This modification is done to take advantage of the larger (1-7/8” ) inlet valves on the later cylinder head, although the earlier heads can also be modified to accept the larger valves. Issues which need to be addressed are as follows:
1. The inlet manifold on 3.8 litre motors differs from the 4.2 litre manifold. This article assumes you have the 4.2 litre inlet manifold.
2. The later cylinder head has extra holes for coolant flow between the block and the head. The two rear holes have to be welded closed. Any cylinder head reconditioner should be able to do this.
3. The later cylinder head casting is more upright at the front. Therefore, if you are using the earlier polished cam covers you will notice that the machined mating surfaces on the later head extend about 5 mm forward of your polished cam covers. Fortunately, if this offends your æsthetic sensibilities (as it does mine), there is enough metal to permit filing back in a nice radius followed by polishing. The fit of your early cam covers will then look just as it should.
4. You will also find reduced clearance or interference between the U-shaped aluminium breather and the cylinder head. You’ll have to take a file to the rear of the breather in the appropriate places.
5. If you want to retain your old tachometer generator, you’ll need to drill and tap three holes for it on the inlet side. Similarly for the camshaft blanking plug on the exhaust side you’ll need to drill and tap two holes.
6. Keeping the tacho’ generator means you’ll have to retain the old style camshaft covers, but for aesthetic reasons you’d probably want to keep these anyway.
7. The rear inlet camshaft bearing cap on the earlier head is longer than its later counterpart. It extends rearward out through a machined half circle in the polished cam cover, whereas the later shorter bearing cap does not. (This is because the later cam cover is not machined to accept the tacho’ generator.) The old bearing cap can’t be swapped onto the Series 3 head because it won’t mate correctly with the head. Therefore the half annular gap between the polished cam cover and the tacho’ drive has to be filled. The easiest way to do this is to cut the end off an unwanted early bearing cap. Alternatively, you will have to get an appropriately dimensioned ring turned up in a lathe, cut approximately in half and filed to accurate size.
8. If you don’t want the original tacho’ generator, but do want the old style polished cam covers you’ll have to arrange for an aluminium plug to be turned up.