Originally Posted by
ProFail
I believe the greatest difference possible between chainrings is 14t. So, You could have a 53/39 but no lower. You could run a compact 50/36, a triple or get a new cassette. I personally run a 16-27 to stay in line with junior gearing restrictions. Quite the lamesauce gearing.
technically it depends on your front deraillieur, but I have a 16 tooth gap (50/34) and I think that is actually pretty common as in no special front derailleur is needed.
What the limiting factor is going to be in this case is the span of the bolts on his crank arms.
Keeping in mind that the 130mm standard for full size cranks wasn't always the case for Suntour stuff out of the 80s, he might be able to get as small as a 36 to fit. I did that on a steel frame bike I restored out of the same era with suntour components a couple years ago. You will want to take your crankarm with you to go hunt for gears so you get the size of chainrings you need. It will be non-standard compared to today's bikes.
You can also change the 53 big ring to something else like a 52 or a 50 to get yourself a set of gears that fits the local roads. (like 52/38 or 50/36) When you do that, however, you may have to lower the front deraillieur on the seat tube, so if it is a braze on, you are stuck with it. If it has a clamp, you are in business. Just drop it down and readjust the cable.
Someone also mentioned looking at the rear cassette. You can find 5 speed freewheel/cassettes that go 14x27, but 14x25 is very common. Definitely do that if you are on a 23 in a hilly area.
so anyway, hope that all helps. I had the same thing with vintage steel frame bikes before, and I live in Pennsylvania, so I understand why a 42x23 for a low gear would be a drag.
good luck