-66cc verses the 260/289 combustion chamber of 52-55cc. If you plan to use these heads you can select new pistons to accommodate the larger combustion chamber or have the heads milled. I had my milled .045" and had to adjust the intake bolt holes since the holes heads were positioned closer to the center line of the engine. No big deal, just grind a little off the inner side of the holes. I did have a problem because one of the matched set of heads was found to have a crack between valve seats when the machinist started the reconditioning. It was quite some time before another V8 Explorer was at my local yard so the project was delayed. We verified the combustion chamber size on both heads before did any milling or reconditioning. We figure that my 289 (.020 over pistons and milled GT40 heads) has about a 9.0 or 9.5 to 1 compression ratio. The engine was test run on a homemade stand for about an hour and re-torqued before installing the engine into the car. No problems. The car hasn't been driven yet so I haven't experienced the new power. In addition to the heads, there is a new Summit K3600 cam, headers and an aluminum billet distributor with a GM HEI head for electronic ignition. I expect a boost from 164hp (260cid) to about 260-275hp in the new (289cid) engine. After you mount the heads then you have to start the tedious job of determining the length of new pushrods. I don't remember what length I chose, but expect it will vary from engine to engine. Getting the right pushrods isn't near the problem of determining the length required. There is an adjustable pushrod tool that will get you close.
I'll post more after I get to drive it, soon I hope!
