I'm wondering how different the MF-6208 freewheel on the 87 master differs from the MF-Z012 freewheel on the expert?
I'd like to swap tires on these 2 bikes and wondering if I could just swap the wheels on them. They both use the
same wheels but different freewheels.

Probably close to good if the freewheel cogs are the same tooth counts. I said “close”, as one certainly needs to check and reset the limit screws on the rear derailleur. Most freewheels I mess with have a fair bit of variance as to the exact inner and outer cog positions relative to the derailleur. If tooth counts are different, that then brings into question chain wrap and derailleur capacity issues. As both mentioned are Shimano, you should probably have good luck with a swap.

They Expert would have 105 hubs and the Master 600. My Experts had straight gauge spokes. Maybe the Master had butted? My Turbo (pre IM) had butted spokes. It is more fun knowing you have a 6208 back there while you ride but I doubt you can tell the difference.

I'll probably just take the extra effort and swap tires.

The freewheels are both Uniglide tooth profile (pretty sure), but the important different would be tooth count. If not the same hi-low, it could affect your shifting and require a chain-length adjustment.

Why not just swap the freewheels, if you're concerned about that?

The 1987 Ironman Expert came with a Shimano 105 gruppo. The BioPace chainrings were 52-42T and the stock MFD12 Uniglide 6 speed FW was 13-24T. The dropouts were 126mm wide.

I got my 1987 IM sans wheels and seat. It had been used by some SoCal tri freak, rode hard and put away wet. The seat post was corroded in place and I had to cut it out.



I had a local frame builder check the alignment and spread the dropouts to 130mm. I'm using a 13-30 Uniglide 7 speed cassette with 48-38T chainrings. I had to change the pulleys on the Shimano 105 RD. Now it index shifts all the gears like butter.




The MSR List Price for the 1987 IM Expert was $520 - a lot of bang for the buck.

The 105 and later 600 RDs have a much wider range than the specs indicate. Here's a 600 with a 12-30T 8 speed cassette and 48-38T chainrings. The steel pulley cages shift a little more crisply than the more expensive RDs with alloy cages.



As mentioned above, you may have to adjust the RD travel stops and the cable fine adjustment. Unless there is a big difference in tooth count you should have no problem just switching wheels.

verktyg

When I unscrewed the plastic external "seal" ring from a 600 Uniglide freewheel, I was surprised to see that the locking cone underneath the seal had the markings of an MF-Z freewheel, so I believe that the bodies are identical.

The 600 freewheels got the added plastic shield and the cogs were a brighter shade of silver. Other than that...