Torquing Main Crank Bolt

#1

I have recently re-installed my rebuilt engine and am hoping to fire it up in the near future.

One issue has popped up that I’m not sure how to deal with. I was not able to fully torque the main crankshaft bolt on the front of the engine before installing it back in the car. The Bentley manual says to torque it to 305 lb/ft, which obviously requires a big torque wrench. The Manual also says there is a “special BMW tool” to hold the crank while tightening the bolt, but of course I don’t have it. I figured it would be easier tighten the bolt after the engine and driveline were installed. (It was a bitch to get the bolt loose with the engine out of the car and on a stand because the whole stand wanted to turn over.)

Anyway, I got a big torque wrench, put the car in 5th gear, fully applied the emergency brake and had a buddy apply the brakes. I was only able to get up to 240 lbs. before the wrench stopped “clicking,” even though it kept turning. It almost felt rubbery because it would “rebound” when I took the tension off the wrench. Not sure what was going on there.

Any idea how I can get this tightened to full torque?

#2

Since I got no replies to my original post, I don’t know whether no one cared, or no one knew the answer.

For what it is worth, I asked at the local BMW shop and the mechanic said they just “wedge a screw driver in the flywheel” to hold it when they torque the main crank bolt. I was skeptical, but since I had no other choice decided to see what I could do.

I first removed the access plate on the bottom of the engine and had a buddy turn the engine over a full revolution so I could see if there was somewhere on the flywheel I could insert a bolt so I could bolt a piece of flat steel to the flywheel and to the transmission housing, but no luck.

So, I took a big screwdriver and wedged it in the teeth of the flywheel gear and then wedged a steel chisel under it at an offset in the bell housing to hold it. My buddy torqued the bolt pretty easily and (hopefully) all is good.

#3

410Nm (302.5 ft-lb) seems high as heck, but that’s what the Haynes manual says too.

I’d have to borrow a torque wrench from work to get that high.

I’ve always used the screw driver on the flywheel trick.

#4

The wrench I borrowed was from a heavy truck facility and was a monster. About 4 feet long. No problem generating the torque with that.

#5

Haven’t rebuilt an M20 yet, but have had the crank nut off on an M42 and a M30. I just said screw it and bought the tools. Looks like this in case you were interested. Long bar that secures against the front frame, while on the other end is bolted to the crank.