MK3 Oil Return

On the MK3 I am not seeing the typical oil drain return to the sump in the timing side of the case.

Where does the oil return in a MK3?

Is lowering the oil return feed at the bottom of the oil pump a safe procedure on the MK3?

Here is a photo of my timing side case.

maaseyracer said:

Where does the oil return in a MK3?

Click to expand...

As I see it, with the engine in the normal forward-leaning position, not set upright in an engine stand as it is in your picture then the hole near the camshaft boss would be at a similar level to the previous drain hole, also the 06.7526 "sealing washer" doesn't exactly make an oil tight seal but seems to act as a baffle and wouldn't prevent oil passing from the timing chest to the crankcase because it rotates with the crankshaft and needs clearance between it and the crankcase opening.

maaseyracer said:

On the MK3 I am not seeing the typical oil drain return to the sump in the timing side of the case.

Where does the oil return in a MK3?

Is lowering the oil return feed at the bottom of the oil pump a safe procedure on the MK3?

Here is a photo of my timing side case.

Click to expand...

Will you be racing it?
I concur with LAB

concours said:

Will you be racing it?

Click to expand...

No. Though it will be a fairly high compression build with as aggressive of a cam as I would want to run on the street.

maaseyracer said:

No. Though it will be a fairly high compression build with as aggressive of a cam as I would want to run on the street.

Click to expand...

What cam is planned?

Irrelevant drivel: My old upright pre-Commando motor has in addition to similar holes shown on your cases a small hole directly below the crank where the blue tape is. It also has a big hole above the feed and return lines, but that is intentionally blocked off in the casting for your cases. The small hole mod was also done on early Commandos I think, before somebody decided most of the holes in the timing case needed to be blocked off. I did not come up with the idea for the small hole, so it may be safe.

A cNw reed valve is located where the magneto used to be. This motor now has a JS2 cam in it and it is a nice street cam.

Schwany said:

What cam is planned?

Click to expand...

JS2, I want a reasonable idle. I have run the Sifton 460 grind on a Triumph and the idle was not what I consider reasonable so the JS3 is out of the question on this build. I have talked to Jim at JSMotorport.com extensively about this build and this seems to be the way to go.

Has anyone here lowered the oil return feed at the bottom of the oil pump on the MK3?

Someone sent me this photo. See the two oil return holes in the photo of a 69 Cmndo left of the upper oil pump stud. The hole circled in red is the one in question. Do they both join together with the same oil return hole?

jseng1 said:

Someone sent me this photo. See the two oil return holes in the photo of a 69 Cmndo left of the upper oil pump stud. The hole circled in red is the one in question. Do they both join together with the same oil return hole?

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And later posts.

Info from that article is still confusing:

"extra timing cover hole next to oil pump is mystery")
But I also do not understand how that lower hole could be connected to the return?


The Combat case below has the mystery hole and no other - if this isn't the drain back hole then where is it?



Its the bottom hole in the photo below I am concerned about. I don't have an early case like that to inspect. Is that lower hole a drain back hole? If not then what is it?



For a reed valve breather you only want one small drain back hole. All the other "vents" should be plugged in order to make the reed valve work more effeciently.

jseng1 said:

Info from that article is still confusing:

"extra timing cover hole next to oil pump is mystery")
But I also do not understand how that lower hole could be connected to the return?

Click to expand...

jseng1 said:

Its the bottom hole in the photo below I am concerned about. I don't have an early case like that to inspect. Is that lower hole a drain back hole? If not then what is it?

Click to expand...

There's been no further discussion about the mystery hole since that thread as far as I can remember so according to 'Bobolink', the upper hole was the drain to the sump and the lower hole connected to the pump return drilling, therefore, logically it should have some form of blanking plug to prevent the scavenged oil from being blown into the timing chest.

jseng1 said:

Info from that article is still confusing:

"extra timing cover hole next to oil pump is mystery")
But I also do not understand how that lower hole could be connected to the return?

For a reed valve breather you only want one small drain back hole. All the other "vents" should be plugged in order to make the reed valve work more effeciently.

Click to expand...

What are the negative engine performance symptoms associated with an inefficient reed valve breather on the back of the timing chest where the vent holes into the crank case are "not" plugged?

The little 3mm hole below the crank in my timing chest is a drain back into the sump.

Would a 3mm hole down there be large enough to vent the crank case?

Edit: I originally got the idea to put all the holes in my P11 case to reduce positive crank case pressure when I installed a 2S cam in the P11. The diagram and description of what to do on a Combat motor may have come from an article on Old Brits. Bob Raber told me about it. This information is now on the Andover site. I winged it for how I thought it might all work on the P11 cases like I do everything else. It seems to work, but maybe it is not ideal.

Interesting finds here. I pulled the cases back apart and reviewed where the crank sits with the lowered the oil return feed at the bottom of the oil pump. Looks like the crank is still moving through a considerable amount of oil.