Rb25\30 Marked Bores - Anyone Have An Idea On Cause?

[The Mafia]

Hi Everyone

My Specs:

RB25 Head Shaved 20 thou, 8.45:1

Precision Pistons (Cast replacements)

Hastings Rings

Standard Rods

King Bearings

ARP main and Rod bolts

ARP Head studs

Cometic Head Gasket

Bores 20thou over

Engine has done 1000kms, 800kms on 10psi, and 200kms on 16psi.

Tune was 14.2:1 cruise, 11.5:1 load.

We removed the head to install oil restrictors (coughed about 1L into the catch can overall, oil was always topped up, ran 5L in the sump). This is what we saw:

Cylinder 1:

Cylinder 2:

5 and 6 look ok. Haven't turned the engine over enough to see 3 and 4.

Any idea on what could have caused this?

Here are the specs:

RB25 Head

Precision Pistons (Cast replacements)

Hastings Rings

Standard Rods

King Bearings

ARP main and Rod bolts

ARP Head studs

Cometic Head Gasket

20thou over

Engine has done 1000kms, 800kms on 10psi, and 200kms on 16psi.

Tune was 14.2:1 cruise, 11.5:1 load.

Had a word to the engine builder, and he immediately went onto the defensive. Was quite rude actually. Says he's going to drag me through all the bullshit (sending parts to some forensic mob etc) and I can take him to court and he'll fight it etc.

Funny thing is, I never mentioned court, I asked him what he thought was the failure, and that I would like to know how we can fix it. (Was pretty good when speaking to him actually)

Anyway, let me know your thoughts. Probably won't bother taking this further, because there are too many variables and he'll just cost me no motor for another 12 months. Plus it seems like he's good at worming out of it (due to experience in these matters) if it was his fault.

Just want to know so this mistake isn't made again.

Bottom end hasn't yet been disassembled.

Thanks guys

Jono

Edited October 27, 2010 by The Mafia

[The Mafia]

To add to this, I just took some advice from another thread, and slipped a feeler guage between the piston and the bore.

I'm able to get 0.279mm - 0.255mm feeler guages on either side of the piston without force or pushing the piston to one side.

Thats 10.98thou to 10.03 thou roughly (I understand that this is a crude way of measuring)

Anyway, thanks again everyone.

[DVS JEZ]

where the bores oiled up when assembling? how long after oiling the bores was the motor started?

did you pour some fresh oil into the cylinders before starting? did you crank with no sparkplugs for a little bit before starting?

what ring gap did he use?

[The Mafia]

1. Not really sure - Engine builder did this.

2. Probably about 2 months.

3. No, I wasn't advised to.

4. Yes, for a little while until I got oil pressure.

5. No idea - When we disassemble we are going to check. Will definitely post results.

Edited October 27, 2010 by The Mafia

[fatz]

what oil pump did you use?

[The Mafia]

Oil pump is a standard RB25 pump, with standard springs, with a 2mm shim. Plenty of pressure.

Cold the needle was about a width from the top of the gauge.

Warm, its always about half way under 3000rpm. Over, was 3\4 to nearly top of the gauge.

Engine never lost oil pressure.

[DVS JEZ]

next time you start a fresh build put some oil into the cylinder, it is best to wipe fresh oil on the bore when assembling then get the motor together and in the car ready to start asap ( when possible)

putting oil in the spark plug hole sometimes wont work depending on the shape of the top of the piston.

[zebra]

What were the AFR's at idle? and did you leave it idling for an extended period or stay at a constand rpm for a while (eg driving down the highway)?

What oil did you use on start up?

and can you feel the scores,

Gimmie a sec and ill check on the box of Precision RB30 pistons I have here for the piston-bore clearence.

But from memory it was 14thou.

[The Mafia]

next time you start a fresh build put some oil into the cylinder, it is best to wipe fresh oil on the bore when assembling then get the motor together and in the car ready to start asap ( when possible)

putting oil in the spark plug hole sometimes wont work depending on the shape of the top of the piston.

Can you get good assembly lube that won't run away for this in case you don't start the motor for a week or two?

What were the AFR's at idle? and did you leave it idling for an extended period or stay at a constand rpm for a while (eg driving down the highway)?

What oil did you use on start up?

and can you feel the scores,

Gimmie a sec and ill check on the box of Precision RB30 pistons I have here for the piston-bore clearence.

But from memory it was 14thou.

The AFRs were in the mid 14s.

Was never left idling for more than about 5 mins. Always up and down on the throttle, and no constant revs.

It had Fuchs 20w50 (recommended by the builder) full mineral oil.

You can't feel the scores, except the one I have circled in red.

Thanks heaps for your help mate.

