More head gasket questions....

[Dave M]

All the talk about head bolts steam from exhaust post (http://www.bimmer.info/forum/showthread.php?t=18080) has raised my own question. When re-assembling the M50, I did it as per Bentley (below) and simply went throught the bolts in the specified order without running the vehicle. Am I in for a surprise or have I done it correctly? I didn't read anything in the manual that eluded to a warm-up period.

http://img.photobucket.com/albums/v11/dave_macisaac/Bimmer/Engine/head_bolt_specs.jpg

Thanks again,

Dave M

[632 Regal]

interesting point!

bump

[jjw]

Provided you use new bolts, and lube the thread with oil. You should have no problems. No need to warm up the engine to torque the bolts.

[Jon K]

A lot of people recommend retorquing the bolts after 500 miles

[jjw]

Hmmm, interesting. When I have my 525 head remove for broken valve cover stud, there was no need to re-torque after whatever miles. And it's been thousands of miles ago. Those bolts should not be reused, so if you losen them, then they are junks.

[Jon K]

This could be true - i am only schooled in ARP head studs... not OE bolts.

[Dave M]

Well, unless someone thinks differently, I'll leave it as is.

Thanks,

Dave M

[Bill R.]

retorque them after the initial torque. Retorqing is not needed and can cause damage. The whole point of torque to yield is that the bolts are torqued until they reach the elastic point where they are stretching. If you retorque later then they odds are you'll break a bolt since they have already been stretched.
Normally on a non torque to yield bolt the aluminum head and block expand as the engine warms up. They expand much faster than the steel bolts do so as the engine warms up the gasket is squeezed harder, then when the engine cools off it shrinks back down and the gasket gets looser, this compression and release over and over tends to lead to the head gasket failing an leaking eventually, this is how the m30 is without torque to yield bolts. With torque to yield like you have, you tighten the bolts down to the point where they start to stretch and become somewhat elastic. With this type of bolt the clamping force of say 90lbs cold when the engine heats up the bolts stretch more and the clamping force stays the same 90lbs, this keeps an even pressure on the head gasket all the time and prolongs the life of the head gasket. So when you initially torqued these to that point the bolts are now at the ideal point for this to happen as designed.




Well, unless someone thinks differently, I'll leave it as is.

Thanks,

Dave M

[stx133]

bill is on the money,
recently torqued up some bolts on a hollow stub shaft to 1800 nm (used a torque multiplier and lots of extention bars). running steam at 180deg c through the shaft. once the bolts are tight and stretched leave them alone.

[Dave M]

retorque them after the initial torque. Retorqing is not needed and can cause damage. The whole point of torque to yield is that the bolts are torqued until they reach the elastic point where they are stretching. If you retorque later then they odds are you'll break a bolt since they have already been stretched.
Normally on a non torque to yield bolt the aluminum head and block expand as the engine warms up. They expand much faster than the steel bolts do so as the engine warms up the gasket is squeezed harder, then when the engine cools off it shrinks back down and the gasket gets looser, this compression and release over and over tends to lead to the head gasket failing an leaking eventually, this is how the m30 is without torque to yield bolts. With torque to yield like you have, you tighten the bolts down to the point where they start to stretch and become somewhat elastic. With this type of bolt the clamping force of say 90lbs cold when the engine heats up the bolts stretch more and the clamping force stays the same 90lbs, this keeps an even pressure on the head gasket all the time and prolongs the life of the head gasket. So when you initially torqued these to that point the bolts are now at the ideal point for this to happen as designed.

Phew, thanks for smart people. Now I can sleep well.

Dave m

[rob101]

anyone ever head of the porsche 2.7L 911 engine's head studs, they break or pop out of the block eventually....