Time For a New Head - Part 4

[old guy]

A continuation from this thread: Time-For-a-New-Head-Part-3

So I thought a little follow up would be nice. I replaced my AMB head with a AWM head a few weeks ago and couldn't remember if I torqued the head properly. I have been debating the best course of action.

Rocket1420 recently replaced his head using the same head bolts and head gasket that I used. He meticulously recorded all of the torque readings for each bolt after both the 90° and 180° turns. Using these torque values I was able to determine an average value for each position.

He started at 40Nm per specification. He added 90° of rotation. At 90° the average torque value was 69Nm. He then added an additional 90° for a total of 180° of rotation. At 180° of rotation the average torque value was 86Nm.

So here you can see how the stretch bolts are reacting to the increased turns. The first 90° increased the torque by 29Nm. The second 90° only increased the torque by 17Nm. Basically what is happening is that the bolts are beginning to stretch. Sorta like a rubber band. This stretch is what helps the bolts maintain a constant clamping force even with the differences in expansion between an Aluminum head and Iron block.

Today I pulled my valve cover and checked the torque on my head bolts. I set my torque wrench to 80Nm. I assumed that if I had applied the second 90° turn that I would quickly hit the 80Nm limit. If I had not applied the second 90° turn I would be able to get most if not all of a 90° turn in before hitting the 80Nm limit.

I'm glad I did this! I guess I really did forget to add the second 90° turn. Apparently the first 90° is enough to initially hold the head gasket but would probably cause a leak in the long run. I put close to 1,000 miles on my engine with only the first 90° with no problems. I used my block tester to make sure I wasn't getting any exhaust gasses into the cooling system and came up clean.

Again - A special thanks to Rocket1420 for providing me the information necessary to put this to bed. Now I can quit worrying about having to do this all over again in the near future.

Now it's time for a stiff Bourbon;-)

[All_Euro]

That is good info - thanks for the follow up and glad to hear you didn’t have any short-term issues with only the first 90*!

[diagnosticator]

Are you using stock OEM head bolts or other?

[old guy]

Elring head bolts and head gasket. I had originally considered ARP bolts but at a 5X cost factor and considering the OEM stretch bolts lasted 225k miles it just didn't seem worth the expenditure.

[diagnosticator]

Yeah, that is reasonable. However, applied torque plus angle turn vs developed clamp load is not a linier relationship. See http://www.boltscience.com for additional info. Text explains how to audit torque settings to confirm correct torque on fasteners,

[Furly]

Pretty sure my bentley only had 1 90° stretch. If it requires two stretches perhaps that has something to do with my head gasket leaking oil out the front passenger side.

[customa4]

Wow I read the title and saw it was from Old Guy and though to myself Oh No!! I'm glad this thread isn't what it sounds like. I thought you actually needed another new head!

Here ya go Furly, torque specs are in here. Click

[rocket1420]

Pretty sure my bentley only had 1 90° stretch. If it requires two stretches perhaps that has something to do with my head gasket leaking oil out the front passenger side.

It is technically 40 Nm + 180 degrees (two 90 degrees allowable).

Unfortunately I had to do my head all over again. My AMB head had .007" distortion, and I put it on anyway. Good news, though, is that I have a freshly machined AWM head ready to go, and my second head gasket and head bolts arrived today. So tomorrow I should be caught back up to Fletcher. Hopefully. Just glad I could help!

Lesson learned - when the manual says .004" allowable, it really means that.

[old guy]

Applied torque plus angle turn vs developed clamp load is not a linear relationship.

Exactly. That's why TTT (Torque To Turns) also know as TTA (Torque To Angle) bolts have a low base torque value followed with an angle (turns) value. The base torque value assures that the bolt is bottomed out. Then the calculated turns value assures that the bolt has been stretched to the proper length to keep it within the acceptable elastic region.

Measuring rotation in degrees instead of an applied torque figure ensures that friction does not overcome the tightening process and that an even clamp load is achieved. Too little stretch and the clamp force can drop out of the holding capacity required to contain the force requirement. (ex: engine firing events). Too much stretch can take the bolt out of its elastic region and cause the bolt to reduce in holding force or even go to the point of bolt failure.

By using calculated length values it takes frictional torque out of the equation.