There has been quite a debate between needle bearings and bushings.
http://juicemotoparts.com/bearvsbushing
http://corvaircenter.com/phorum/read.php?1,27783
It seems that this set up is usually used for 4 stroke engines.
I've tested hooking up the wrist pin to a power drill and spin it on the connecting rod with the inner ring, and the connecting rod. It seems there is not much friction heat, so i heated it up with a small torch to see if it would seize, but still seems to be ok... (tested with a infrared gun)
In terms of thermal capacity, steel retains more heat than bronze.
In terms of thermal expansion, bronze expands more than steel.
If you haven't experienced needle bearings getting disintegrated in an engine. Those little needle bearings are a real pain.
If you have been riding for more than 3,000 kms or so with the engine on, then maybe you need to check those needle bearings. Because, it's usually at this point they start to disintegrate. It would feel as if the engine has more power, then starts to stall. It would still start but would not sustain an idling rpm. If you're experiencing some of these symptoms, maybe you should check those wrist pin bearings.
There are 2 manufacturers for this inner rings, INA and KOYO. INA is a German company, while KOYO is Japanese. The ratings are not really specified because it depends highly on the type of lubrication.
When using Bronze bushings, make sure it is cast bronze coz it's rated to work 500 deg Celsius, powder bronze is rated only at 120 deg Celsius. There are also Oil less bushing designs. Most needle bearings are also rated to perform at 120 deg Celsius.
I'll try to post some pics and possibly a video of the comparison. I'm haven't reassembled my engines yet...
