The DOHC 250 Velo Engine: Securing the Cylinder Head and Cambox

Having completed the eccentric studs, the next step was to check the compression ratio – up to now I have had no means of bolting down the barrel and head properly.

The crankshaft assembly and piston + rings (the old rings – I have a new set but I’ll come on to that at a later date) were reassembled into the crankcases. The valves, without springs, were then inserted into the head and the head bolted down and checked for clearance at TDC. Plenty of clearance here - there would be no chance of the valves and piston making contact during the valve overlap period so the head was reassembled,

the piston was positioned at TDC

and with the engine held on its side with the spark plug hole uppermost, the combustion chamber space was filled with oil (R 40) from a burette.

The combustion chamber volume was measured to be 30.5cc which gives a compression ratio of slightly over 9:1. My guess is that this is probably around the value of when the engine was originally built and, although fairly high, should be OK with modern 98 RON pump fuel.

The next step was to determine the dimensions of the cylinder head studs, which also provide pillars for the cambox, and the cambox securing bolts. Like the eccentric studs for the cylinder, these were made in EN24 steel for strength and took quite a few hours of machining, for example, the 2 long bolts on the drive side of the cambox

and milling multiple hexagons.

Anyway, they all worked out well and the cylinder head and cambox fitted perfectly.

At this stage, I had intentionally left excess material on the studs/pillars so that there was a gap between the valve pushers in the cambox and the valves.

The height of the studs was then reduced to give a running clearance between the pushers and the valve stems with the cambox seating properly on its 4 contact points. I have set the clearances to 0.020” on the exhaust and 0.012” on the inlet. With care, I am hoping to build this part of the engine without resorting to shimming.

The final step was to chemically black all the bolts and studs ready for final assembly.