427 Head Bolt Tensile Strength

[Ned T.]

I might need to get new head bolts for my engine builder. I want to use bolts with the original head markings. The builder wants to use bolts that are 180,000 PSI Tensile Strength. This is what he uses in normal high horse motors. I realize 180,0000 is very high. I have other bolts with this rating but not with the correct head markings. I was thinking of reusing the good used ones where it shows and new ones where it doesnt show. My question is: What is the Tensile Strength on stock head bolts? Can I buy new ones with the correct head markings?

[Michael H.]

Re: 427 Head Bolt Tensile Strength

I might need to get new head bolts for my engine builder. I want to use bolts with the original head markings. The builder wants to use bolts that are 180,000 PSI Tensile Strength. This is what he uses in normal high horse motors. I realize 180,0000 is very high. I have other bolts with this rating but not with the correct head markings. I was thinking of reusing the good used ones where it shows and new ones where it doesnt show. My question is: What is the Tensile Strength on stock head bolts? Can I buy new ones with the correct head markings?

Ned,

In my opinion, if that's worth anything, the new super bolts that your builder wants to use are a complete waste of money and absolutely unneccessary.
If your old/original bolts don't have any issues, (serious rust or rounded hex head) I would use them over again.

The load applied by a 7/16-14 bolt at 75 lbs ft is probably less than 20,000 lbs.

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Correction: The load applied by a 7/16-14 bolt at 75 lbs ft torque is just shy of 26,000 lb

[Duke W.]

Re: 427 Head Bolt Tensile Strength

I dont know the yield or ultimate tensile strength of the OE big block head bolts, but unless it is necessary to use higher than OE tightening torque, greater strength bolts will do absolutely nothing, and greater than OE tightening torque is not necessary unless the engine is highly modified.

Using higher strength bolts with greater tightening torque increases the risk of pulling threads, which is why if higher than OE tightening torque is required, hardened head studs are preferred.

This is another example of "engine builders" wanting to use aftermarket parts that offer no benefit over tried in true OE parts, but they will likely be more expensive.

From an engineering perspective, the greater a bolts' UTS the more brittle it is. As a general rule, head bolts do not call for a great amount of torque relative to their shank size and in many cases may be no more than Grade 5.

Stock or mildly modified iron block/iron head SB and BB engines have no head bolt/head gasket issues that I know of. If it works don't "fix" it!

Duke

[Mike M.]

Re: 427 Head Bolt Tensile Strength

agree with mike and duke. only advantage to using the HTS head bolts will be seen in the engine rebuilders bank account.mike

[Ned T.]

Re: 427 Head Bolt Tensile Strength

Thanks for the replys. i will pass this info on to the builder. I think he is more used to very high horse power engines. He has been doing these for over 30 years that i know of, and also has a school. I dont think his intention was to run up the bill, hes been a friend of the family for 50 years. I will go with the stock bolts unless there are issues with rust.

[Duke W.]

Re: 427 Head Bolt Tensile Strength

The elastic modulus of steel and the thermal expansion rate for typical bolt alloys is essentially the same. Therefore, regardless of a bolt's UTS the amount of strain (stretch) for a given torque is essentially the same for the same bolt geometry.

For example if you load either a 120 kpsi or 180 kspi UTS steel with a 90 kpsi load they will both stretch about the same amount. If you want to increase clamping force commensurate with 110K psi load, then the higher strength bolt is required because the lower strength bolt (say it yields at 100 kpsi) will yield and will not provide additional clamping force beyond the yield stress.

As far as thermal expansion is concerned the slight difference between cast iron and steel is not enough to significantly alter the stress on a bolt between a hot and cold engine.

The above reason is also why there is usually barely any measurable difference between hot (engine idling) and cold valve clearance because the cast iron block/head expand about the same amount as the steel pushrod.

Again, the only reason to use higher than OE strength head bolts (or studs) is to increase clamping force with higher installation torque, and a higher YTS bolt may be required because the design requires that the loading remain in the elastic range, however, this is not true of all engines.

Some modern engines are designed with head bolts that are torqued to and slightly beyond the elastic limit. Some of these are one use only, and some can be reused more than once, but have a replacement overall length and must be replaced when this limit is reached.

Duke

[Michael H.]

Re: 427 Head Bolt Tensile Strength

The elastic modulus of steel and the thermal expansion rate for typical bolt alloys is essentially the same. Therefore, regardless of a bolt's UTS the amount of strain (stretch) for a given torque is essentially the same for the same bolt geometry.

I WAS going to post and say basically the same thing but decided to pass.

Amazing that some engine builders go to extremes like double trick head bolts and rollerized everything but then, they hone cyl's with a $20 auto parts hone without deck plates installed. ???? Just amazing, some of the "super duper trick" JUNK that flows through the aftermarket HP world but the important basics are swept aside.

[Duke W.]

Re: 427 Head Bolt Tensile Strength

Yes, and they also rarely measure deck clearance and select a head gasket to optimize the CR for premium fuel.

Most still recommend "low compression pistons" and just throw the engine together without measuring anything, and you barely end up with something between 8 and 9 to one, which leaves a lot of torque/power across the entire rev range sitting on the table and not in the engine.

Duke