First, I would like to apologize for inadvertently breaking one of the Forum rules. Thanks to Bruce for editing my post so the dealer's photo of the spare sprocket I purchased is still visible. Now an update on what I have found out about this problem over the last week.
A very helpful engineer at Masport unearthed some technical drawings for this component and sent them to me. I have uploaded them in this post for others to see. I have also uploaded pictures of the "old" sprocket I am replacing and the new replacement part I was sent from Australia which shows that visually they appear virtually identical.
The first 2 drawings are for the "re-introduced" sprocket which Masport tell me was brought back as a spare part in 2008. They show that this sprocket has a M16 (metric) left hand thread at 1.5 mm pitch. The third drawing is older and shows the reel/cylinder spindle or shaft for the older Morrison Olympic mowers. This drawing shows the thread on the end of the spindle is 5/8 inch 18 TPI Left Hand UNF. So it seems (despite the earlier post in this thread) that Masport did in fact change the spec for the reel and drive sprocket from Imperial to Metric around 2008 but (unfortunately) retained the same part number for the new spec sprocket. I have asked Masport if there is anyway to differentiate between the 2 sprocket versions when ordering but have not had any response.
I am therefore left with a "genuine Masport" replacement sprocket of the correct part number (004298) shipped to me in the UK all the way from Australia (at a considerable cost) but which unfortunately does not fit my reel/cylinder. I cannot blame the supplier (although I will let them know about the problem) and I suspect it is unlikely they will have a old spec sprocket even if they were prepared to swap it for the one supplied. So I am investigating other possible solutions.
My best option is to find an "old-spec" spare by asking the supplier to confirm the thread spec before purchasing... but that may be a long shot. Second option is to re-tap the metric thread to imperial - the M16 1.5 metric thread size is pretty close to the old 5/8" 18 TPI UNF (which explains why it does screw on for a few turns before jamming). Another solution would be to re-use the old sprocket "boss" and get someone to weld on a new blank sprocket to replace the one that is damaged - it's only the smaller of the 2 sprockets that is damaged (you may be able to see this in my new and old sprocket pictures).
Thanks to everyone who responded to my initial inquiry and I hope the results of my investigations will prove valuable to others. Any brilliant solutions to my problem or further advice will be gratefully accepted!
