Prop extension length???

[DeltaSierra]

Do any of you old timers who know a little about early model RV-3's tell me what length prop extension is required, described below.

I'm installing a Lyc !0-320 in a RV-3A and Sens metal prop. It was previously equipped with a C/S prop and Lyc 360 on the same dynafocal mount. The 4" extension I purchased was too short and the prop rested against the cowl opening. I'm assuming Vans provided longer cowlings for builders specifying a constant speed propeller.

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DELTA/MIKE