Will a modern Shimano crank set be compatible to replace an oval tech Shimano set from the 1980’s.
It’s for a Raleigh Montage Mountain Bike.
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Shimano make a number of triple cranks for 7 or 8 rear speeds. These can be found in Shimano's Acera, Altus and Tourney lines.
The chainline standard for triple cranks is 47.5mm (chainine is how far the center of the middle ring is from the center line of the frame, see here for details). I'm guessing an bike from the 80's might have used a narrower chainline. If possible, measure the chainline of the original crank. If it's within a few mm of 47.5mm a modern triple crank will work.
I think the chainring spacing should work fine with the Suntour thumb shifters. I'm guessing the front shifter isn't indexed, or has many index points for fine adjustment of the front derailleur.
You will need to replace the bottom bracket along with the crank. 'Three piece' cranks for a square taper or Octalink axle need a specific axle length to get the correct chain line. The Montage has a square taper axle but it's very unlikely to be the correct length for a replacement crank.
From what I can tell the Montage frame has a standard 'English' threaded (1.370" X 24 tpi) bottom bracket shell. You will need to check the width, it should be 68mm, if it is, a new cartridge bottom bracket will fit.
Focusing on the oval part of your question - here's a pic of a 1987 Montague clearly showing the oval chainrings it had stock:
They're listed as "SR Ovaltech Alloy triple chainset" so if you're going for a concourse or a period-correct rebuild, that's what you need.
Another option is any non-round chainring. Shimano Biopace would have been available between 1983~1993. All these not-a-circle chainrings look "wobbly" while spinning, but don't stand out when the bike is at rest. Simply keep watching your local auction sites for "biopace" and they will turn up eventually.
Most of these chainrings will be built for 5 bolt spiders, so you will need a 130 or 110 mm BCD spider, and a bottom bracket to suit those crank arms. It is 99% likely that you need a common english 68mm bottom bracket, and it will use a square taper to connect to the cranks.
I personally have biopace big and medium chainrings on a road bike, and I find them unnoticeable while riding. I only got them because they were almost unused. So if you're after a functional bike, use what you have and forget originality.
While you could fit an Octalink bottom bracket cartridge in a standard English thread, then you would require three-piece octalink cranks, not a more modern 2 piece crankset. However this would probably let you run with whatever 4 bolt spider and then 4 bolt chainrings.