I think I know the answer but just wanted to check. If mating an 1800 kombi (type 4) motor to a 1600 squareback IRS (type 1) box do you need to replace the input shaft with one from a 1800 kombi box because it's longer? Or am I completely confused?

as long as you keep the 1800 215mm flwwheel and machine it to take a spigot bush everything will work fine inc stock starter

Yes, basically the type 1 gearbox input shaft remains, but you need to modify the type 4 flywheel to accept a type 1 bearing.

The detail then becomes a bit harder to track down but it goes something like this:

You will need to remove the old bearing from the Kombi flywheel. One way to do that is to get a steel rod thats a snug fit through the bearing. pack the bearing recess full of grease, then insert the rod about half way into the bearing. If you then bash the rod inwards the hydraulic action will force the bearing outwards. this is the technique outlined in shoptalkforums.com
I havent got around to doing it yet but I've got 2 of them to do soon. Otherwise good quality internal bearing pullers may work, if you can get hold of a set the proper size.

The bearing is the same as the type 1 needle roller bearing, ( I believe). The ones supplied to me by German Motor Works in the US are part No 111 105 313A, and they are marked with HK1512, and they cost me $7 each.
The centre of the flywheel then needs to be machined out to suit the bearing, 21mm in this case, and for 2 thou" crush, press fit. That crush info is also on shoptalkforums and is Jake Raby's advice I believe.

The bearing should be inserted with the thickest/strongest edge to be flush fit on the clutch side of the flywheel.

Too easy hey? But i suggest you do some checking of your own.

Grease and spare input shaft works great when removing bearing from type 4 crank.

Thanks guys. I remember that the crank I put in at the time had the bearing removed and I replaced it thinking I was going to use in the kmobi at the time. Bugger.

I've gotten them from local bearing shops for $11 ea , tried the grease method and modified a few tools to get bearing out but for the small $ for a brand spanker i ended up crushing them to remove

i believe there is also a small difference in the length of shafts. 10mm longer on the kombi i think. cant quite remember as its been a few years now since we fitted the motor.

yep thats why the spigot bush needs to be fitted into flywheel so the irs t1 box one will be supported by it