Bicycle Mechanics - Upgrading 1985 crankset?

The chainrings on my 1985 Trek 400 are about shot. It is a Mistral crank with beautiful crankarms. But cannot find replacement chainrings without spending more than I should.
My question is: Will a modern crankset work with my 6 speed freewheel setup? I have seen a number that use my square taper BB, but most say 9 or 10 speed compatible. I am leaning toward a compact setup.

Yes, it'll work fine. You'll most likely need a matching bottom-bracket as well since modern cranks are designed to work with symmetrical BB-spindles (the one you have is most likely longer on the right side). The 9/10-spd compatible spec. is just narrower chainrings to fit inside narrow 9/10-spd chains. You can still use your 3/32" 6/7-spd chain without any problems and the 6-spd freewheel won't even know you've changed the cranks.

that was a great looking crank, what is the BCD? if it is a standard pattern (130,135,144) I can't believe it is cheaper to replace the cranks and BB rather than buying new rings.

Measure the distance between the centers of two ajacent chainring bolts and compare it to this chart: http://www.sheldonbrown.com/cribsheet-bcd.html

If the distance matches either a 130 mm or a 110 mm bolt circle, you can find replacement chainrings many places, including Harris Cyclery.

Sugino makes great inexpensive rings for the 2 common 80s sizes, 110 and 130.

I just looked over on Velobase. this has a 144BCD. still a great looking crankset but if he wants a compact drive (which I never understood) he has to replace it.

http://velobase.com/CompImages/Crankset/F9232C75-6CEB-426E-BD39-3AB672FB4B3B.jpeg

I just looked over on Velobase. this has a 144BCD. still a great looking crankset but if he wants a compact drive (which I never understood) he has to replace it.

http://velobase.com/CompImages/Crankset/F9232C75-6CEB-426E-BD39-3AB672FB4B3B.jpeg
If the crank on the OP's bike is really a Campy Mistral then you are right that it's a 144 BCD. He is going to either have to replace the entire crank or pay a lot for replacement rings. And, you are also right it won't come close to a compact set up.

It is also a replacement crank itself. The 1985 Trek 400 came OEM with an SR 110/74 mm crank. As eqipped it had 52/40 chainrings but the ability to add a granny ring as an option. Here is the Trek catalog from that year with all the specs:

http://www.vintage-trek.com/images/trek/85TrekSport.pdf

If the crank on the OP's bike is really a Campy Mistral then you are right that it's a 144 BCD. He is going to either have to replace the entire crank or pay a lot for replacement rings. And, you are also right it won't come close to a compact set up.

It is also a replacement crank itself. The 1985 Trek 400 came OEM with an SR 110/74 mm crank. As eqipped it had 52/40 chainrings but the ability to add a granny ring as an option. Here is the Trek catalog from that year with all the specs:

http://www.vintage-trek.com/images/trek/85TrekSport.pdf

PSST PSST *whispering to Hillrider* Ofmega Mistral not Campi

PSST PSST *whispering to Hillrider* Ofmega Mistral not Campi
OK, my mistrake. :) Actually since Ofmega was a lower price Campy knock-off the same conclusions apply; the chainrings will be expensive and it's a replacement for the OEM crank on the Trek.

Your replies have been very helpful and informative.

Any 144mm BCD ring designed for 3/32" chain will work on the Mistral arm; it just won't look as nice as the Mistral ring.

I have the same crank on a mid eighties Raleigh but branded Raleigh rather than Ofmega. It has 144 SR rings denoted as SR #106 if that helps any.

Historical note: in the '80's, Avocet (remember them?) imported Ofmega components with their brand on them too.

Historical note: in the '80's, Avocet (remember them?) imported Ofmega components with their brand on them too.
Yes, but Avocet never imported the Mistral crank. AFAIK, it was only available as an OEM part in North America.