There is a strong bias toward replacing the 28 amp (actually make between 28 and 32 amps by my measurements) with a 42 amp (air cooled) which make either about 45 amps or close to 60 amps, depending on whether they are actually the 42 amp version or the 55 amp version. I just shipped another repaired "42 amp" back to the owner which looked to have been in service for a long time so would assume it was an OEM install. It made 55 amps on my test rig which cannot spin to full speed to expect it will make close to 60 amps on a bike.
There are up sides and down sides to both options as have been explored in detail many times but the correct answer depends on your own perceptions regarding risk, service endurance, output requirements, etc. There is not sufficient sample size, IMO, to understand clearly where the advantage lies. With the greatest respect and affection to postings, most are simply the extension of personal bias without a conclusion which can be defended.
Just to expand on some of the views expressed:
1) "Tick, tick, tick....your 28 amp unit is about to fail" I cannot see that this can be defended logically because there are so many 28 amp units still in operation from 1991. In the UK, the failure rate of the 42 amp type seems to exceed that of the 28 by a significant degree.
2) The 28 amp lacks sufficient power output....here's where someone will likely hoist me on my own hypocrisy because I swapped 28 to 42 soon after acquiring my 1994. I think that I understand hypocrisy quite well, having continued to practice it for so long.
: the 28 amp produces more than adequate output for most users, having over 30 amps output (32 to 33 amps is typical) which allows 3 amps over my heavy lighting (100 watt high beams + 2 x 55 watt driving) so the need for more output may not be important. You need to decide but please do not do so on the basis of subtracting estimated Watts of loads from the estimated alternator output. Measure current and work from there. On the other hand, one could solve much of the electrical load problem for most users by swapping to HID headlight bulbs at much lower expense and trouble. Explore all the options before making the decision.
I recently swapped WhiSTler to HID because the headlights seem to me to be more "eye catching" than were the 100 Watt H4 bulbs even with wiring upgraded. I don't need the power conservation but changed for the other reason.
3) 28 amp "failures" generally are to two types: oil leakage from the wire grommet, and VRR connection burning. Sirepair, makes solid points regarding the refurbish problem as these units are not well understood or well supplied with replacement parts. I haven't had one apart for some time so cannot advise very much as to the problems in disassembly/reassembly. There is no question that the 42 type is better understood because it is cousin to so many automotive ND alternators.
Servicing of the external VRR wiring is critical for any motorcycle using this type of strategy and neglecting this on a 30 year old bike is certainly a ticking time bomb.
4) I think that I know of more 42 amp failures than that of 28, especially if one considers only the catastrophic failures which leave someone stranded. The 28 is actually a more advanced design in that it is sealed to contaminants.
5) I am beginning to understand the degree to which the alternator repair industry has changed since my time in the industry. It has become more difficult to obtain service to the 42 amp type than one would have expected even 10 years ago. A practical problem for a repair shop is to drive the repaired unit for testing purposes and this requires a purpose intended test rig because the unit cannot be belt driven. In retrospect, it may have been better for me to have a pulley adapted to fit the drive spline rather than using the rig but could still do that in future. The volume of repair units is tiny at this time, much smaller than ABS1 modulators, so not sure whether it would be worth the bother.
What would I do now if had a 28 amp which failed....first thing, that answer would be difficult to defend, given the small sample as said earlier so each should look to one's own wallet and biases.
Point 1, however is that the swap is not a particularly difficult exercise so that should not deter one unless it needs be done in a limited time frame.
Simply reassessing the problem with your 28 may answer your question. Is it showing some oil dampness or dripping? You may wish to try cleaning and sealing by an epoxy or silicone to see how that pans out.
What is most valuable is to hear what others have done and why they made their decision. We do lack the sample size to assist in making blanket decisions in this matter but more context from savvy people could help in that matter by improving the quality of the data which we do hold.
FWIW, the failures I usually repair on 42 amp units have been regulator, bearing, rectifier. Corrosion expansion and failure of the stator core is an not infrequent cause of catastrophic failure which usually also destroys the brush-end housing. I see these periodically. Brushes are a wear item but due to the low average alternator load, don't wear out as quickly as one might fear. There is a significant history of rotor winding failure to the 42 amp unit and this is a problem because replacement of the rotor is not an option = not available. Replacement of the rotor winding runs about $100.00 and is very specialized. There have been several excellent threads dealing with the repair, in place, of the rotor lead by die grinding, soldering and epoxy.
At this time I am wrestling with parts supply issues for the 42 amp as some of the parts have become more difficult to obtain in this market. I am exploring a bulk purchase from over seas but it is difficult to rationalize this from an economic stand-point. I repair these bike components (modulators, pumps, alternators, starters, etc.) to help others but there is no profit from which to insure against some failure which would require that I cover the cost. In addition, there is no profit or economy of scale to absorb the investment of stocking a larger parts supply so will likely just continue as is for now unless a repair fails.
The Suzuki Bandit uses what looks like the same series of alternator but cannot obtain a sample to do a direct comparison as this might be useful information for either the ST or Bandit people.
Sorry for the long tome but thought it might be of interest to throw out my appreciation of the alternator question. There will be valid differences in point of view which is good as these provide additional perspective and help me to check out my own biases.
Off to take son out to practice his riding. I plan to move him from the GS850 to the ST today so that he has another experience. He has been practicing the cone maneuvers in preparation for his parking lot learner's test and don't think I can match him. Time to get back to that task as skills erode...
The very best to all!
Norm
