cylinder boring

davenichole

Member
Apr 1, 2011
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Illinois
what is the difference between attempting to bore a stock cylinder to 72mm and a factory made 240 BBK? why is it that you can not bore a stock cylinder over say roughly 68mm? a 240 kit has a very thin cylinder wall after all.
 
There actually is not enough material inside of a stock sleeve to go to 72mm piston bore size. A stock cylinder can have another sleeve installed that CAN be "bored" to 72mm but it basically involves completely cutting out the old sleeve and installing a new one into just the aluminum left after the iron sleeve is gone.
 
There actually is not enough material inside of a stock sleeve to go to 72mm piston bore size. A stock cylinder can have another sleeve installed that CAN be "bored" to 72mm but it basically involves completely cutting out the old sleeve and installing a new one into just the aluminum left after the iron sleeve is gone.

Nuff said. I:I
 
from what i have seen, it is my understanding that the outside diameter of a stock or big bore sleeve is the same diameter. with the stock one being a lot thicker. what am i missing here?
 
same outside diameters, just thicker walls.
 

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Devil is in the details....

Stock sleeves are actually cast in place, are "keyed" into the aluminum, and not removable. They are also not large enough to run a 72mm piston inside of.

Stock replacement sleeves (made for making a worn bore into a new, 66mm cylinder) are small around and uniform in thickness and OD. They are designed for partial removal of the original sleeve (but not complete!) and to be interference fit into the original sleeve using heat.

Sleeves designed to completely replace the stock sleeve (in case part of the sleeve itself is damaged for instance) are larger around (OD) but still have small ID's. They are considerably thicker at the top (the OD of the skirt area that slides down into the cases is thinner to allow clearance) and can be bored to 72mm and still support the piston.

Big Bore cylinders are deceptive. Like the sleeves for doing the work, they have thin skirt areas. However, they are cast into place with the "top" half being much thicker than that tiny little skirt piece. They can generally be overbored to 73mm.
 
The part you are looking at in those pictures is the "skirt area" I'm talking about ^^^^^. Because it has to slide down into the cases, both the big sleeves and stock sleeves are the same OD.

The part that makes the Big Bore sleeves runable is the top half the sleeve (the part inside of the aluminum). Stock sleeves barely go up to 68mm before the iron sleeve is too thin to work.
 
Exactly. The sleeves capable of running the 72mm pistons are "stepped" so that the combustion chamber and port area are thicker. Strength where it's needed, extremely thin where it HAS to be to fit on the engine.

Luckily, that lower "skirt" area isn't really a load bearing surface or have any large holes punched in it (ports) so they can get away with making it thinner down there to allow the big sleeve to fit inside the stock cases.
 
Interesting information there, Civic. I didn't know that about the big bore sleeves. I just never really thought about it but if the sleeve was bigger all the way down, the cases would have to be bored too. I know some of the Honda 250 BBK require the cases to be "opened up", I guess that's why. On the Blaster engines that are 265cc, are the cases bored to accept a sleeve that has a larger OD sleeve down into the cases?
 
Interesting information there, Civic. I didn't know that about the big bore sleeves. I just never really thought about it but if the sleeve was bigger all the way down, the cases would have to be bored too. I know some of the Honda 250 BBK require the cases to be "opened up", I guess that's why. On the Blaster engines that are 265cc, are the cases bored to accept a sleeve that has a larger OD sleeve down into the cases?

I believe once you go beyond 270cc, you have the bore out the cases to accept the sleeve on the TRX250R engines.

I haven't ever got my hands on a complete trinity 265 kit but I don't see how they can fit that big a piston into that small a hole! I figure the cases would have to be bored to accept the sleeve "skirt".