251 / 301 Conversion

[Andy_C]

Having briefly - all too breifly, owned a 301 I was wondering about converting my 251.

So can anyone help with information.

1. I know that the head needs to be re machined to suit the bigger bore.
2. 301 cylinder may not fit into the 251 crankcase mouth - later model 251s had a larger bore for the liner than the earlier ones so I understand
3. Can a 251 cylinder be machined out to take the 301 piston - or is the liner not thick enough
4. Carb - have a feeling that jetting is not a lot different to a 251
5. Silencer - think they are the same

Just thinking about it as a possibility - the 251 goes pretty well, but I would really like it more as a 301.

Anyone have any thoughts on the subject ?

[Blurredman]

As someone who has a 300cc upgrade on my ETZ250, I can confirm the following:

Cylinder needs re-boring,
Head needs to be re-machined to accomodate extra height of the piston, plus squish band,
I believe 251 cylinder is fine for 300cc piston,
I never changed my jets, and it ran fine,
Silencer is fine as stock.


As far as extra power goes I really cannot tell. It had negligable effect. Perhaps it was slightly quicker?? But what difference is 1-2hp really going to make?

At the moment, the particular engine which has the 300cc had a mean bearing go, so unfortunately my cylinder and cylinder lining are pitted a little from metal filings but I believe It can still be fine. That engine bearing had been grumbling since I got the bike and that was since 2012 and I covered 8k miles.



I will stress however, that the 300cc I have, I bought and didn't make myself. It is an overbored 251 cylinder, meaning that oversize is pretty much impossible as the lining is on the edge of being comfortably thick enough. But, still comfortable and totally usable.

[Andy_C]

Blurredman

Thanks for your input.

Been giving the subject a lot more thought, to be honest the bike has been running so well of late I have asked myself why mess with a bike that runs perfectly well.

Having owned a factory 301 I know that they do have a little more torque than the 251, but is it really worth all the expense I ask myself.

The 251 seems to thrive on revs - it feels like the harder you ride it the better it feels. Bottom line is that I'll stick with the 251 format for now unless something else comes along.

Cheers.

[Blurredman]

Good choice. The output is negligible.

I just had a good deal on the piston/barrel/head set seeing as it was damaged in the post (never use cheap despatch companies)..


That said, at least I had a spare barrel and piston that I later put on a seized engine whereby the piston was totally unusable and barrel requiring honing. Which I have yet to get around to

[Andy_C]

I might just keep an eye out for the various bits and pieces that are needed to do the conversion, but going to leave it as is for the time being.