Can any one tell me what this tube with the red is? This is a photo of a 1977 Cessna 172 engine. Looks like the tube is directly connected to the top of the crankshaft.

  • $\begingroup$ Your other question asked about identifying parts for your checkride. For PPL and under, a DPE is not going to ask you about this part. If this part is not on the preflight checklist and/or FAA ACS don’t bring it up in the checkride. You are not an A&P. What is the real reason you are poking around this airplane? faa.gov/training_testing/testing/acs/media/… aviation.stackexchange.com/questions/78803/… $\endgroup$
    – Dean F.
    Commented Jun 11, 2020 at 2:44
  • 4
    $\begingroup$ He's just looking in the cowling inlet and wondering what it is. I would certainly want my students to get to know their airplanes mechanically. $\endgroup$
    – John K
    Commented Jun 11, 2020 at 2:45
  • $\begingroup$ @JohnK - Knowing the aircraft mechanically is great. I volunteer at my A&P’s shop for my own knowledge. But, per his other question, he is about ready to take his checkride. His time is better spent on other things besides this minutia. And, he has a CFI who he can ask questions to fit the DPE’s gouge. After his checkride, he should ask the local A&P if his CFI can’t give him the answer. But, not right now, IMHO. And, that part is on the firewall, not the cowling inlet. He is doing a lot of digging in the wrong places. $\endgroup$
    – Dean F.
    Commented Jun 11, 2020 at 2:51
  • $\begingroup$ Fair enough. On the part, what he is asking about is the orange-y elbow which is at the just forward of #1 cyl, front left, and you can see that in the inlet as it's right near the front (that is #1 cyl because you can see the hoisting fitting on top to the rear a bit, which is between the cyls). The bulkhead at the back where the oil line passes is the engine baffling bulkhead not the firewall. $\endgroup$
    – John K
    Commented Jun 11, 2020 at 3:07
  • $\begingroup$ Are there really CFIs who DON'T train students on this? It's part of understanding the basic operation of a four-stroke engine. This is basic PPL knowledge. $\endgroup$
    – acpilot
    Commented Jun 12, 2020 at 20:06

1 Answer 1


NOTE: Revised in response to poster comment: The engine is a Lycoming O-320 D2G, part of the series that replaced the disastrous H2AD engine which that '77 172 originally came with, in 1981 (the H2AD was '77 to '80). The H2AD engines (I have to think some joker at Lycoming put the AD designation in as a gag — it had a number of ADs against it), among other problems had a very small valve lifter contact face that would spall very quickly and trash the camshaft (on the plus side, you can remove the lifters without splitting the case). Most 77-80 172s have had their H2AD engines replaced with Ds, which went back to the old lifter design, which also applies to the airplane pictured.

Anyway, the fitting is tapping into an oil gallery that supplies the valve lifters and has a plug at each end. Someone has removed the plug at the front and installed the elbow to run oil pressure back to a cockpit indication, possibly because the port normally used on the accessory case is being used for something else.

  • $\begingroup$ One of the airplanes I fly is a 1977 C172N with the Lycoming 0-320. It has the constant speed prop STC. I always figured it was more of an odd duck than the norm. Have you run into many of these, yourself? I’ll have to check that tube next time the A&P pops the cowling. Also, do you have issues with this particular models prop control? The knob on ours is always very, very stiff. Even after warm up. It rotates easily. But, back & forth movement is sometimes an issue. $\endgroup$
    – Dean F.
    Commented Jun 11, 2020 at 3:13
  • $\begingroup$ Myself I've never seen a 172 with a CS STC in person. There will be about a 50-75 lb weight penalty when you do that due to the extra weight of the prop and governor and those STCs never include a compensating gross wt increase. Not something I would do if I owned one, just to get a bit of extra performance. The tube will be visible just looking in the right cowling inlet when you check for birds nests and such during a walkaround. The prop control will be some generic vernier control. $\endgroup$
    – John K
    Commented Jun 11, 2020 at 3:44
  • $\begingroup$ There was no GTOW increase to account for the prop, hub and governor. We never had more than two people in it anyway. The performance increase was decent, but nothing to rave about. On the other hand, the fuel economy was outstanding if you leaned it right. Very much worth the weight penalty if you were renting it by dry-tach rate for time building. If it did not have alternator issues, and had a better panel & stack (an ADF, 1 non-WAAS 430 and a LORAN), I would still be flying it today. I moved up to a 172SP with a G1000 for my instrument training. $\endgroup$
    – Dean F.
    Commented Jun 11, 2020 at 13:58
  • $\begingroup$ In Canada when you do an instrument rating, there are separate ratings for single (Group 3), multi (Group 1), and center line thrust (Group 2 - Skymaster et al). You have to do your IFR check ride in the type based on the rating you are after. So if you want a multi-IFR, you so a multi rating, then finish the hours for the multi-IFR and since you're doing the check ride in a twin, you end up doing most of the hours in a twin. The check ride will include an engine failure while executing a missed approach. When I did my mult IFR some years ago I only did cpl hrs in a 172, rest in a Seminole. $\endgroup$
    – John K
    Commented Jun 12, 2020 at 21:03
  • $\begingroup$ @JohnK Thank you for your answer. But the plane does have a fixed pitch propeller. The engine is actually a Lycoming O-320 D2G. After digging through the installation directions I found that it is an oil pressure reference line. $\endgroup$ Commented Jun 14, 2020 at 19:11

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .