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RB25det help needed, oil entering 1 cylinder..

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Hey guys,

-I have a quick question and id appreciate a few expert and experienced opinions from here.. So the story is, i have a RB25 in my s14 drift car, i built it myself 2 years ago and it ran flawless for 2 years, the brief spec is: running e50 fuel, 770cc injectors, CP carrillo 8.5:1 cr pistons 87mm, GTR oem rods, polished and balanced crank with superforma oil pump drive collar fitted, 1.2mm cosworth head gasket, ARP head studs, ACL bottom end bearings, has OEM cams, still on hydraulic lifters, got the head skimmed and pressure checked when building it, valve stem oil seals were replaced and it was assembled using the oem valve springs, N1 oil pump, N1 water pump and i fitted 1.1mm head oil restrictors to keep more of the oil in the sump.. Turbo set up is a 6 boost manifold, turbo smart progate 50 external wastegate and an MMP turbo which is basically an enhanced gt25 size turbo as i want a good mid range power and something responsive..

- issues encountered in the 2 years: So on the dyno the car ran ok and made a very responsive 370bhp.. On the first day out drifting i did notice on / off throttle quickly it was losing all power on limiter and sounded like it was losing compression and the sound my tuner recognised straight away and it was valve float.. i got a set of after market Piper valve springs and fitted them with the head in place.. i knew it may lead to premature valve stem oil seal failure with the 2 days of valve float.. using my bore scope there was no sign of piston / valve contact and compression was perfect.. I fitted the springs and went back to the dyno and all went 100% so happy days and from then on on track days that issue was resolved..

- the issue: running e50 fuel you guys know how sweet it smells, on start up you get the lovely smell of the ethanol, one day i noticed the it was smelling ok for 30seconds or so then started smelling oil like, i removed the spark plugs and found CYL 4 plug wet and oil in the cylinder using my bore scope.. i changed the plugs, blew out the cylinder and went for a spin and all was ok.. The issue persisted (only when cold) and that oil like smell turned into smoke and that proceeded to get quite bad, i did a compression test and had perfect compression across all 6 cylinders.. i narrowed it down to valve stem oil seals because after the smoke starts, when you drive the car in boost once or twice hard it clears out and remains perfect until let cool down / parked for a few days, so i replaced the valve stem oil seals in cylinder 4 with the head on and found 1 torn so i was confident thinking i had the fault found and confirmed but i reassembled it and it was the exact same.. i just did 2 days out of the car and it was driving perfect but embarrassingly smokey and smelly after started from cold until its cleared out and driven..

- i have had a few 'experts' here tell me its oil rings, others are saying it may be a valve guide issue and most guys tell me its defiantly coming from the head down since it clears when hot and only happens as the car cools.. But what i cant grasp then is why is it not smokey and small nice for the first 30-60 seconds like there is no oil in the cylinder until the engine is running?

Of course i am going to remove and inspect all engine components but i would like a good insight and peoples opinions before hand? i think it will enable me to diagnose the issue more accurately as i take apart and inspect the parts as i don't want to skip over or neglect and aspect of the engine.. Im sorry guys if that was a little long winded but i wanted to give as much details and history as possible.. And of course thanks in advance for any information or feedback from your experiences..


- so i stripped the engine bare, pistons have a few operation marks in my opinion and have a few dings on the crown from small particles passing through but nothing that would case my issue.. I have measured the bore and pistons and all are still in spec, i even checked the ring gaps and all were still in spec and ring gap orientation ok.. i was half expecting to see a siezed ring or cracked ring land but no.. See the pics of the head attached, wet in around valve area on exhaust side on cyl 4, one worse than the other.. I took off the 2 springs in valve on exh cyl 4 to move around the valve to see was there play in the guide, i had no play at all in them.. im hoping maybe i just fitted the valve stem oil seal not 100% correctly? I am dropping the head to a reputable builder / machinist but im afraid ill get head built, re assemble it with new rings / bearings / etc and it'll be the same?! any feedback will be greatly appreciated..


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Hey guys, since writing this post and thinking about it ill try re-torquing the head.. Has anyone ever seen a faulty head gasket let oil into the cylinder? or a cracked head with RB's (i'm hoping not)? Considering the rear most block to head oil restrictor is fitted between cyl 4 and cyl 5 maybe thats a factor..

Pics added !!

Hi Shane, that's quite a mess you've got on your hands. It can be a little tricky accurately diagnosing the reason for oil consumption which is frustrating. One thing I would note is that your oil ring rail gaps aren't actually aligned correctly - These should be about n90 degrees apart. However in reality the rings do tend to rotate in operation so I wouldn't pin everything on that.

With valve guide seals you'll normally see a trail of oil up the roof of the port that starts at the guide on the exhaust side, while on the intake side you'll end up with oil burnt to the back of the valve. I can't quite see these aspects from your pics. It's also typical with badly leaking valve guide seals to have the engine smoke from initial startup since the oil will already have made its way into the cylinder.

While it would be difficult to attribute it to just 1 cylinder, is there oil in your breather system or intake track from the turbo?

Hey Andre, thanks a million for your detailed reply.. Yeah i have quite a mess here but i think its looking up.. I took the block, head and pistons to Westward engineering here in Ireland, Martin there has 40 years experience with race engines.. His opinion was that the bores were in spec, the previous ring gaps were a little too tight and lightly scored the block but nothing alarming, pistons were not perfect but the marks on them were not enough to cause the issue, he stripped the head bare and said that the previous head builder had 75% of the valve stem oil seals forced on too far and hard damaging the seal tip where it meets the valve guide and the ones i fitted were driven home too far again so he is pretty confident that the valve stem oil seals were my issue..

So now the pistons are cleaned up, the ring gaps are done by him, i got the block decked and honed, the head skimmed and pressure checked and all new valve seals fitted and valve to head surface re done and also the crank polished and measured and balanced so thats all good..

well spotted with the oil ring rail orientation, but i had them off and moved about so i think thats just the way i put them back on roughly..

My next issue is choosing head gasket thickness, previously i had a 1.2mm cosworth MLS one but now the block is after its third decking and the head in my ownership is after 2 skims so i think maybe ill be leaning toward a 1.7/1.8mm gasket i must get a deck bridge and measure how negative my piston protrusion is..

My breather system is from rocker covers to a catch can which is vented to the atmosphere by a baffled filtered catch can, i dont have any rocker cover into turbo intake pipe like the OEM set up.. But what i do have plumbed like day 1 is the driver rocker cover into the inlet manifold with the 1 way valve, would you recommend leaving that there or vent that into the catch can? i know that at idle the engines vacuum will suck out some gasses from the rocker cover via the oem 1 way valve but at high rpm and high inlet mani pressures that the rockers cant breath from that 1 way valve if the mani pressure is greater than the rocker cover pressure.. It is worth mentioning i have ordered a Franklin Engineering external head oil drain kit so that should help any crank pressure and breather issues..

Hey guys, thankfully it was just a valve stem oil seal.. Just off the dyno making a fresh 505BHP after running out of z32 with my power FC..

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