0-300 NEW STYLE Accessory Case
Posted: Wed Feb 07, 2024 1:38 am
New style accessory case? What am I talking about. Well as much time as I've spent looking through the C-145/0-300 IPC, I never noticed this before. There on page 16 is the good old accessory case we all know but what's this on page 18? Must be a mistaken duplicate of page 16 but wait, there it is in the description, "Figure 10. Crankcase Cover Assembly (NEW STYLE).
For the last few days I've been corresponding with an individual from outside the US who owns a 172H with an 0-300-D which he had overhauled. During overhaul it was discovered that his accessory case was cracked about the starter mounting area and so a NEW accessory case was ordered.
Holy cow. Did anyone know you could still buy a NEW O-300 accessory case. Yessiree you can.
Engine was assembled, test run and sent back to him. With a few months delay the engine was installed and started. Much to his surprise he had low oil pressure and his tack was being driven backwards. He found my email in a web search no doubt linked to a thread or threads I've posted which convinced him I might know a thing or two about the 0-300 and he sent me an email. Did I mention english is not his native language (I'm guessing) and he is blind from an airplane accident so he can't see the picture and illustrations I sen him and he can't go take requesting pictures without relaying my request to a helper.
Initially there was some confusion as he thought his maintenance co told him they installed a IO-360 accessory case and that is what he told me. Holy cow is that possible I thought as I downloaded the IO-360 IPC. I knew the tack drive was integral to the oil pump gears and if his was running backwards it's no wonder he's got no oil pressure. Low and behold the IO-360 accessory case is somewhat similar to the 0-300 but one difference is the oil pump runs opposite the normal 0-300. I thought how does this compare to the GO-300 which is a geared engine which runs in reverse and so must it's oil pump. Quick download the GO-300 IPS and overhaul manuals. But wait a minute, whats this NEW STYLE 0-300 accessory case. The study began.
As it turns out he did not have a IO-360 accessory case, he had the NEW STYLE 0-300 accessory case with an oil pump that ran backwards JUST LIKE the IO-360 his maintenance co. chief commented.
So sure enough the NEW STYLE case has new oil pump gears and they are driven by another gear the runs on the inside cam gear teeth and this gear drives the top pump gear not the bottom as the original style did. The original style lower pump gear is driven directly from the center of the cam gear and the lower oil pump gear turns in the same direction and speed of the cam. The NEW STYLE gear being driven by a gear running inside the cam gear also turns at least 2 times, maybe more, faster than the cam. To correct the direction of the tach drive a 90° gear box is used to change directions and slow the rotation speed back down. This reduction box is listed on page 18 with the accessory case.
But how does the oil flow from the sump to the pump to the rest of the engine if the oil pump is reversed. Well I wish I knew. The illustration in the IPC on page 18 looks pretty much like the one on page 16. Sure it is at a different angle and not shaded, and has a few more ports that are plugged but besides the oil pump gears, loos the same.
Lets look at the GO-300 accessory case illustration. Yep looks the same. Wow ,wait a minute now it's not. The 0-300 accessory case has the suction screen on the bottom left and the filter on the bottom right and the GO-300 has there positions swapped. Thats how a reveres oil pump could work. Problem is this in not how the NEW STYLE accessory case is illustrated. Of course we've seen IPC illustrations wrong before.
I asked for pictures of the installed NEW STYLE accessory case and thought sure I'd see the suction screen and filter locations swapped like the GO-300 but no, it's not.
You will notice the casting looks cleaner and what I think more modern from the castings of most accessory cases we see. Around the filter adapter you see a wide flat machined area with threaded holes which look like there may have been some kind of filter manifold available to be bolted on. This is not what you find on a old style accessory case. And look to the left of the filter adapter. In the picture is an open suction screen hole. Problem is this engine, I think, has been run and there would be oil in the sump. How is it not running out of the suction screen hole. Unless it is modified internally.
Still so many questions how does this accessory case work with the oil pump running backward.
One question that seems to be answered though is the low oil pressure seemed to be a combination of low adjustment of the pressure relief spring or maybe something holding the relief plunger off the seat and also oil trying to make its way through and filling the oil cooler you can see plumbed of the filter adapter under the filter. So while I don't understand how this accessory case with a reversed rotating oil pump can work, apparently it can.
