Mk IV and V radiators

Are the radiator blocks for a 3.5 Mk IV and V interchangeable? The support frames are different of course, but can the blocks be swapped? Also, is the rad. block for the 2.5 and 3.5 Mk V the same? The Mk IV has a different rad. for each engine which meant that the rad shells, mudguards, and bonnets are model specific.


Hi Peter,

What do you mean by 'block’s? Are they the rubber mounting blocks or core?


The MkV 2.5 and 3.5 radiator cores are different and I don’t think the MkIV 3.5 core is the same as the MkV 3.5 either.


Hi Tim and Peter,

I’m using the terminology in the parts catalogue. It is for the radiator unit complete, not the core. It is part number C170 for the 3.5. I don’t have a Mk V catalogue for a comparison of numbers, not that this would necessarily be conclusive.


I don’t think they are, small differences here and there, although of course technically a MKV radiator would work on a MKIV. I have a MKIV engine (#SL2199) in my MKV DHC.

The radiator cowl and connections on the 2 1/2 Litre and 3 1/2 Litre MKV are the same, unlike MKIV where all three different size engines have a differnt radiator cowl / chrome surround and different size radiators.


The Mark V radiator is wider by a couple of inches.
My Mark V 3-1/2 core measures 17" wide x 17-1/2" tall.
My '38 SS 2-1/2 core measures about 15" w x 17" t.
Here is the relevant info from the Mark V SPC.
I’m not clear why there would be a difference between the 2-1/2 and 3-1/2 blocks, but the difference between early and late Mark V is the early had an oval hole in the left side for air intake for the heater on the scuttle shelf, where late eliminated this hole because the heater was inside the cabin.
The earliest cars also had a remote drain tap operating rod.

Thanks to all for your feedback. I need to do a full recon. of my IV 3.5 rad. but I have an option on a V rad. on which I was contemplating doing the overhaul, and keeping the original as a spare. It looks like I need to do a side-by-side check first as it is very difficult to confirm accurate compatibility whilst installed.

I think of the tooling cost for the fronts of the different IVs. Each one has different shells, bonnets and guards. It is appropriate for the 1.5 to be unique, but hard to justify just for the subtle difference between the two radiator blocks of the 2.5 and 3.5.

I have another query. I have a larger fan available, 16", which will fit behind the 3.5 radiator with a bit to spare, and will obviously pull a lot more air. I do not know what car it was for, possibly a later early XK engine. Has anyone any experience with these? I have also seen a picture of a IV with an 8 blade fan. Maybe these were after-market options.

The 1.5 litre and 2.5 litre have difference cores but the grills are the same width and the same width as the coachbuilt 2.5 litre. When the 3.5 was added to the range with the introduction of the all steel cars it was found that cooling was marginal using the 2.5 litre radiator and so a wider version was used. This of course required a wider bonnet. You may ask why was the wider radiator and bonnet not also used on the 2.5 litre all steel cars and one can only assume that there was a reluctance to lose the elegance of the slimmer tapering line when not demanded for pragmatic reasons.


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Apparently Lyons decided that with the Mark V the wider grille would do for both engines, as you notice in my post above the shell is the same for both.
So I don’t think a V rad will do you any good, definitely wider.

The 8 blade fan is from XK140 or Mark VII/VIII/IX.
The 16" six blade fan might also be from a Mark VII, although I thought they were 14".
There were different center hole IDs so these may not fit your water pump.
I wouldn’t think a bigger fan was necessary if your cooling system is reasonably clean.

MKV radiator showing the air intake for the early heater.

I have an eight blade fan on my MKIV which appears to be original.


That’s interesting. I wonder if we should investigate fans.

According to the SPCs, pre-war L and M engines used a 14" fan C.1160 with six C.1137 blades and C.472 center.
Post-war P and S engines used fan C.529 with the same C.472 center but six C.530 blades.
I imagine the blades were a different length?
Then Mark V T and Z engines went back to the 14" C.1160 fan.

The 1-1/2 Litre used a 4 blade fan.

XK120 and Mark VII initially had a 5-blade cast aluminum fan but then went to a 6-blade, XK120 being similar to Mark V but 13-1/8" diameter, and Mark VII was 14" diameter but the blades were bent at a greater angle than Mark V.

Hi Rob,

I found this photo on an old thread of an SS1 Ed Nantes was restoring which had a six bladed fan.


Would it make much difference to the cooling?


That’s an interesting fan on the SS1 as I see the six blades are evenly spaced.
The fans on my '38 SS, '50 Mark V and '51 XK120 all have unevenly spaced blades.

The idea of unevenly spaced blades is to alleviate a phenomenon known as “beating”, wherein blades passing a stationary flow obstruction such as the fan belt set up an oscillation, the same principle as a musical instrument, but which is objectionable in a passenger car. The uneven blades dissipate this oscillation.
Perhaps William Heynes was aware of this and changed the fan design when he came on board.
I have seen it on some modern cars with plastic fans.

That’s fascinating, thanks Rob. I’ve always wondered why they were set apart, albeit with counterweights. It also makes a difference when they are painted correctly! :grin: :wink:

Stay well,


The most prominent ‘beating’ sound we are all familiar with is the helicopter, as the blades pass over the body.

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