Converting the 28/38/48 chainrings on the commuter/touring bike to 28/42/bashguard. Shimano Alivio 104/64 BCD crank and chainrings.

I put the 42T meant-for-outer chainring in the middle position with the bashguard on the outer position and the gap between the 42T and BG looked too close. Re-mounted the 28T inner and the gap to the 42T was too big. I realize now that the outer chainring curves outward from the bolts to the teeth.

Anyone know off-hand the thickness of spacers needed between the crank spider and the 42T chainring to put the teeth at the same relative chainline as a true-middle chainring would produce?

The answer about the spacer size is that it depends on the exact chainring, and you probably need to use some trial and error. However, even when you get that right you may still have a problem with the chainring bolts protruding out of the inside of the chainring, and the chain might rub on these when it is on the inner ring. Real middle rings would be recessed on the inside to allow the bolts to be flush with the ring. TA Specialites make a 42 tooth 104 BCD middle ring.

you shouldn't have to space it, rather flip it 180 degrees- make sure the same side of the ring in contact with the spyder as before.

If you're still planning on running a FD and a granny gear, you might run into trouble if you have a chain keeping peg on the chainring which would run aligned with the crankarm.

you will also lose (if you have them) the ramps and pins on the big ring.

Edit: on second thought, nevermind, this will never be a permanent solution if you want to keep an FD.

Damn. I think I'll return the 42T and stick with the 38T, then

All that was based around a [false] assumption that the 42t was the stock outer chainring. What kind of ring is it? got any pics?

I've done tons of stuff manipulating chainrings with spacers, bolts and position, with impunity - now have a quad + bashguard and some of the rings are from cogs. The dedicated spacers tend to be expensive. Normal procedure would be to start tweaking ring positions using machine washers. After you are happy with the setup, you can get the dedicated spacers - bike shops keep set boxes at hand. Honestly though, machine washers can work just as well. The bolts normally leave you quite a bit of space for adjustments. The rule of thumb is to have at least 4 threads engaged, before needing to switch the length.

Outer modern derailleur chain ring wont work well flipped over,
if you buy a non ramped , single speed chainring, it will work
albeit up shift is a bit less automatic.

A friction shifted derailleur will have your hand accommodating the shift feel change,

big ka-thunk of STI shifting may not go as well ..

as to Thickness of spacers, get out your measuring tools,
you can determine much thru careful measurement ..

I decided to just stick with the 38T. I don't need the top-end as much, but was hoping to increase the range of the two chainrings. I was hoping it would be an easy swap, but it's just not worth the hassle. Thanks for the input everyone.