Bicycle Tutor

Hi Folks, my Bike has a shimano Biopace crankset with a 52 teeth and 40 teeth chainrings, these are worn and are stretching my chain so I want to replace them. I have been able to source a 52 Teeth and 42 Teeth chainring, but unable to find a 40 Teeth chainring. Will this 52/42 combination work or should I resell the 42 and keep trying for a 40? The Biopace says "for narrow chain only", but measuring across the chain is around 7.1 mm which I think is the normal size chain which was fitted 9 months ago by my local shop - any clues as to what chain I should buy?
Thanks for the tutorials Alex, so far I've serviced my headset and replaced my pedals and repaired 3 punctures and put a new tyre on! all thanks to you!
My Bike is an old Dawes Galaxy circa 1985 and I think the crankset is probably original.

I have fitted "normal" rings to a Biopace crankset. As long as the hole centers are the same then you can fit most rings. The holes on the Biopace are the same standard BCD (bolt circle diameter) as other Shimano chainsets.
Regarding narrow chains, I think they are only narrower because the rivets don't stick out the sides of the chainlinks. I assume the inside dimension is standard for 7/8 speed chains.

The new chainrings are Biopace chainrings they are new old stock - it was just the change from 40 to 42 I wasn't sure about.

You (might) have to move the front derailleur up a bit, though the increase in radius is just 5%, that should be in the order of 3mm.

Thanks Joe,
I'll check that when I install - just watched Alex's tutorial on it ;-)

Neil, I was only pointing out you could fit an ordinary round ring if you really wanted to get a 40. You should have no problem with 52/42.
The front mech' height is set by the large ring (52) so you shouldn't need to move it. The only drawback is your lowest gear is now 5% higher.

Sorry, I misinterpreted your reply, I assumed you thought the chainrings I have bought are normal ones. Your right about the large ring and the derailleur adjustment, but I'll still check it as it hasn't been checked in 25 years! I can live with the 5%, I seldom use the lowest gear and then its usually on the 52 ring anyway.
I'm still not sure about the chain, I think its a 7/8 chain thats on there, but I'll probably remove the chain and take it with me to compare when I get the replacement chain to be sure.
thanks for your help.

Well chainrings arrived from Norway and I've fitted them. So far cycled 10 miles and they seem fine, no problems shifting and no slipping even though I haven't replaced the chain yet.
Thanks for everyones input.
Regards
Neil