What Has Caused This? Marks On Bore.

[James_03]

Hey guys,

Pulled my rb30/26 down over the last 2 weeks and today finally got to taking the pistons out. Basically I have 87mm rb30et pistons in there now, and I have new higher CR pistons to go back in which will yield 9.5:1

I had the bottom end built by an engine builder back in the day. I pulled them out to find this, marks were on every cyliner and on both inlet/exhaust side of the bore:

Now I have two questions:

1. Is this caused by incorrect piston to bore clearance? If not what is likely to have caused it?

2. Can I give the bores a lite hone and re-fit my new 87mm pistons and be OK? The bores are 87mm at the moment, and the new pistons are too, hence the issue.

Some people have told me a lite bottle brush hone to get the marks out won't hurt, others have told me to ditch the block all together and hone out another 30 block to suite my new 87mm pistons.

Issue with that is I spent many a hours modifying this block for the 4wd adapter plate

Anyways what be SAU's opinion.

[Sheriff Wan]

I wouldn't imagine piston to bore would play as a factor here given it is on all cylinders in the same spot.

Potentially, could it have been a problem with lubrication, as in the squirters were not getting to those points in the bore?

Or a more unlikely cause could be too many revs while the engine is cold and the pistons have not expanded fully causing the bore wear?

Just some ideas...

I would speak to the bloke that built it though - how long had the motor been going before the pull down?

[ascenion24]

First off you can still see the cross hatching in those dark areas, so the finish is still fine. It doesnt look like any abnormal wear, if it was you would see straight up and done scratch marks in those dark spots, it looks like something has stained your bores. But always hard to tell just looking at photos.

Is it carbon build? will it scratch off? does it still feel smooth, try and describe it to us a bit more.

Also a second interesting clue is that the dark strip goes down below the line where the top of the piston would be at the bottom of its stroke.

There is also a very clear line at the top where it stops, it looks like the line could line up with one of the rings. Figure out where the top of that line lines up with the piston when its at TDC that might give you another clue to what is causing it.

Also do the same at the bottom of the stoke.

But it almost looks like the oil squirters are squirtering at that spot at the bottom (hence the rounded stain at the bottom) and the piston is dragging it up to the top. Over time it could of just stained the bore.

Then again like I said its still very hard to diagnoise properly without having a proper look.

Edited February 28, 2010 by ascenion24

[[200]]

Would it be the oil control rings?

rb30's dont have oil squirters unless they are installed in to the block ( I think)

[GTScotT]

in thinking piston temp vs oil rings......

that could be tune related, did you ever log EGT?

[Cartman]

when the engine was rebuilt last time, was the block torque plate bored and honed? have you taken messurements in all different areas of the bore to see how round it is...

[GTScotT]

when the engine was rebuilt last time, was the block torque plate bored and honed? have you taken messurements in all different areas of the bore to see how round it is...

valid point

[tridentt150v]

Cartman has it spot on, you have blow by because the bore is out of round. It should be nice and shiny like the rest of the bore because your rings are scraping, they are not scraping on your black bit. You probably didn't even notice this when driving, may have used just a tad more oil, but not much, and would have vented a bit more that usual into your catch can

[DVS JEZ]

are the marks on the pistons where the skirts are???

this is the largest part of the piston, i reckon the piston to bore clearance was too tight,

another possibility is the pistons have swelled during an extreme heat session, big burnout, too many hard laps on circuit etc.

[James_03]

Thanks for all the help guys.

Yeah it did burn oil ever so slightly, according to a mate following me once under load it was evident. It ran a 100% atmo catch can too so this probably explains the burning oil. Has a brand new head so I knew it was not stem seals that was causing the issue.

I think your right in that the bores are out. I don't know how it was done last time unfortunately, I just took it to a local mob with the pistons and they bored it out to 87mm and assembled it with the pistons I supplied.

I ran a bottle brush through no6 this afternoon very quickly and it took the marks right off and it looked brand new again. Obviously I still have a major problem but.

Waiting on a mate to bring some equipment around so we can measure the bores properly.

So where too from here? Another block?

[DVS JEZ]

take off your old comp rings and keep in order of cylinder, carefully put them in there matching cylinder and check the ring gaps, you will also be able to see if the bore marks are out of round, this is not accurate you still need to measure with a bore gauge

[tridentt150v]

a new block depends on the meat you have to remove to get it round again, it may still be within spec and OK you need to mike up the bore both ways and then check that against clearance spec's

[Swiper the Fox]

Thanks for all the help guys.

Yeah it did burn oil ever so slightly, according to a mate following me once under load it was evident. It ran a 100% atmo catch can too so this probably explains the burning oil. Has a brand new head so I knew it was not stem seals that was causing the issue.

I think your right in that the bores are out. I don't know how it was done last time unfortunately, I just took it to a local mob with the pistons and they bored it out to 87mm and assembled it with the pistons I supplied.

I ran a bottle brush through no6 this afternoon very quickly and it took the marks right off and it looked brand new again. Obviously I still have a major problem but.

Waiting on a mate to bring some equipment around so we can measure the bores properly.

So where too from here? Another block?

James, were the pistons installed the correct way around? Did the piston skirts appear to have excessive wear. The pin offset the wrong way around could have the rod angle incorrect at TDC.

[r33_racer]

It looks like the skirts were rubbing.

What did the seal line around your rings looks like?

[T04GTR]

since the marks go well past where the rings sit, to me looks like to much piston to bore clearance. since the skirt has rubbed it up aswell. they were installed the right way round? little dot or arrow facing forward?

its fairly normal to get those marks up top with forgies, but not the whole way down past the rings. what pistons were they? and what ones are you putting in?

[r31 25t]

I've had marks like these in an engine before and it was due to excessive piston to bore clearance. Do the pistons have polished areas above the top ring on the thrust faces? You get this when the piston "tips" in the bore. Must've been noisy on cold starts..

[STATUS]

it looks like too much piston to bore clearance, another cause can be incorrect timing causing it to rock on the way down. what are the piston crown and skirts like?

[James_03]

Hey guys,

I think your right, piston to bore clearance. Pistons were all around the right way, according to the mark.

Pistons that it had were CP.

Measured it all up tonight, I think they have added the piston to bore clearance to the actual hone itself.

Bores were showing a fairly consistant 87.07 across the board. Plus whatever CP recommend on top of that again which is like 3-3.5 thou I read.

I think that is what has happend anyway. Skirts are all showing signs of wear yes.

Sound plausible?

Edited March 1, 2010 by James_03

[r33_racer]

Unless I worked that out incorrectly, thats only 2.8 thou of clearance. I assume the piston size is 87mm at the skirt? .07 is only .0028" Unless your measuring devices are not accurate enough???

What if the piston to bore was too tight, not too excessive? Would the rubbing show up much the same? or would it be in four corners instead of just two?

[tridentt150v]

If it was too tight then it should have scored the bore and the piston should have score marks on the top skirt above the top ring lands due to 'unaccommodated' heat expansion.....well thats my take.