The latest and greatest from Longman is a cylinder head that has the same valve sizes as the early BIG valve "S" head (1.4" Intake, 1.21: Exhaust), AND that now is setup for unleaded fuel! The 21cc combustion chamber can be made smaller but this is slightly smaller already than stock; thus everything else being equal, the compression should be slightly higher.
The GT7 cylinder head has performance that you can not match for twice the price! Flow bench tests and a survey of competitive price in either the UK or the US have proven the GT7 head from Longman is the champion.
All Longman heads are fully ported and polished and come assembled with Longman's valves, silicone bronze guides, springs and retainers, Valve seats are flame hardened for use with unleaded gasoline.
Combustion chamber is 21cc as supplied but can be order to your specs.