If the miles are original at 11,000 and the engine never suffered a series of catastrophic failures or something... there's another reason it might be on its last bore...

Some owners have the mistaken idea that "boring out" the engine to the maximum available oversize would somehow double their horsepower or something.

I've known people who've done it... especially when these bikes were new... even to engines on standard bore that were in perfect condition. It was very common when I was racing. I spoke to one engine builder I knew who "recommended" this when he would port an engine. I asked him if boring to the maximum really gave any measurable increase in horsepower? He said he didn't know but it definitely kept him busier porting cylinders... at least he was honest.

One thing to watch out for when oversizing the bore of a two stroke is that since the ports are angled into the cylinder, the port timing and size can change significantly. And always to the side of smaller and "lower performance". The rear transfer boost port on reed valve Yamaha's comes in at such a steep angle on some models that even a couple of sizes overbore can lower it a couple of mm which will lower performance if not corrected. Exhaust ports also end up slightly lower and sometimes narrower as cylinder is bored oversize. Very few shops correct for the porting changes that occur as the bore gets bigger so most people I know that "overbored" and did nothing else actually ended up with less power than they had stock.

Mark

Is that the "chamfering" that I often read about on re-bores?

Have you tried the Bike Boneyard for a jug?

you may want to consider geting your cylinder nicasil plated , you can dedcrease your bore size ,020 and keepit there for a long time , its much more durable then the iron . call Power seal USA 866-845-1531 its worth looking into .

Rich

No. Chamfering the port edges must be done no matter what or you risk snagging or breaking a ring.

Take a look at the picture... I drew an angled line in the rear boost port... if you can imagine as the bore is taken oversize, the distance to the top of the port to the top of the cylinder is going to increase.

The steeper the angle of the port as it enters the cylinder, the more the port height will be affected as the bore is made bigger. The transfers and exhaust tend to angle up... so as the bore is made bigger the ports get lower which not only makes them smaller but "detunes" the port timing.

In fact, if you look at the other two transfer ports you can see how the larger one of the left looks like it's a bit shorter than the smaller one on the right. I'm pretty sure that if you looked at a new DT400 cylinder, the transfers would all be the same height.





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Mark the transfer ports from the factory are not equal nor is the boost port , When I port a cylinder I do make them equal which makes for much better response . Although I do agree what you say about bringing the bore out alters port timing .

Rich

Rich when you plate a cylinder dont you have to run a different type ring? Of a different material than cast iron?

Nick

Being a nosy so and so, I decided to ask about the barrel on the engine (+ bits) on ebay. Shown below is the chap's reply.

Hi Alan,
The engine no.starts 501 which I believe makes it a 1975 dt400b. This engine was in a ty250 frame which I have now fitted with the correct engine. It started easily and ran well then. I believe it has a big bore cylinder on it. Hope this is of some help to you.

Regards Nigel.

Good luck with whichever way you go.

Alan

Well i suppose you've already got a big bore as yours is now but i'd guess whoever had the Ebay motor before the present owner looks like they've removed every second fin to make it stand out in a crowd--which it certainly does. Interesting what people do--even if it's not a good idea.