Header bolt torque

[boat737]

Oh so close to mounting the headers (and first start). The engine builder said 35 ft-lb torque for the headers, but that sounds high to me. 3/8-16 x 1 inch bolts (Percy's) with anti-seize, into Edelbrock aluminum heads, no gasket (RTV Copper instead). I'm thinking more like 20 ft-lb. What do you say? Thank you.

[NAZ]

As much as 90% of the torque is used to overcome friction and anti-seize reduces that friction so you must reduce the torque value approximately 20% -- 25% when using a lubricant or anti-seize. If you don’t, you will stretch the fastener past it’s yield point and damage it & or the aluminum threads. This is a rule of thumb reduction, to get an exact number requires evaluating clamp loads using a Skidmore gauge but 20% -- 25% is a conservative number that will work for most applications. Use the anti-seize on the flange (area under the hex head) as well as the threads.

[Chuck Sloggett]

Verify the depth of the threads in the block, I had to use a bottoming tap to give the necessary room to avoid bottoming out before the bolts were tight. -- Chuck

[NAZ]

Good tip Chuck. I've had to do the same thing on even new heads that should have been machined properly from the get go. Also it's important to note that a steel fastener in aluminum threads will require a thread engagement ~1.5 times the fastener diameter.

[boat737]

As much as 90% of the torque is used to overcome friction and anti-seize reduces that friction so you must reduce the torque value approximately 20% -- 25% when using a lubricant or anti-seize. If you don’t, you will stretch the fastener past it’s yield point and damage it & or the aluminum threads. This is a rule of thumb reduction, to get an exact number requires evaluating clamp loads using a Skidmore gauge but 20% -- 25% is a conservative number that will work for most applications. Use the anti-seize on the flange (area under the hex head) as well as the threads.

To be clear, that is to reduce the 35 ft-lb BY 25%, or 35x(1-.25)=26.25 (or even 35-(.25x35)=26.25). Not 25% OF 35, or 8.75. Looks like I answered my own question. Never mind.

[boat737]

Good tip Chuck. I've had to do the same thing on even new heads that should have been machined properly from the get go. Also it's important to note that a steel fastener in aluminum threads will require a thread engagement ~1.5 times the fastener diameter.

Just ran the bolts all the way in. The 1" bolt has about .175" (to washer) sticking out. With a .375" header flange width, that leaves me about .200" leeway. I could almost have gone with a 1.25 inch length bolt. As it is, I calculate the bolt will be in the head .5625 (or 9/16"), which is the 1" bolt minus the 3/8" flange and the 1/16" washer thickness-.4375" (or 7/16), which is right at your 1.5 x bolt dia. rule. Whew...

[NAZ]

You should be good to go.

[CraigS]

With the variation in tools you will probably use to tighten the bolts in different positions I don't think you will be able to actually torque them. Remember the torque wrench must be in motion when it clicks or it is not a valid torque. I use my special ground down 3/8 inch box end wrench, which is maybe 4.5 inches long, and push or pull as tight as I can. Done.

[first time builder]

What Craigs said !

Kenny