The Restoration 1922 OEC 550cc Blackburne engine

Amid all the frame work on the Scarlet Runner, I found time to strip and check the OEC engine. Blackburne increased capacity from 500 to 550cc in 1922; I’d date this engine as 1923/24 and although outwardly similar to the 500cc there are several differences between it and the Runner’s original 1920 engine...

1 The 500 and 550cc engines share an 85mm bore; the 88mm stroke was extended to 97mm to increase capacity. For 1923 the head was much more generously finned.

2 Cranks are very different. The 500’s onepiece, forged crank and plain big-end was replaced by a built-up one with roller big-end; full circle flywheels arrived in 1923.

3 Cam design changed between 1920 and ’23 with a longer dwell on the later cam (left). Blackburne used this improved profile on all engines – side and overhead valve.

4 The single cam lobe operates two ‘bellcrank’ followers; their position and radii give different opening/closing periods – it’s simple, but limits options for valve timing.

5 Valve overlap is split 2:3 over TDC (if the inlet opens at 10 before top, the exhaust should close 15 after). Two pinion keys allow vernier (‘half a tooth’) adjustment.

6 This is TDC – it’s not a hi-comp engine! This one had a thick compression plate, suggesting it came from a mower – identical to a bike engine but usually less worn!

7 Vintage engines often use bonze pads rather than circlips to secure the gudgeon pin. Check there’s still plenty of material at the ends – they do wear away slowly.

8 The right spanner for the valve caps helps avoid damage. It’s worth renewing the copper/asbestos washers, available from Lloydhansa Ltd (07973 259319).

9 Coming along nicely! The next big jobs are fitting a front mudguard, making a brake pedal, sorting the clutch and trying to deal with a bottomless fuel tank...