Apparently all the "small chassis" Triumph cars apart from the Spitfire and the very early Herald 948 use the same suspension turrets. The Spitfire has a slightly different design which makes the front of the engine sit lower down to clear the low bonnet line. I'm told that the early Herald has a completely different design but I've not seen an example of those.
Thanks to all those I spoke to about the problem and those who posted in reply to my query on the Club Triumph web forum. Also thanks to Pete at Yorkshire Triumphs who let me loose in his yard with a tape measure to compare the parts fitted to various cars.
In the end it became obvious that (early Herald excepted) there are two different left hand turrets and two different right hand ones. To fit a Spitfire engine, you need a matched pair of Spitfire turrets. To perform the common conversion of fitting a six cylinder engine in a Spitfire chassis ideally you need a matched pair of non-Spitfire turrets. It does seem though that this conversion can be performed if you only change one side which probably means there are more cars out there with mismatched turrets.
There seems to be very little information published on what the differences in these parts actually are. (Either that or I've been looking in the wrong place!). Having got both parts together I took some photographs and measurements in the hope that they will be useful for anyone else with similar problems.
First of all, here's the two turrets side by side and viewed from the inside, that is the side that normally faces the engine. The Spitfire part is on the right of this picture.
The only difference appears to be the engine mounting on the front of the turret. You can see in the picture that the Spitfire engine mounting is visibly lower.
Of course that wouldn't be obvious if you don't happen to have both parts to hand to compare them. Fortunately there seems to be an easy way to tell which kind of turret you are looking at.
On the Spitfire turret, the engine mounting face is flat with a small raised flange at the end. The non-Spitfire one has a crease running vertically down the centre.
|Non-Spitfire. Shorter top flange|
|Spitfire. Longer top flange|
|Non-Spitfire. Taller from chassis to engine mount|
|Spitfire. Much lower from chassis to engine mount|
Hopefully this will help prevent someone else falling over the same issue I had. Once the correct part was fitted the engine bolted in easily.