I had put this post on the honda atv forum, thought I might get some different readers here.

I have a chance to buy a 95 300ex. It supposed to be in well kept condition all but the motor. Apparently the owner bored the engine .005 over (something like that). I’m thinking he decked the head or something, but anyway he had a mix up with gas and ran lower octane fuel than what they had modified the engine for. I don’t know all those details. It sounds like the head has a hole possibly, from a rod or something.
I’m a machinist with a lathe and a small mill. We have done numerous mods to are farm machinery as well as made several different “homemade” farm equipment. I have a 250r and have rebuilt the top end as well some work to the atv. (so I have some experience)
I’m just wondering if someone has some ideas for this atv. I’m kind of guessing the engine has had it. What are some other engines that can possibly be used or that have been used successfully. Or If you have leads on another stock engine (new & used) I could find? I can probably get it for $1000 or possibly less. Is the price a deal or not ? Or what about repair I’m sure its quite involed if the rod is screwed up. I know very little about a 300ex engine ……..so far.

Thank you for any ideas, I’m supposed to see it this Monday and make a deal it, or not .

 

Well...this is a tough one. If too low of octane gas was ran, I would say detonation occured(pressure induced self ignition), that may have done some damage to major damage. I would say the motor will need to be totally torn down and rebuilt from there. If the piston did experience detonatation and fracture the possibility exists that piston pieces may have damaged everything from valves to cracking the case. If the piston fractured then yes, the rod may have gotten bent. Even if all it did was "hole" the piston, it should be torn down and the crank windage thoroughly cleaned out. I'm not an exact expert on the 300EX motor so I'm unsure if a rod is available (Honda says that 250R cranks are un-rebuildable, hence a rod goes bad you buy the crank, where we all know that there is a host of manu's that sell rods). The machine work would be simple. Parts wise, again that's tough to say, but simply for parts I would say under $500. So...if you can pick this thing up and the rest of it isn't wasted--for a decent price (<$1000), but...also find out what he did to it exactly to know if he was simply "trying" to do some "good" or had a planned and proven package he was attempting to create himself. Sounds like this may be a good project...almost has me interested...