I have a 300ex with a 385 kit (all bore). It started puking oil out the crankcase vent tube after it warms up, with an obvious loss of power. My suspensions are that I have a broken or cracked ring, but I have not tore it down yet. My question is, does anyone know what piston this would be? Pro-Fab ATV did the work about a year ago and he is now out of business. I don't know anything about the work he did other than its a 385 displacement with a stock stroke (65.5). I would like to try and find a set of rings before I tear it down if possible. Any help you can give would be truly appreciated.

Thanks
Bill

ps
Can you figure the size of the piston with the info above or do you need something else???

 

If the stroke is 65.5 you are around 86.5mm on the piston. the 400ex is stock at 85 and the 416 piston is 87. I have heard of the 400 piston into the little ex. We have a 305 on our 282 cc'd 300ex so I have done some research into that motor.

I would say take it apart and mic the cylinder and see if your bore is good. If it is get a 400ex bigger piston.

 

just for the record, thats a huge overbore, and a 86.5 mm bore with a 65.5 stroke would produce a little over 384cc's but i've never seen a piston that big made for the 300ex and im not sure, but i dont think a 400ex piston would work, i mean im not an expert on a 400ex but it has a longer stroke, so i'd think that the 400ex uses a piston in which there are more differences than simply the bore size, most likely the skirt of the piston is either longer or shorter depending on the rod length, and if put into a 300, even with teh correct bore size, this piston would cause 1 of 2 problems, either and extremely low compression ratio, or it would ram the piston into the head, and well thats bad, i would highly reccomend doing an extremely large amount of research,math, and thought before putting a 400 piston into it, i'd look into the possibility that your really running an overbored 300ex piston which are commonly available in .020,.040,.060,and.080 oversizes, but these produce nothing like a 385cc i dunno the guy that sold it to ya, but i've never heard of a 385cc 300ex motor that has been done with sticly bore, sounds kinda hoaxy to me. but hey, dont listen to me, do your own research so you don't mess anything up, cause that would well......suck.

 

Hey, thanks for the opinion. Thats why I asked on here so I could get some different ideas. He resleaved the cylinder and that required that the case halves be cut to make room. He had the description up on his website. The guy built two other motors for me that I was very happy with, and he came highly recomended from two others I talked to. The mystery is HOW he did it, and what piston did he use.

 

I just went out and checked. I have a 400 piston as well as a 300. They are way different. About a quarter inch difference in wristpin height and diameter.

Try wiseco's web site and see if you can talk to some one about it. There is alot of Honda pistons they make, it would just be a matter of finding the right wristpin stats.

 

You pretty much have to pull the cylinder and see what kind of piston is actualy in there. That might not be the problom. Most likely is though.