BMW M42 crankshaft assesment

[Boyracer]

Here is crankshaft of my BMW M42 engine project (1886 cm3 16 valve 8500 rpm). It is forged, stroke is 81 mm.

Few questions:

1. Is there any modifications that should be done to oiling holes etc to improve lubrication? Engine will rev quite a bit higher than in stock setup (8500 vs 6800)

2. Crankshaft + vibration damper are going to be balanced soon on local machine shop. What else should be done while the crankshaft is there? Polishing journals etc?

3. What is rough assesment for crankshafts suitability for high revs? Stock piston + pin 480 grams, rod 610 grams (total 1090 grams) while new piston + pin 380 grams and rod 540 (total 930 grams) so there is about 14% loss of weight.

I hope to post here more as the build progresses. Thanks!

[Guy Croft]

Crank polishing:

http://guy-croft.com/viewtopic.php?f=13 ... aft#p10619

Oilway mods:

Your crank already has generous chamfers. Do not alter the general form but you should work on the all sharp corners inside the oil holes where they have been chamfered to radius them. Sharp edges are not good for flow. I use small tapered burrs followed by tiny grindstones, then small tapered abrasive points at 120 grade and then 400 grade carburundun by hand and then finish all the outer edges with fine ScotchBrite by hand. I don't unfortunately have a decent photo of this but could do on if you're not sure.

Balance and rpm

You have not mentioned the balance of the flywheel. This should be lightened and balanced on its own. Balancing a damper pulley can be interesting because if it's not in great condition the rubber joint can cause shifting between the two components. I don't know why it's fitted but on a 4 cylinder unit it's usually because the FW is heavy and they're worried about the crank 'winding up' and snapping under the constant torsional loading. I wouldn't say it's a weak-looking crank though. I wouldn't bother balancing the damper myself because its diameter is so small and I would be very surprised if any (or much) balance correction is needed.
The crank should be balanced on its own and then with the (lightened/balanced) FW bolted up (and position marked) and the pair balanced as an assembly. They mus then stay together as a pair. You could have the clutch done on the FW and then with the crank FW assy too and one always does that if it's a none standard clutch and FW mods have been carried out. That is absolutely critical. The weight of the rods has no impact at all on a stright four crank, the crank has 'internal balance' ie: can spin in dynamic balance without any external weighting (unlike a Vee item).

As far as rpm goes, if you do all that there is no reason at all why that assembly should not be able to withstand 7800 or more.

Hope that answers some of your questions,

G

[AndrisV]

Actually, there is one more thing. Have to apologise, I did a M42 build a while back and can't remember exact data, but as far as I remember, M42 had only a 180 degree grooved OEM main bearings, we swithed to another BMW bearings (360 groove), with minor key modifications. And I really struggle to remember about oil restriction jet in oil channel from engine block to head or M42 had some sort of restriction jets in head already. If there is a thought about 8500, it clearly has to be solid lifters, so don't see any reason for a lot of oil pressure in head.

[Boyracer]

Actually, there is one more thing. Have to apologise, I did a M42 build a while back and can't remember exact data, but as far as I remember, M42 had only a 180 degree grooved OEM main bearings, we swithed to another BMW bearings (360 groove), with minor key modifications. And I really struggle to remember about oil restriction jet in oil channel from engine block to head or M42 had some sort of restriction jets in head already. If there is a thought about 8500, it clearly has to be solid lifters, so don't see any reason for a lot of oil pressure in head.

Yes, OEM main bearings have something like 200-220 degree oil groove. I think that means that oil will flow to con rod bearing only during that period of crankshaft rotation which might be disadvantage in high revving engine. I think main bearings with 360 degree oil groove can be taken from M20 inline six engine, maybe that is what you used? How about thrust bearing, M42 OEM is only 180 degrees?

Good info about oil restrictor, I haven't thought about that! Lifters will be solid with shim under the bucket adjustment so oil requirement in cylinder head is definitely lower than standard.

Can you give any information about the M42 you built?