M42 Crankshaft play acceptable?

[Dj Buttchug]

wrong.

not wrong.

but whatever. just buy arp and be done with it

[mikeedler]

I'm not to sure about BMW thrust play, but all of the Chevy small blocks I've seen with excessive thrust play always have a crank you can now throw in the trash. It's an M42, throw a new thrust bearing in it and hope it will hold another 50,000miles. A new bottom end can be had for a few hundred bucks.

[nando]

not wrong.

but whatever. just buy arp and be done with it

yes, wrong. they're torque to yeild. that means they stretch past their yeild point. you can't retorque them.

[bwawuz02]

yes, wrong. they're torque to yeild. that means they stretch past their yeild point. you can't retorque them.

shop that machined my bottom end said the same thing. BMW engines everywhere in this place too ;) Rebello racing in antioch.

[Dj Buttchug]

yes, wrong. they're torque to yeild. that means they stretch past their yeild point. you can't retorque them.

Im well aware of what that means. There is some misconception and theory about bolt stretch, clamping force and tensile strength.

so maybe the manufacturer says buy new. if you have less than .002 stretch the fastener will still be effective.

A buddy of mine was in a bind for rod bolts once on a m50. it was the weekend and we didnt have time to wait around to order new, so we measured the bolts, mixed and matched the shortest ones with leftovers from other engines and installed them. Yes, corners were cut and it was a budget build but that was 20k miles ago. weekend track days too. no issues to report.

so yes you can re-use them and it will work provided you dont have excessively stressed hardware. The real question is SHOULD you... Those who are perfectionists would say no. I as well would shy away from the idea, especially on my own engine. with that said. not trying to be a dick. just explaining my past experiences...

[nando]

you obviously don't understand if you think that a TTY bolt can be retorqued.

[Ryann]

BMW rod bolts are TTY. you must replace them.

m10/m20/m30 rod hardware is reusable, no?

[nando]

No! They are all tty!

[twistednut]

Back to the original question, Yes Luke you probably have a 200# paperweight. To be sure you need to pull the pan and the #4 main cap and see if there is any shoulder left on the crankshaft for the thrust bearing to ride on. I have seen them wear to the point that it was flush with the counterweight. The block isn't worth much either since the piston skirts have been banging against the piston walls which are probably oval by now.
As for replacing the rod and main using this as a reference,

http://tis.spaghetticoder.org/

It definitely says to replace the rod bolts but not the main bolts.
When I did a complete rebuild in my red 318 I referenced this

http://m42club.com/forums/showthread.php?t=2247&highlight=torque+specs

I really didn't know shit at the time and reused the rod and main bolts and I haven't had any problems in 45,000 miles. (except for the rear main seal which I replaced with the updated one)
The last motor I checked the thrust bearing on I played with the torque setting on my torque wrench on the main caps until the bolt barely moved. It was right at 50#