why does the manual call for the subframe bolts to be replace once taken out?

 

I think Honda must be taking the cautious road. Anytime a bolt has been torqued, it stretches. Perhaps they think is has stretched to a point where it's integrity has been jeopardized. Perhaps they are going over a bit overboard. I don't know. Just take a look at almost everything else of this nature that they tell us to do whenever we replace something:
Replace the cat nuts when working on the exhaust. Replace the header bolts. Also, the diff output shaft bolts, prop-shaft bolts and any number of other nuts and bolts. Heck, they even have us replacing drain bolt washers. I have re-used many of these to no ill effects. I've re-used the output shaft to 1/2 shaft nuts and bolts and so far, nothing bad. I believe you need to assess your own situation to decide how far you want to go in replacing any nut or bolt you undo.

 

The two Honda dealerships where I have talked to the mechanics do not replace the bolts (I bet most do not). My transmission was out numerous times without a bolt change but I finally replaced them the last time I did the clutch.. but actually the bolts looked fine.

 

I'm a retired aircraft engine design engineer. We would typically not require replacement of used fasteners on aicraft gas turbines.

If a bolt is torqued to its yield strength, it will stretch a little. If it is reused and torqued to the same level, it will not stretch an additional amount. (The only situation where stretch can be cumulative is exposure to temperatures high enough to cause creep. Most bolts we deal with on the S2000 aren't anywhere near that hot.)

Denis