Another build/rebuild thread Sierra + MX5

[Ianr]

I've posted this elsewhere but have had no reply yet, can anyone here help?
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I'm building a Haynes Sierra/MX5 Roadster with a Mazda MX 5 1.6 Mk 2 steering column. I bought a steering wheel and boss from ebay which had been fitted to a 1.8 Mk 1. Ok I knew there would be issues with the airbag and horn but I was surprised to find the splines on my Mk 2 column are about 15mm dia where the boss I have bought is about 25mm. Looking around many suppliers say a Mk 1 boss will fit a Mk 2 but with no airbag or horn connections.

Does anyone know what the column diameters are on the various models - or have a boss that will fit my Mk 2 - I don't want to buy a Mk 1 boss to find it will not fit again.

Thanks
Ian