The 1275cc rear main bearing cap is larger than the 1098cc rear main bearing cap so when fitting the 1098cc rear engine plate to a 1275cc engine some relieving work has to be done to the 1098cc rear engine plate to enable it to fit the onto the rear of the 1275cc engine.
The main problem is the lack of metal in the 1098cc rear engine plate between the hole for the 1098cc oil pump cover and the enlarged hole for the 1275cc rear main.
The pump cover is of a larger diameter (93mm) on the 1275cc engine than the one fitted to the 1098cc engine (89.2mm) and the cover hole centres are set slightly different even though the 1098cc/1275cc camshaft and crankshaft centres remain the same (see photo below).
Whilst it can be made to fit great care needs to be taken so as not to remove too much metal between the 1098cc oil pump hole and the 1098cc rear main bearing cap hole. Therefore it may be beneficial to remove a little metal from the 1275cc rear main bearing top cover to compensate. Too much metal removed from the rear engine plate can lead to cracking between the oil pump and rear main bearing cap holes which is a persistent problem on the 1275cc especially the 1275cc Midget.
The 1098cc rear engine plate has been placed underneath the 1275cc rear engine plate for comparison purposes in the photos below.
As the oil pump covers are also of different diameters the 1275cc oil pump can be a tight fit under the 1098cc oil pump cover.[frame]
