4 scribed marks for the valve alignment and line boring of the seatings for the bushes
I decided it was time to look inside the engine, as it was obvious from the outside that some work was needed to the crankcases, and at the least, bearings would probably need renewing. The barrel and head are all one piece, as was the practice at the time. The inlet valve is housed in a separate removable assembly (a bit like a valve rocker box), that splits from the head by a bayonet 'peg and twist' lock arrangement, together with a screwed lockring. This type of fixing was also used on the 3hp twin in 1913, altered April 1914.
Lifting both barrels off revealed one scored barrel (tricky, as the cylinder walls are only about 3mm thick), and ALUMINIUM pistons.
Removing and dismantling the external automatic recirculating oil pump revealed 2 gear wheel operation (again carried over to the early 3hp twin). However, it also features an oil feed to the base of the front cylinder, a feature that only carried over onto the 345cc racers, and the 3hp TT Replica.
Inside the timing chest, after removing the cam wheels and followers, original 1912 scribing and alignment marks can be seen setting out the valve gear! Hopefully the photo shows sufficient detail.
The drive side main bearing retaining plate is held in place by 6 screws. Each of these has the head crossdrilled (racing practice) with a tiny hole so that all can be wired in place. We're definitely into prototype territory here.
Unfortunately, the scribing/alignment is offset to the bush housings, so boring out is also off-centre and a few have cracked when the bushes were pressed in. The hole to top left of photo was for a screw to hold the bush in place! Will consider welding crack, and using Loctite with a new, non press fit bush.