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.
- Original style accessory case
- NEW STYLE accessory case
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.
- GO-300 accessory case
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.
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.
The OS case has the relief valve on the right and it is located at the end of the oil flow through the engine, not the beginning. The NS case and other model cases such at the IO-360 case have the relief valve at the beginning of the flow as it exits the case prior to oil passing through the engine.