mark kibort

I need some help for my BMW racer buddy.

He has a M5 V8 which has SiAl engine block, as we do. it has the coated pistons that are shorter than ours, but haunting simular.

the engine developed a dead hole, cyl 7, and he pulled the engine. he said he did a leak down, but no compression check. leak down was reallly bad. said, it was not going through valves, exhaust or intake and I took his word for it.
he gets the engine out of the car, heads are caked with carbon, pistons are caked so much that it probably raised the compression 1 point ( )
BUT, they look perfect from my standards (limited of course). skirts are lightly worn as ours are when we disassemble. cylinders are perfect, with the dead hole having a slight scratch that you cant even feel with your finger nail. he is **** (remember) so he thinks that little mark is the point of failure. I say, we have a lot more of those in a 928 engine and we just re assemble it with no worries. his rings look brand new, and the gaps were offset 120degrees, when I saw them, but who knows when he pulled them out (thats one of the reasons I wanted to be there. I still think its valves, not pistons or rings as an issue)

so, here are the picks. you can see the cylinder walls look great. there is a little scratch , 1' down from the top. you cant feel it. there are little marks on the pistons, but those for sure, can not be felt .

is there anyway, that the leak down, showing a dead hole could have a problem with rings or the piston sealing?? i am inclined to think with all that carbon, he had a piece lodged in the valve seat and that was the problem, but he swears that no air was coming out the intake or exhaust pipe.
Is there any way that our pistons,rings or anything else can cause a bad leak down by something that really isnt obvious (i.e. broken ring, scored cylnder wall, gauled wall, etc)??

check this out. also notice that the cylinders share the same 5mm wall. its a really compact block. very light and small. stroke is near 90mm and bore is only 94mm. Its a 5 liter!!

anyway, Greg B......... do you think this is worth re processing with the SiAl process to clean up a scratch you cant even feel, way up in the cylinder?
thoughts???

anyway, check out the squirters! are those like the S4 blocks that have them??

mk

EDIT: I put a picture of that carbon'ed piston! also a 928 block for contrast.

docmirror

From what I can see, and what you've described, I would concur. The hole and the piston and rings look nearly flawless. I'm also betting on something holding one of the valves open, and he didn't know how to check it very well.

The air has to go somewhere, and a valve is a high candidate. If the head isn't cracked, and no evidence of the gasket blown out, it was going through a valve. could have been held open by a stuck lifter too.

Fabio421

Mark, I exchanged PM's with Greg B. regarding a 928 engine with a scratch you could barely feel with your finger nail and he said not to worry about it. That many engines have these slight scratches and not to worry.

It's safe to assume that a scratch you can't feel is of less concern than one you can barely feel. I'd say have the heads checked.

blown 87

Carbon on the valve seat or a pumped up lifter, your problem is not with the rings or block.

I think AST or BAUM makes a plate so you can leak down the block with the pistons in and the head off.
He will be throwing his money away if he buys one.

Gregg K

Could a ring be carboned up so that it's stuck inward?

I've heard that big ring gap or ring gaps all lined up can cause a poor leakdown.

I have an engine that had a horrible leakdown. I pulled it, rebuilt it, and guess what, it still had horrible leakdown. It's just a strange design. I will never figure it out.


Edit- whoops, I see Greg's reply above.

terry gt

You might check out the bmw forums , some of the V8s had alusil block issues .

mark kibort

wasnt the bmw issues with the Nicasil blocks?? unless the pistons are out of oval ( ) i have a hard time imagining leak down issues with this engine block . the problem is, he rushed down the leak down test before really narrowing down the issue and where exactly the leak was. one test I suggested, was compression test. if that would have been near normal, that would confirm valves to a certain extent. but he said that was a usless test after doing a leak down and finding a dead hole.
Im thinking and have always thougth valves. his heads are done and just came back and look real nice, so that will not be the issue when he puts it together.

mark kibort

what is a scratch that you can barely feel with your finger nail?? .0001? his scratch you can barely feel with the nail so i think its just beyond superficial.

docmirror

Two hours of diagnosis could have saved him a few thou it sounds like.

mark kibort

and his rings move nice and easy. Unlike the 928 engine we are building for Petty. his are all gunked up. lots of cleaning needed. (dont worry, we are being careful not to touch the skirts much. ring lands are a mess though!

blown 87

do not be alarmed if you have zero compression after you put the cams in, they have to sit and be turned to get the lifters back down.
I almost always leave them for a day before trying to start them and try to put every lifter on the cam lobe for at least 1/2 hour.
The problem with the block was fuel and fueling issues more than a problem with the blocks.

Rob Edwards

The E34 530i and 540i Nikasil motors had problems with high-sulfur gasoline doing a number on the cylinder lining, leading to rough idle and bad compression. The E39M5 motors' (which I presume is the one being discussed here) Achilles heel is carbon buildup in the combustion chambers and cylinder heads, that throws a CEL codes and costs $8k to scrape the crap out of the heads, only to return later. BMW knows not why.

http://www.m5board.com/vbulletin/e39...q-summary.html

Not qualified enough to judge where your buddy's problem lies, so I'll just throw up a +1 on it being a valve-related issue. Based on my N of 3 or so, those cylinder bores look fine to me.

blown 87

We have "Fixed" a few of them by doing back to back BG service on.
The other issue is that VANOS systems and the crank vent systems.

928 at last

I've got nowhere near the experience, but I'm with Greg on this one. If there was as much carbon in there as you say, then I'll put money on the carbon on the seat theory......

UncleMaz

I would consider sending photos and inquiry to Frank Fahey of Frank Fahey Motorsports http://www.frankfaheymotorsports.com/. He is the Greg Brown of BMW's.