Vacuum Pump ?

[170C]

While checking the Illustrated Parts List for our O-300 series engines I was wondering what the opening was just above where the generator/alternator fits. Must be a oil return passage, but above it is a plug that is identified as # 15, part # 2025, plug, 3/8 NPT, Vacuum Pump Oil Return. Were our engine accessory cases originally manufactured with the plan to incorporate a rear mounted vacuum pump?

[blueldr]

If your airplane was originally a C-172 with an O-300D engine, didn't it have a vacuum pump?

[170C]

My '56 C-172 has a O-300-B engine. The current accessory case was one that replaced the original one at overhaul, so I don't have any idea what series of O-300 it came from, but I believe the D engine uses a different accessory case from A-C series. This is just an opinion, not something I am sure of.

[cessna170bdriver]

My '56 C-172 has a O-300-B engine. The current accessory case was one that replaced the original one at overhaul, so I don't have any idea what series of O-300 it came from, but I believe the D engine uses a different accessory case from A-C series. This is just an opinion, not something I am sure of.

The only difference between "5-bolt" accessory cases among the different model engines (that I've been able to discern) is the length of the studs for the starter, and that oil return hole. The accessory case currently on my engine originally came from an O-300D, and was retrofit with the studs for an O-300A, and a plug installed in the vacuum pump oil return line.

Miles

[Bruce Fenstermacher]

Miles

The biggest difference and one that will definilty stop one from using a 3 hole accessory case when they need a five and visa versa is the number of studs holding the accessory case to the sump.

There is either 3 or 5 studs and you can't use the wrong one. Of course you can change the sump to match the accessory case.

There are other differences but the stud count at the bottom is the major difference.

[cessna170bdriver]

Miles

The biggest difference and one that will definilty stop one from using a 3 hole accessory case when they need a five and visa versa is the number of studs holding the accessory case to the sump.

There is either 3 or 5 studs and you can't use the wrong one. Of course you can change the sump to match the accessory case.

There are other differences but the stud count at the bottom is the major difference.

I agree 100%. I just pointed out the difference between the various "5-bolt" accessory cases.

The only difference between "5-bolt" accessory cases among the different model engines (that I've been able to discern) is the length of the studs for the starter, and that oil return hole.

Miles