the original cast iron head has 18mm plugs and works well, but provides just 20bhp! Unfortunately the 72 year old water outlet has corroded and is difficult to seal.
the original plugs for the cast iron heads are short reach (½") types. 18mm heads use Champion D16 or NGK type 1110 A-7 fitted and set to 22thou.
correct plug for the alloy head is a 14mm long reach (¾") type - Champion N8 or I used NGK B6ES
Oh dear!!! The gaps on the plugs were reduced to almost zero when the valves punched them!
so - all plugs were packed out with 14mm inner x 20mm outer by 1.5mm thick aluminium sump plug (oil drain) washers to give clearance. No wonder it initially ran poorly on two/three!
Piston swept volume = 1125/4 = 281cc per cylinder
Cast Iron = 51cc + 5cc for 95thou gasket plus 2cc for 25thou at top of bores; giving a compression ratio of (281+58)/58 = about 5.85:1
Aluminium = 42cc + 5cc for 95thou gasket plus 2cc for 25thou at top of bores; giving a compression ratio of (281+49)/49 = about 6.73:1
No adjustments were needed to the carburation, and the clean response to the throttle continued unchanged.
The Lichfield does seem to run a little bit better, with maybe a little more power. I've not bothered to quantify it as it hardly matters how many minutes it takes to 40mph!!!
However it does now go up most of our steep hills around here in 3rd gear instead of 2nd and is much more pleasant to drive in this hilly rural area.
More importantly - I notified the change to my Insurance company. They noted the change but have not altered the conditions or premium.
Cylinder compression pressures at 5,100 miles from rebuild and after fitting aluminium head and spacer washers have increased from 95psi to:
1: 110psi 2: 105psi 3: 105psi 4: 110psi