Edited October 27, 2010 by The Mafia

[zebra]

Also, did the engine see any det at all?

the Moly coating on the rings chips off when there is det present.

[The Mafia]

Also, did the engine see any det at all?

the Moly coating on the rings chips off when there is det present.

No detonation worth worrying about. I logged diligently and had the timing wound right out for run in.

3 or so times it might have hit 40-50 according to the powerfc while tuning, but that is very minimal.

[zebra]

Can you get good assembly lube that won't run away for this in case you don't start the motor for a week or two?

The AFRs were in the mid 14s.

Was never left idling for more than about 5 mins. Always up and down on the throttle, and no constant revs.

It had Fuchs 20w50 (recommended by the builder) full mineral oil.

You can't feel the scores, except the one I have circled in red.

Thanks heaps for your help mate.

You can buy "penetrating oil" which you could squirt down the plug holes, but i wouldnt use assemble lube in the bores.

[The Mafia]

You can buy "penetrating oil" which you could squirt down the plug holes, but i wouldnt use assemble lube in the bores.

ok no worries, I will look into this on the rebuild.

Just wanted to thank you guys for all of the input so far. I really do appreciate it.

[zebra]

Ok just went hunting thought the garage, but cant find any of the info sheets for the psiton or rings

But looking at the photos I would be checking the crank thrust or if the bores are perfectly round.

The scores are inline with the crank, so if the crank is moving back and forward it will take the pistons with it a little.

[The Mafia]

Ok just went hunting thought the garage, but cant find any of the info sheets for the psiton or rings

But looking at the photos I would be checking the crank thrust or if the bores are perfectly round.

The scores are inline with the crank, so if the crank is moving back and forward it will take the pistons with it a little.

no worries mate, thanks very much for your effort.

Here is my checklist so far:

Piston to Bore Clearance

Ring Gaps

Crank Thrust

If bores are round (Should be, was bored 20thou over)

[r33_racer]

20thou over with a torque plate??? If not, then Id put money on your bores not being round.

How come those pistons have such large piston to bore clearance? 14thou seems huge if thats what it is.... However, if your piston to bore is too tight and that coupled with out of round bores that easily can be your piston grabbing issue, poor ring sealing issue and why you were dumping so much oil into the catch can.

Assemble on inox or similiar spray lubricant as zebra said. It works well. Dont use oil. That trick was great 30 years ago....once you understand how a ring seals and what happens when you are bedding your rings in, you wont ever drive around for 1000km or do any of that crap again.

When you check your ring gaps...if you get to inspect the rings yourself, have a look at the ends to see if they are polished/shiny spots, if he was good when grinding the rings he would have only cut one side if he even needed to grind them. Eitherway the shiny spots are ring butting marks. Also if you look carefully around the sealing point of the rings you should be able to see a fine line thats even and goes all the way around the circumference of the ring. This should be especially noticeable on coated rings. Should tell you just how well the rings were sealing.

[STATUS]

Forget the suggestions it was not enough oil on startup etc...

WD40 or even a wipe with auto fluid on a rag is enough.

It would be interesting to see the rings (side, back and end gap), it even looks kinda looks dusted so put a eye piece on the rings.

Explains your catch can, restrictors are not the answer here.

Its gotta be pulled and rebuilt so measure the skirts and bore and see where thats at also, may explain your scoring if the rings dont..

When it was first assembled what was the:

Ring end gap

Ring side gap

Ring back gap

Piston to bore clearance

[r33_racer]

Dont suppose you have some micrometers and a dial bore gauge? I thought cast pistons with high silicon content usually run tight piston to bore clearances. Perhaps its .0014" or thereabouts...

[zebra]

Dont suppose you have some micrometers and a dial bore gauge? I thought cast pistons with high silicon content usually run tight piston to bore clearances. Perhaps its .0014" or thereabouts...

You could be right!

I do remember it being 14 something for the piston (I had the same ones)

[The Mafia]

Forget the suggestions it was not enough oil on startup etc...

WD40 or even a wipe with auto fluid on a rag is enough.

It would be interesting to see the rings (side, back and end gap), it even looks kinda looks dusted so put a eye piece on the rings.

Explains your catch can, restrictors are not the answer here.

Its gotta be pulled and rebuilt so measure the skirts and bore and see where thats at also, may explain your scoring if the rings dont..

When it was first assembled what was the:

Ring end gap

Ring side gap

Ring back gap

Piston to bore clearance

Thanks for your reply - This is really good to know.

As for the gap questions - I have no idea. I had a workshop assemble the engine.

I will remove the pistons and rings and get some pictures of what they look like. Its a time thing so I will organise it ASAP and get back to you.

Thanks again for all of your replies.

Also interested to know what the proper Piston to bore clearance is supposed to be.