27" on 700c
 

Are there any issues with running a 27" wheelset on a frame built for 700c as long as there is clearance?

higher bottom bracket, brakes

Nope.

The bike will sit a few mm higher and this will affect the handling a little... brakes will need room to slide the pads up or new brakes will be needed.

Finding tyres won't be as easy as it is with 700c wheels as the selection is far more limited.

but why would you want to use 27" instead of 700c?

I have 3 sets of 27" wheels and was hoping to use one of them rather than having to buy another set or rims to rebuild.

good enough reason for me.

it's not that different compared to using a larger/fatter tire on the wheel, and assuming the frame has clearance for 27", it was probably built with fatter tires in mind.

The biggest issue is that you'll have to search high and low for even half-decent tyres.

You may even need to discover some NOS to avoid having to fit something really crappy.

I've had no problem finding fine tires for 27". But, I live in the U.S., which may have more of a market, and the bikes on which I run 27" wheels aren't my nicest bikes - I'm just looking for something durable and not too heavy.

You'll need to move your brake pads up by 4mm. If your brakes allow this and your frame has clearance for the tires you want to run, you'll be fine. If you have horizontal dropouts, push the wheel all the way to the rear for a bit of additional clearance.

the 4mm difference in brake pad placement comes from 700c rims having 622m diameter, and 27" rims having 630mm diameter. 8mm diameter difference = 4mm radius difference.