RB25 drilling block - oil returns off centre?

[R34-GT-T]

Hi everyone,

I'm currently in the process of building my RB25 Neo. I've read the entire oil control thread and have been implementing all the recommendations. I'm wanting to enlarge the 5 oil return holes in the head and block from 8 mm to ~9-10 mm.

I found when aligning the headgasket using the dowels to the underside of the head, that the 5 oil return holes line up perfectly. It's just a matter of opening up the existing holes. Not too difficult.

However, when sitting the headgasket (again aligned with the dowels) on to the block the 5 oil returns are all completely off centre (see picture below). If I were to enlarge the exisiting holes much more, they would protrude under the gasket. I have a spare R33 RB25 block, and it's the same thing. I also check using the original headgasket and it's the same.

I thought the idea was to align the drill with the existing hole (on a mill or I've even read people doing it by hand) and enlarge it to 10mm to match the headgasket. Am I missing something?

Any help or info would be super appreciated! 🙂

 

 

 

[TurboTapin]

This can be milled out regardless with a long enough cutter but finding an end mill, etc. long enough is the issue. I've heard of 12" end mill cutters, and I'm presuming someone somewhere makes them longer but you may still need to hit it from both sides which makes it more of a pain for alignment. 

For the reason above, I only ended up enlarging my head to 10mm. My S1 RB25 passages look identical to yours. I'm presuming it was slightly off in certain castings. 

[GTSBoy]

When you have to re-centre a hole you can start with a mill and then change to a drill.

I wouldn't bother. The real conclusions in the oil control thread are that drilling the returns achieves 3/10ths of f**k all. You are better off providing large external vent lines from sump to breather system to avoid the need for the oil and blowby gases to have to travel countercurrent to each other in the same (oil return) holes.

[R34-GT-T]

Appreciate the info. I was wondering because all the photos I've seen show the holes in the centre, glad to hear mines not unique.

I was thinking similar ideas, end mill the whole way thru, end mill to start and follow with drill, or make a thick steel guide plate to bolt to the deck with the holes in the right spot to guide the drill. All of which have their pros and cons but seem like much more effort than they're worth.

Glad to get both your input, think I'll leave the block (and maybe just drill out the head while I do some other work to it). I've got an enlarged sump with an outlet on the intake side above the windage tray. I'm going to run this to the catch can as a crankcase breather as you've suggested GTSBoy.