So how many here have seen a NEW STYLE accessory case in person. Anyone have one uninstalled they could share pictures of?
Would be nice to see one and solve this mystery.
For the last few days I've been corresponding with an individual from outside the US who owns a 172H with an 0-300-D which he had overhauled. During overhaul it was discovered that his accessory case was cracked about the starter mounting area and so a NEW accessory case was ordered.
Holy cow. Did anyone know you could still buy a NEW O-300 accessory case. Yessiree you can.
Engine was assembled, test run and sent back to him. With a few months delay the engine was installed and started. Much to his surprise he had low oil pressure and his tack was being driven backwards. He found my email in a web search no doubt linked to a thread or threads I've posted which convinced him I might know a thing or two about the 0-300 and he sent me an email. Did I mention english is not his native language (I'm guessing) and he is blind from an airplane accident so he can't see the picture and illustrations I sen him and he can't go take requesting pictures without relaying my request to a helper.
Initially there was some confusion as he thought his maintenance co told him they installed a IO-360 accessory case and that is what he told me. Holy cow is that possible I thought as I downloaded the IO-360 IPC. I knew the tack drive was integral to the oil pump gears and if his was running backwards it's no wonder he's got no oil pressure. Low and behold the IO-360 accessory case is somewhat similar to the 0-300 but one difference is the oil pump runs opposite the normal 0-300. I thought how does this compare to the GO-300 which is a geared engine which runs in reverse and so must it's oil pump. Quick download the GO-300 IPS and overhaul manuals. But wait a minute, whats this NEW STYLE 0-300 accessory case. The study began.
As it turns out he did not have a IO-360 accessory case, he had the NEW STYLE 0-300 accessory case with an oil pump that ran backwards JUST LIKE the IO-360 his maintenance co. chief commented.
So sure enough the NEW STYLE case has new oil pump gears and they are driven by another gear the runs on the inside cam gear teeth and this gear drives the top pump gear not the bottom as the original style did. The original style lower pump gear is driven directly from the center of the cam gear and the lower oil pump gear turns in the same direction and speed of the cam. The NEW STYLE gear being driven by a gear running inside the cam gear also turns at least 2 times, maybe more, faster than the cam. To correct the direction of the tach drive a 90° gear box is used to change directions and slow the rotation speed back down. This reduction box is listed on page 18 with the accessory case.
But how does the oil flow from the sump to the pump to the rest of the engine if the oil pump is reversed. Well I wish I knew. The illustration in the IPC on page 18 looks pretty much like the one on page 16. Sure it is at a different angle and not shaded, and has a few more ports that are plugged but besides the oil pump gears, loos the same.
Lets look at the GO-300 accessory case illustration. Yep looks the same. Wow ,wait a minute now it's not. The 0-300 accessory case has the suction screen on the bottom left and the filter on the bottom right and the GO-300 has there positions swapped. Thats how a reveres oil pump could work. Problem is this in not how the NEW STYLE accessory case is illustrated. Of course we've seen IPC illustrations wrong before.
I asked for pictures of the installed NEW STYLE accessory case and thought sure I'd see the suction screen and filter locations swapped like the GO-300 but no, it's not.
You will notice the casting looks cleaner and what I think more modern from the castings of most accessory cases we see. Around the filter adapter you see a wide flat machined area with threaded holes which look like there may have been some kind of filter manifold available to be bolted on. This is not what you find on a old style accessory case. And look to the left of the filter adapter. In the picture is an open suction screen hole. Problem is this engine, I think, has been run and there would be oil in the sump. How is it not running out of the suction screen hole. Unless it is modified internally.
Still so many questions how does this accessory case work with the oil pump running backward.
One question that seems to be answered though is the low oil pressure seemed to be a combination of low adjustment of the pressure relief spring or maybe something holding the relief plunger off the seat and also oil trying to make its way through and filling the oil cooler you can see plumbed of the filter adapter under the filter. So while I don't understand how this accessory case with a reversed rotating oil pump can work, apparently it can.
So how many here have seen a NEW STYLE accessory case in person. Anyone have one uninstalled they could share pictures of?
Would be nice to see one and solve this mystery.