Ah, thanks for saying about it, that saves me putting a valve in line, I've just had a look at Rupert Ratio's book and found it. I wouldn't have thought of looking up the scavenge pipe for a ball and spring. I guess it keeps the pump primed. The trick now is to make sure it works.
My new present to my B25 is the high delivery oil pump from SRM which I hope cures any oil pressure problem they inherited.
I would have thought regular oil changes and perhaps use a hi detergent diesel engine oil would stop any valve sticking .... ?
Perhaps RON's experiance was something other than the valve itself malfunctioning, there could have been something that caused bits to float around the system, which caused the valve to lock up ?
I must admit i keep to the KISS principle (Keep It Simple Stupid) and beleieve firmly in the 1st law of Engineering (if something can go wrong - it will, usually at the worst possible time - i.e. the YORK flood barrier!!!)
HORROR - my nice CI 1972 oil pump has now started to wet-sump - a bit.
The smug answer is to keep the oil clean, but i have (very) regularly changed the oil and ......
Anyway, it due to be stripped later this month once i hear back from the DVLA - so perhaps i will find something holding a valve open somewhere?
FYI - best piston to bore out a B25 seems to be a STD Yamaha XT250 piston - 75mm, this allows a spigot thickness of 2.5mm.
Just enough - assuming you don't want to bore out the crankcases ....
Also leaves the oil drain holes clear of the liner, so you don't need external drain lines (like a CCM).
Gives a 10:1 compression - MAX, though.
You do have to make 'top-hat' sleeves to reduce the piston wrist-pin holes from 20mm down to 5/8".
An alternative is a std MORGO 75.5mm piston (about £78 new) which has a 5/8" wrist pin as standard, but the compression height is lower - which probably nicely compensates for the crown 'hump'.
It does leave the spigot thickness a bit marginal ?
Grandpa's solution to the wet sumping was adding the oil tap to the engine feed line. It's been on the bike so long now it's just part of the starting routine.
I did something similar to Ian's suggestion with the bungie cord though, with attaching something obvious to the oil tap - as you can see if this pic.. :D