Took my TV175 cylinder to an engine rebuilders for a rebore. The company is used by several Lambretta enthusiasts and also a dealer so when we discussed tolerances I asked them what was the norm and they said 4Thou when I was thinking more 2.5thou as suggested in the book.
I bought the cylinder for my engine rebuild at the last Melton Parts Fair for a very reasonable £50 with piston and head but after a closer examination I found a hairline crack in the spigot! Got this welded by a specialist and they did a grand job so a lot of work time & money invested in this already.
This week I was ready to start thinking about fitting the cylinder to the engine so I chamfered the transfer ports and then proceeded to fit the rings in the bore and check the gap which was 20thou!
Cylinder has been bored out to 63mm which equates to 197.92 circumference
minus 63.1mm = 198.234
= 0.31449 converted to imperial = 0.01238
So based on the difference and what the gap is at 63.1 then at 2.5thou the gap would have been 20 thou minus 0.00619 = 0.0138thou which would also have been too much?
Makes me wonder if the rings are the right size and more worryingly if the bore to cylinder tolerance is too much??