Oil bypass mods - help please!

[bloxidgp]

Does anyone know the optimal size of the hole to be drilled in the oil bypass modification to the crankcase (2.7ltr, pre-1977) as suggested in Wayne Dempsey's excellent book?

[dan67]

interested! draws
bloxidgp see here: http://it.youtube.com/watch?v=lKuceDywEJg

[bloxidgp]

Dan
Thanks for the link - really useful and very clear EXCEPT I still don't know what sized drill bit to use for the mod ........looks 6 - 8mm-ish? What do you reckon?
Thanks again
Paul

[dan67]

...mmm i think 1/4 inch (i hate the anglo-saxon measures )))) = 6,58mm....mmmm i work 7-8 mm.......

bruce anderson book pag 137 see:

http://books.google.it/books?id=KjuPioKlSJkC&pg=PA135&lpg=PA135&dq=oil+bypass+crankcase+porsche&source=bl&ots=3JotUhrZ6W&sig=Gz0yQlww4TcCKwi33yJczZ35kpA&hl=it&sa=X&oi=book_result&resnum=1&ct=result#PPA137,M1

[MBEngineering]

HI bloxidgp
use a 10mm drill, any less will give you oil press' prob's.

regards mike

[bloxidgp]

Thanks guys..........it seems that the advice ranges between 7 and 10mm. I guess that I'll try 9mm and see how I go. Thanks for the link to the video and to the book Dan, most useful.

Paul

[MBEngineering]

HI Paul
do not go less than 10mm, been there(less than 10mm) and had prob's,

regards mike

[cgarr]

It looks like in the youtube vid that may be an older case? If your doing a 7R case there is another web near the hole that has to be tapped (not much room) you may have to cut down your tap to get it in there.

[nimskov]

To the trained European Metric Eye (c), the drill looks like a 10mm or so.

The case pin-bolts are 8mm (right...?), and the drill is surely larger.

[daepp]

As seen in the video, what was the hole (that got tapped) for originally, and what screws in to those new threads?

[cgarr]

That hole was where the oil dumped into the sump from, it now gets a 1/4 pipe plug.

[cgarr]

Here is the fixture G2 made which helps with drilling the bypass port. Its pretty simple, The use of the pilot drill may not be necessary but I think it does help.


The Fixture and pilot sleeve, final drill size is 3/8


Drilling the Pilot:


Drilling the 3/8 passage:

[daepp]

So why do you drill down to the lower port and then plug it?

[notmytarga]

The reason for the Oil-bypass modification is that it directs shunted oil from the pressure relief piston to the inflow of the pressure portion of the oil pump rather than into the engine case sump. Reduces the risk of the oil tank running dry, allows the scavenge side of the oil pump to be smaller in favor of a larger pressure pump - and other things that I can't remember. See page 63 of Bruce Anderson's book, 2nd edition.

Of important note is that the pistons for the two oil bypass valves need to be the updated pistons - the ones without the holes.

Word on the street (my local wrench) has it that racing teams were doing this modification when tearing down and modifing race engines in the 70's and the factory adopted it in 1976 and changed the oil pump ratio.