Retract system: including tires with aluminium hubs, struts and the SLR electric retract units.
Best in class retracts for e.g. the P-51B and P-51D 20cc.
The Electric Retract kits consists of the retract units from SLR, electronic contoller, struts and tires with aluminum hubs.
Main tires included with the P-51 electric retracts are 4".