This also works with aftermarket tin, although you usually have to do more trimming.
Clearancing your tin for a wider engine and big intake manifolds
This article discusses a method to make your OEM German tin fit on an engine that is up to 1/2" wider than stock. Anything else, like wider engines or crappy aftermarket tin is out of bounds.
So, you just got a big engine put together, and you had to use longer cylinders or spacers, and you're going to be using big intake manifolds. Now your tin won't fit.
Bummer.
Here is what you can do.
See the offending space between the tin and the case above?. This not only will leak air, it will look like crap, and will make it hard to get a good seal on the fan shroud.
Start by evaluating the points where the tin is being held back. The first place to look is the deflector tin right above the head.
Once you figure out how much space you need to make to get the deflector spaced as much as the tin gap at the case, add about 1/16" to that, and trim the deflector. Then you know when you move it down it'll fit. Don't try to move the tin down yet, you still have some more you'll have to do.
The next order of business is to find the other points where it is getting held back. Trim these if you have to. Here is a common spot below:
You will probably have to use a grinder to carefully remove a little material from the head here to make space.
The point below is probably going to be your limiting factor. Once this is butted up, that's about as far as you are going.
Now take a piece of 2x4, and lay it down over the flat edge of the tin where the cylinder numbers are stamped. push it up against the flat area where the spark plug holes are. Get a rubber mallet and give it a good whack in the direction it would take to smash it down against the case. It should take a few good whacks to get it there. Make sure you make provisions for the intake studs so you don't damage them .. might have to notch the wood out or something more fancy to get there, or just whack from each side of the tin.
Now you have another situation to deal with. The intakes won't fit because you had some maniac hog out your intake ports and you are of course using huge intakes. Well, never fear, newbie.. There is hope. See the situation below:
First thing you have to do is push the back edge inward. If you have really large intakes you will have to slit the tin on the bottom edges of the spark plug holes. Then it will push in easily, and of course you will have to trim the cylinder head deflectors some more. Note that on some intakes, you may be able to grind material off the intakes themselves to allow them to fit without totally butchering the tin. Big Beef manifolds for instance can be ground quite a bit back on all but the most major port work. You may have to trim the outer edges too to allow the other side of the intake to seat on the outsides properly.
Once done it will look more like below:
Much better. Things are looking up.
Now the tin should look more like this too.