Stuart How many blades on the prop? Variable pitch? What
Stuart balancer are you using? Is it nose wheel or tail dragger?

It's a 2 blade metal prop (Sensenich) and I am using the spectrum mode
of a Smart Avionics PB-1 balancer to look at the vibration spectrum.

As I am actually the manufacturer of the PB-1, I believe it to be
trustworthy!

The spectrum display very clearly shows big peaks at 0.5 and 3.5 crank
speed and not much elsewhere.

A previous post suggested re-indexing the prop and that sounds like it's
worth trying.

What I really would like to find out is if there are any known engine
problems that can give rise to that particular vibration frequency.

Mark