I just pulled the motor/ trans from my '62 1600 Spider to correct a nagging oil leak. Noticed that a previous owner/ mechanic had installed a 4" long 3/8" threaded pipe stub for the dip stick extension. I didn't see it before as it is tucked under the distributor. Seems to work ... but what should the correct extension length measure? The pipe stub presently extends 100 mm. I don't want to over/ under fill the sump. Thanks.