I  have been modifying pre-unit pistons for a long time now. Sometimes unit 650 as well. When things are a bit smoky and it is not the valve guides noted by a small puff that clears after idle or accelerating after a closed throttle. It may be rings noted by a constant smoke trail at all throttle openings. What happens with pre-unit 650’s from 1950 to 1962 is a problem with oil ring seal when using overbores of  +.040 but more likely at .060 and definitely at .080. The cylinder head bolt pattern leaves a thin wall from the bolt hole to cylinder. This allows distortion of the cylinder wall under head tension and the rings when at TDC have to contend with a high spot ¾” down the cylinder, right where the rings are trying to do their sealing. It particularly affects the one piece cast oil ring but some blowby from the compression rings may be apparent. I have been using a 4mm- 3 piece oil control ring set, a scraper with 2 rails, since when I started to go big bore on my Triples in the late 80’s. This involves using a 650 Tri piston regrooved from 1/8” to 4mm for the 3 piece, then flycut for a bigger valve and finally radiused at the bottom, changing the shape from a pot on my belt sander, for the required crank clearance. These 3 pce work very well as the bigger sleeves may see some a bit more distortion than normal in an alum triple block. A one piece cast oil ring wouldn’t cut it. Must have had the top end off ½ a dozen times trying to come up with an answer. Also I have found some rings available nowdays seem to be left wanting and getting a seal can be a problem. Always check sump capacity if smoky. Take for a decent run and drop sump oil, 2-300ml max is about all std oil rings can cope with. I found with 3 sets of rings from 3 makers that the oil rings contacted only in 2-3 spots around the oil rings circumference. This on a fresh bore in a 9 bolt block, so no distortion.  Poor.

Garth from Enoggerra Cyl heads has been able to help with supply. Before that I dealt with Total Seal in LA. US firms Hastings and Grant also make suitable rings.

Triumph originally used 3 pce oil rings in the triples and the 750 TR7/T140 but not so for a while and nowdays one can only find a one piece cast ring with coil expander, they work well but are a bit fragile. The 3 piece helps out when a bore is a bit out of round or slightly tapered due to wear, where the one piece may struggle, so hone and go again may be ok. In the 8 bolt +.040 may be ok but I find at .060 and definitely .080, the 3 piece is superior. I will make 4 studs to replace the outside 4 head bolts and hone them in place if going to .080. Generally I’ll hone the top of the bore a tad more with bolts in and pulled up some .  Nice to get the extra mileage out of a cylinder.

I have also done this procedure to A10 BSA , not because of bolt pattern as there is plenty of space from bolt to cylinder wall but because of wet sumping. All BSA , singles ,twins and the triple do it . It’s a gear pump thing. Wet sumping can be messy especially if one tops up the oil tank before startup. I tend to take a hose from the breather on singles and twins to the oil tank so if it wants to purge itself, there will be no mess on the ground.

Check out a 10 minute video I did on this subject that  I have on Yootoob  (british bikes by brian ) where I also show some of the non matching mongrels built from swap meet junk.