At Twin Cam engine number 2251 (rather late in production), valve springs were changed to increase spring pressure and increase engine speed before encountering valve float. The new valve spring part numbers are AEH668 (outer) and AEH669 (inner). Final specification can acheive up to 7800 rpm before valve float.

Any Twin Cam engine rework should include later specification valve springs and careful checking of spring force at installed height, spring force at maximum compression height, and test to assure they never go to solid height. Engine maximum speed may then be increased from 7000 to 7500 rpm, but in no case should it ever be allowd to run higher than 7500 rpm. Valve spring specification can be found in the Engineering Technical Data book, engine section.