would you use this crank???

[digger]

the smoother it is, the thinner the oil film can get (more load capacity) before you get metal to bearing contact because the peaks on the surface are genrally smaller. note that RA doesn't explicitly define the peaks but its the most common used. when people look at honing there are a whole bunch of other terms to properly define the surface valleys and peaks

Smoother also does less damage to the bearing on cold/dry start where the oil isnt flowing very well yet or you have lack of lubricaton from giving the engine too much rpm and load with cold (thick) oil

some places also do REM and super finished cranks but that has other benefits besides journal performance

[zaq123]

I don't agree with (1) as heavily loaded journals require a lower RA

(3) to add to this cast cranks b25 and eta require specific grinding and polishing rotational directions where forged steel not so much

alright, I'll step away from google and try to use my head regarding #1... just want to get it. My understanding of the crank riding on the bearing covered by the oil film is like tire hydroplaning on water: less tire tread (smoother) = more hydroplaning (good in the crank case). But there are probably some limitations due to clearances, oil type, oil pressure...

[digger]

I don't agree with (1) as heavily loaded journals require a lower RA

(3) to add to this cast cranks b25 and eta require specific grinding and polishing rotational directions where forged steel not so much

[zaq123]

A few more things I've learned by researching about crankshaft journals...maybe it will be useful to someone who stumbles upon this thread:

Do not over polish the crank:
1. if journals are too polished, there is nothing for the oil to get stuck to. Journals that are too smooth will leak oil too fast and crank will ride the bearings without the oil film.

2. be careful when shop polishes the thrust bearing journal. Tape it or something. If belt sides are touching, it can leave crosshatched marks on the thrust area of the crank which will act like "wipers", dispersing the oil from the bearing and eventually destroying it.
Thrust bearing finish has to be 10ra max and only circular pattern is acceptable.

3. Do not polish crank oil seal journals while running polishing belt in longitudinal direction. Polishing spiral pattern will promote oil to be pushed out past the seal and develop leaks (shouldn't be an issue on BMW cranks since oil seal journals are narrow and the polishing belt will basically cover the whole width).



some good reading:http://www.mahle-aftermarket.com/med...s-brochure.pdf

[digger]

makes sense there is a dirty big angled hole in the much smaller rod journals which seems to work ok

[zaq123]

Another shop said that this journal would be just fine as long as nothing is sticking up. He said one can drill a hole in the journal and it would be fine as long as its edges aren't raised (obviously it won't be good idea due to some decrease in the bearing area). Kind of makes a sense.

[ForcedFirebird]

Case hardened, but more importantly, the cranks are long. I have pulled apart m20 motors that were rebuild a decade ago that had .010" under bearings, but they looked rough. Not like the short 60°v6 cranks we do.

[zaq123]

Gents, thanks for the advise. FF, I asked the shop to mic all journals for me. That shop is very reputable too. I brought it back and was told that polishing won't take this off. I will find another shop to discuss this. Are these journals coated or just heat treated? If coated, how much can be taken of without the need to recoat?

[ForcedFirebird]

that's too deep for polishing the crank, according to them anyway. Needs regrind. This journal marked green from the factory and after polishing, it still sits at 2.3613.
Deck needed about 5 thou removed to get it flat. Was too pissed about the crank, didn't want to get into the deck with them....probable should have.

I'm doing 2.9L: 85 bore, 84 stroke


Can someone explain me what would be the issue with running something like that? It's flat with the journal surface.

You have a tenths mic to measure to the 5th decimal?

Let's convert mm to " for the sake of this thread. 59.971mm is the minimum stock journal size = 2.3610", so you have 3/10th before you even meet that (white bearings). A 2.36" journal means you need about .0023" of oil clearance following the 10's rule (.001" for every 1" of journal on the mains).

So, take a polish (probably closer to .0005" than .005"), and you are still very close to acceptable in a race-oriented engine. You are splitting hairs when talking about one or two ten thousandths of an inch (now you are talkin RA finishes).

In short, I agree with Digger, get a second opinion. A mic itself can "scratch" tenths on a polished journal. I bet a polish will take that right out. The machine shop shouldn't have an issue with re-polishing it, even if they did and blame you, most shops only charge ~$50-70 for a micro polish, so offer to split it with them if they balk.

My $.02

[digger]

the oil film is very thin, that is why the metal needs to be smooth such that imperfections arent thicker than the film which means they will touch the soft bearing.

i would get a second opinion from a machinist who does this day in and day out. if the guy doing the work stuffed something up then his opinion is tainted because he will be out of pocket so less inclined to fix it

[zaq123]

that's too deep for polishing the crank, according to them anyway. Needs regrind. This journal marked green from the factory and after polishing, it still sits at 2.3613.
Deck needed about 5 thou removed to get it flat. Was too pissed about the crank, didn't want to get into the deck with them....probable should have.

I'm doing 2.9L: 85 bore, 84 stroke


Can someone explain me what would be the issue with running something like that? It's flat with the journal surface.

If your shop is reputable and you are spending money with them, they should be able to lap that crank back. We recently had one with deeper grooves and polish took it out (it's a main, so you have a little more leeway for sizing), and they didn't even question if it happened during transport back to our shop. Worth asking if they will re-polish it, just take note of the diameter. We make it policy to NOT turn the BMW cranks to .010", we just replace them.

As far as the deck, it appears they scraped it with the hone upon exit, but unsure why the deck would be milled before the hone jobber. How much did you take off the deck? You start new threads often, so not as simple as a scroll up to see your particular build - but - in my experience a .010" scrape off the deck leaves room to do another rebuild and keep a decent quench with a -.005" head. Really until you have both components machined, you have no idea how much room you have left.

[ForcedFirebird]

If your shop is reputable and you are spending money with them, they should be able to lap that crank back. We recently had one with deeper grooves and polish took it out (it's a main, so you have a little more leeway for sizing), and they didn't even question if it happened during transport back to our shop. Worth asking if they will re-polish it, just take note of the diameter. We make it policy to NOT turn the BMW cranks to .010", we just replace them.

As far as the deck, it appears they scraped it with the hone upon exit, but unsure why the deck would be milled before the hone jobber. How much did you take off the deck? You start new threads often, so not as simple as a scroll up to see your particular build - but - in my experience a .010" scrape off the deck leaves room to do another rebuild and keep a decent quench with a -.005" head. Really until you have both components machined, you have no idea how much room you have left.

[zaq123]

correct, crank was selected out of a few and it was perfect. I can't prove that they did it since I took it home (it was wrapped) and I don't really want to start any wars with the shop. It is definitely deep enough to require grinding (finger nail, copper penny etc) but it's flat with the journal. I definitely do not want to grind the crank. Wonder what harm can it do since it's flat with the journal.

i think the deck is fine as long as there isnt a tiny burr that sticks up in which case you can rub it down carefully, though its not ideal being directly under the fire ring and i assume you are at 85mm bore? sitting the gasket on might see how much is in free air?

the crank id get a second opinion from a shop that doesnt have a vested interest (photos can make things look worse than reality) , since it doesnt polish out it might be a little too deep. are you saying that it wasnt there before?

[digger]

can you feel if with your fingernail?

[digger]

i think the deck is fine as long as there isnt a tiny burr that sticks up in which case you can rub it down carefully, though its not ideal being directly under the fire ring and i assume you are at 85mm bore? sitting the gasket on might see how much is in free air?

the crank id get a second opinion from a shop that doesnt have a vested interest (photos can make things look worse than reality) , since it doesnt polish out it might be a little too deep. are you saying that it wasnt there before?