I am still thinking about rebuilding/improving my '86 250R. I have been looking into using a factory cylinder, but by the time I buy the cylinder, have it bored and ported, then buy all the other necesary pieces, it is just a cheap to go with a Pro-X cylinder. So, my newest question is about the rod/case situation. I like the idea of the short rod (faster revs), but would like to be up around 330-350 cc in the distant future. Would it be better to swap the crank and rod to a later style and use a bolt on 330, or go ahead and have the cases split and machined out so that I can use my short rod with a 330-350 cylinder. I have heard of some very nice running short rod 350's, but both options seem like a lot of trouble. What would be a decent price to have the cases split and machined or the crank swaped? Also what would be a good place to have it done? I do not feel like I could do the job, but want it to be done right.

Thanks,
Nic

 

I have been doing some more research, calling engine builders. I did not realize that the bolt on 330's only allow for one overbore. I have also heard that the 350R's are more trouble than the 330's, and typically only make more power for true top-end-ported drag racers. Some builders even said that they don't recommend a 350, it is just better to go on and do a 370 with a stroker crank. As of right now, I am considering having my cases machined and the bottom end checked. That way I can try a ported non-bolt-on 330. I would also like to keep my short rod crank to try and keep the faster revs. I plan on using a 38mm A/S carb. Any suggestions? What pipe, clutch, gearing etc.

Thanks

Nic

 

You are right anything bigger than 310 you will need to machine the cases except for the 330 on a long rod which only give you one overbore.

If it where me I would just go to a 310 and be done with it or machine the case bore/stroke it to 370. I don't see the point in machining the case for an extra 20ccs.

The power is going to come from the porting anyway.

 

you will get more potential out of your motor if you machine the case. You will have better transfor ports.

if you are doing just drags then get an FTZ drag pipe. Get a hinson clutch basket (best for dumping the clutch) with some new clutch plates. you could get the CR500 springs because they are stiffer and can hold more stress.