OK - Thanks Ray.
Please bear with me whilst I get vaguely technical. Cycle gear ratios are calculated with a slightly bizarre formula:
Chain ring teeth / sprocket teeth x 27.
What's bizarre about it is that the "27" is supposed to be a representation of the distance travelled in inches per revolution of the crank and it is not - but that's another story. If you calculate this for all the possible combinations (preferably with the aid of a spreadsheet as it saves lots of time and lets you play around with alternatives) you'd get something like this for the original gears supplied with the bike:
____14___17___20___24___28
52__100.3_82.6_70.2_58.5_50.1
42__81.0_66.7_56.7_47.3_40.5
Compare this with my guess of what have now:
____13___16___20___24___28___32
48__99.7_81.0_64.8__54.0_46.3_40.5
36__74.8_60.8_48.6__40.5_34.7_30.4
My concern about the 1st setup is that there is a fair bit of overlap; 52/20 is pretty close to 42/17 and likewise 52/24 is very like 42/20, meaning that for all those 10 gears, there are only really 6 ratios to choose from (remembering that you should avoid the 52/28 and 42/14 combinations). The second example is little better, as when you change down from 48/28, the next lowest gear is 36/24 - with only 2 more to go.
In practice, of course you don't need to know the ratios or even where the next highest/lowest is - you just change gear to try to maintain your pedal rhythm regardless of the slope/wind but my point is that it is common to end up with far fewer choices than is possible with a little thought. If we assume that the range 30.4 to 99.7 is right for you, consider the following:
____14___16___19___22___26___30
50__96.4_84.4_71.1__64.4_51.9_45.0
34__65.6_57.4_48.3__41.7_35.3_30.6
Not perfect, and I could certainly do better given some time but I think this would give slightly more useable, better-spaced gears with closer ratios around the area that you use most.
This of course assumes that you can find the ratios that you design, and since it's not really fashionable any more to build up your own block (=freewheel) ratios you are often limited by the sprockets that come with the one you buy.
My main point is that chain, block and chain rings wear together and should be replaced as a whole (less so large chainrings, but the principle remains true) so doing your homework whilst grinding the last miles out of what you have can mean that your new drivetrain is better suited to your needs.
I'd stick with 165 cranks if you can, but again choice may be limited. As you have found, integral chainrings designed to look superficially like decent ones are a false economy and a good, forged alloy chainset with a "popular" BCD (e.g. 86mm) will last you forever.
Check back on here before you buy as there are a small number of "gotchas" around like different bottom bracket taper angles, odd pedal threadings and so on, but there are quite a few people here with intimate knowledge of this kind of thing (far, far greater than mine) who will steer you clear of trouble.
Hope this helps and shout if any of the gobbledegook needs better explanation.
Z
PS/ Apologies for the tables - this software doesn't let you paste the kind of thing I need to make it all line up.
