I have posted a large archive to alt.binaries.pictures.aviation with
scans of 12 photos taken of a sectioned pobjoy niagra 90hp engine.

this is an engine worthy of a model engineering reproduction or even a
homebuilt engine in full size for homebuilt aircraft.

from memory the engine as photographed is 21 1/2 inches in diameter.
in one of the photos is a dressmakers tape measure graduated in
centimetres.

other dimensions....
these were taken off the pobjoy R 75hp engine on display in the
shuttleworth trust.

the splined prop shaft appears to be inch diameter. the rear spline on
this shaft that the reduction herringbone gear is positioned on is
40mm diameter. between the end of the inch spline and the 40mm spline
the tapered shaft section is 48mm long.

in the front centre of the motor is a square boss with rounded
corners. the boss is 49mm across. the hole in the centre is 36.5mm
diameter.

the reduction gearbox from front to back is about 150mm.
the width of the rear part of the casting is 60mm.

the four head fins are 120mm diameter.
the cylinder is 100mm from the underside of the head to the aluminium
face of the engine casting.
the bolt down cylinder flange is 9mm thick.

veedubber I've emailed a copy of the scans for your blog.

Stealth Pilot