It should go thru to the air vents between 1-2 and 3-4.
An old cylinder with Henry Abe stepped sleeves had holes to vents. I think sleeve outer was 74.5mm
Same with another cylinder CycleX stepped sleeves 74.2mm dia.
And finally my 970cc cylinder with CycleX sleeves, 74.5mm dia.
All earlier cylinders with small oil return holes. All with holes to vent. Maybe one cylinder only had holes to 3-4 vent.
Oil WILL weap if not sealed.
A 72mm bore will have at least 76mm outer dia for 2mm thick sleeves. I have heard that 1.5mm is minimum.
What about cylinder with all that material gone?
My 970 cylinder got a wider milled part for tight sitting o-ring to seal, 77 mm (1.25mm groove 5-6mm deep). On top of that Three bond sealer (Same as Honda bond)
![](https://uploads.tapatalk-cdn.com/20240627/916fa72a13f580fd39e774ecce5e5d49.jpg)
I filled the bottom of the triangle vent due to the risk that sand and dirt could find its way into engine.
The base gasket is an RCS, 78mm holes I earlier used with the billet block that has ~77.5mm dia "sleeves".
I think that the entire triangular hole can be filled with epoxy to make it more stable.
My cylinder got ca 7 mm thick bottoms in the triangular holes. Might help with stability too
![Grin ;D](http://forums.sohc4.net/Smileys/default/grin.gif)
How will a stock cylinder keep stability after it has been bored to at least 76mm?
77mm sleeves for 73mm pistons?
The Billet block however has no weakening air vents. More meat and wider cooling flanges.
Edit:
I think Brian, "the Aussie side car racer" said they welded the air vents completely shut when boring to bigger bores.
1200cc with stroked crank?