The M113 and all it’s siblings were under a constant re-engineering from day one. As stated they were gasoline powered at first, and then went to the Detroit Diesel engines. Later they also did some with a Cummins six cylinder engine (most went to Israel.) There were three completely different transmissions used as well. The first one really on showed up behind the gas powered tracks, but there were a few behind diesels. The next one was still automotive based (think Turbo 400 on steriods), and was made till the early 1990’s. The last one was completely different, and was a serious gear box designed for tracks (kinda like a scaled down Abrams gear box). The first two used an automotive differential and what was known as the water steer gear box. The last one did away with all that, and couple directly to the sprockets. It’s interesting that in the U.S. Army’s infinite wisdom they decided they did not need the water steer unit in Vietnam!! Lets hope the Aussie’s were not that dumb!
There were a few M577’s in RVN, but still not real common. The main mortar track was the M106, but there were also some of the others. Units often removed the 4.2" tube and base plate for the 81mm tube. This was fairly common and started in the rubber plantations. The zippo (flame thrower) was out there, but most smart folks kept their distance from them. Then there was the M106 that had the tube and base plate stored out of the way. It was used to haul ammo. Then they would mount the tube and your back in business.
Being made out of aluminum, you’d really be surprised at how well the hull held up in combat. They’d take a pretty good hit. Yet a typical booby trapped 155 round would destroy it in a split second (and driver). It just gets uglier from there. A 50 caliber round would penetrate the hull, and a 51 caliber as well. Mine were almost always set off by ground pressure from the tracks, and that’s why the driver’s job was for the FNG! The trim vane was usually trashed after three months of busting jungle (or less). They were almost always back ordered so they made new ones from 3/4" or 1" marine plywood. Double canopy jungle will rip everything off the hull that’s bolted down!!! Bend up everything that stays put. No tracks in triple canopy! During the monsoon season they logger up till it sorta dries up a bit. Otherwise they’ll be stuck in the mud down to the hull bottom. Samething for M48’s and Sheridans too. Doesn’t mean that combat stops, just means we’re doing it differently (on foot). Still in this time of the year, you’ll often see them on the main roads doing mine sweeps and running convoy security. Just not in the bush.