Cylinder Head

I have a 49 p3.I am wondering if the head from a later more powerful engine can fit my block. 3ltr would be great or do I have to fit a later engine as I am fitting a differential from a p4.I'm finding it hard to find any info on the Rover engines. Regards Peter Lynch Perth Western Australia


Well-Known Member
I recall fitting an alloy P4 head off a 75 onto a P3 block, so that shows the 75 blocks were the same. The P4s used an SU carb. Easiest way to check is to compare head gaskets for matching bolt holes, cylinders, and water passages. Not sure if the heads off the larger 90 blocks will fit, try the RSR as above for info on this.
Edit :- the P4 75 (2103cc) had a bore of 73mm, stroke 89mm; the 90 engine (2645?) had the same bore, longer stroke at 105mm. Both had 4 main brgs, so I would suggest the bore spacings are the same. The alloy heads with SU carb should give you improved power - check the head gaskets - and contact some of the Australian clubs, all on the web. If you can get an alloy head on, think about next going to a later diff ratio, from 4.7 to 4.3 to reduce revs when cruising. I doubt the heads off the late engines would fit, as they went to 7 main brgs, which may have increaseed block length and bore spacing, and thereby bolt spacing. Loss of power in early motors could be caused by wear on the rocker shafts and rockers in the cam chest, which was not really overcome until the followers /rockers were fitted with rollers - 7 main brg versions. Locate the rear cover of the camshaft (left rear of block, level with rocker cover) and see if there is evidence of a hole cut in the firewall that lines up - easiest way to pull rocker shafts was through such a hole with an easy-out welded to a tube from inside the cabin. Much easier than pulling radiator and timing cover etc. This will release all the rockers for inspection.....but can you replace them? There are 2 shafts , a short one at rear, longer one after the distributor drive.
Probably more than you wanted to know?
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Thanks for that. My car is not on the road at the moment. The motor runs ok but it has what I think is tappet noise. The block is clogged up mud and has had boiling problems before I got it. The car is not in bad shape but is mechanically poor. I've replaced all the front end bushes & shocks fixed the gear selector as first was reverse most of the time. I have P4 diff to fit I just hope the donk has enough power. Thanks again Peter Lynch
You might need to do some research; as far as I'm aware the vast majority of P4 6-cyl heads won't fit. The early P4 75 up to about 53/54 (the twin carb 2103cc) I think should fit. However, when Rover developed the 6-cyl 90 engine (2638cc), it was a spread-bore engine and, as far as I'm aware, not compatible with the earlier 75 engine.

However, for the 55-year facelift (bigger back window) the 75 engine was changed to a short-stroke version of the 90 spread-bore and became 2230cc with a single carb.. The 105 models were twin-carb versions of the 2638cc engine. The 6-cyls in the later 100, 95 and 110 were different again, based on the 3-litre block and they were 2625cc..

The 110 had a 'Weslake' head as did the 3- litre MKII onwards, which was quite different to that of the 100 and 95.
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Well-Known Member
IIRC the P3 engine had steel plates on part of the block water jackets - below the inlet side. If you have these it might be wise to remove, clean, repair, and clean the insides of the block. Also I think the P3-4 engines specified hot valve clearances, which is a real PITA for the exhausts. Spec clearances are 8 inlet and 12 exhauts, thou. We found , IIRC, that cold settings of 3 and 7 though worked well (on the alloy heads) - cant guarantee the figures, but they will be close.
Gear selector - I recall my Mum getting stuck in gear once due to problems with the remote. Managed to get the top of the selector cavity off, and get home changing gear with a pair of scissors.
All the F-head 6 cyl versions will fit your bellhousing, probably safest with the 4 main brg versions, from P4s. IIRC the 6 cyls used in LRs were all 7 main brg units.

The 110 head was designed by Harry Weslake.